uu.seUppsala University Publications
Change search
Refine search result
12 1 - 50 of 55
CiteExportLink to result list
Permanent link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf
Rows per page
  • 5
  • 10
  • 20
  • 50
  • 100
  • 250
Sort
  • Standard (Relevance)
  • Author A-Ö
  • Author Ö-A
  • Title A-Ö
  • Title Ö-A
  • Publication type A-Ö
  • Publication type Ö-A
  • Issued (Oldest first)
  • Issued (Newest first)
  • Created (Oldest first)
  • Created (Newest first)
  • Last updated (Oldest first)
  • Last updated (Newest first)
  • Disputation date (earliest first)
  • Disputation date (latest first)
  • Standard (Relevance)
  • Author A-Ö
  • Author Ö-A
  • Title A-Ö
  • Title Ö-A
  • Publication type A-Ö
  • Publication type Ö-A
  • Issued (Oldest first)
  • Issued (Newest first)
  • Created (Oldest first)
  • Created (Newest first)
  • Last updated (Oldest first)
  • Last updated (Newest first)
  • Disputation date (earliest first)
  • Disputation date (latest first)
Select
The maximal number of hits you can export is 250. When you want to export more records please use the Create feeds function.
  • 1. Andersen, A.C.
    et al.
    Höfner, Susanne
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Astronomy and Space Physics.
    Loidl, R.
    The Influence of Dust Properties on the Mass Loss in Pulsating AGB Stars2002In: Radial and Nonradial Pulsations as Probes of Stellar Physics, ASP Conference Proceedings, Vol. 259, 2002Conference paper (Other scientific)
    Abstract [en]

    We are currently studying carbon based dust types of relevance for carbon-rich AGB stars, to obtain a better understanding of the influence of the optical and chemical properties of the grains on the mass loss of the star. An investigation of the complex interplay between hydrodynamics,radiative transfer and chemistry has to be based on a better knowledge of the micro-physics of the relevant dust species.

  • 2.
    Bladh, S.
    et al.
    Univ Padua, Dipartimento Fis & Astron Galileo Galilei, Vicolo Osservatorio 3, I-35122 Padua, Italy..
    Paladini, C.
    Univ Libre Bruxelles, Inst Astron & Astrophys, CP 226,Blvd Triomphe, B-1050 Brussels, Belgium..
    Höfner, Susanne
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Aringer, B.
    Univ Padua, Dipartimento Fis & Astron Galileo Galilei, Vicolo Osservatorio 3, I-35122 Padua, Italy..
    Tomography of silicate dust around M-type AGB stars I. Diagnostics based on dynamical models2017In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 607, article id A27Article in journal (Refereed)
    Abstract [en]

    Context: The heavy mass loss observed in evolved asymptotic giant branch stars is usually attributed to a two-step process: atmospheric levitation by pulsation-induced shock waves, followed by radiative acceleration of newly formed dust grains. Detailed wind models suggest that the outflows of M-type AGB stars may be triggered by photon scattering on Fe-free silicates with grain sizes of about 0.1-1 mu m. As a consequence of the low grain temperature, these Fe-free silicates can condense close to the star, but they do not produce the characteristic mid-IR features that are often observed in M-type AGB stars. However, it is probable that the silicate grains are gradually enriched with Fe as they move away from the star, to a degree where the grain temperature stays below the sublimation temperature, but is high enough to produce emission features.

    Aims: We investigate whether differences in grain temperature in the inner wind region, which are related to changes in the grain composition, can be detected with current interferometric techniques, in order to put constraints on the wind mechanism.

    Methods: We use phase-dependent radial structures of the atmosphere and wind of an M-type AGB star, produced with the 1D radiation-hydrodynamical code DARWIN, to investigate if current interferometric techniques can differentiate between the temperature structures that give rise to the same overall spectral energy distribution.

    Results: The spectral energy distribution is found to be a poor indicator of different temperature profiles and therefore is not a good tool for distinguishing different scenarios of changing grain composition. However, spatially resolved interferometric observations have promising potential. They show signatures even for Fe-free silicates (found at 2-3 stellar radii), in contrast to the spectral energy distribution. Observations with baselines that probe spatial scales of about 4 stellar radii and beyond are suitable for tracing changes in grain composition, since this is where effects of Fe enrichment should be found.

  • 3.
    Bladh, Sara
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Höfner, Susanne
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Exploring wind-driving dust species in cool luminous giants: I. Basic criteria and dynamical models of M-type AGB stars2012In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 546, p. A76-Article in journal (Refereed)
    Abstract [en]

    Context. The heavy mass loss observed in evolved asymptotic giant branch stars is usually attributed to a two-stage process: atmospheric levitation by pulsation-induced shock waves followed by radiative acceleration of dust grains, which transfer momentum to the surrounding gas through collisions. In order for an outflow to occur the two stages of the mass-loss scheme have to connect, i.e., the radiative acceleration can only be initiated if the levitated gas reaches a distance from the stellar photosphere where dust particles can condense. This levitation distance is limited by the kinetic energy transferred to the gas by the shock waves, which imposes strict constraints on potential wind-driving dust species. Aims. This work is part of an ongoing effort aiming at identifying the actual wind-drivers among the dust species observed in circumstellar envelopes. In particular, we focus on the interplay between a strong stellar radiation field and the dust formation process. Methods. To identify critical properties of potential wind-driving dust species we use detailed radiation-hydrodynamical models which include a parameterized dust description, complemented by simple analytical estimates to help with the physical interpretation of the numerical results. The adopted dust description is constructed to mimic different chemical and optical dust properties in order to systematically study the effects of a realistic radiation field on the second stage of the mass loss mechanism. Results. We see distinct trends in which combinations of optical and chemical dust properties are needed to trigger an outflow. Dust species with a low condensation temperature and a near-infrared absorption coefficient that decreases strongly with wavelength will not condense close enough to the stellar surface to be considered as potential wind-drivers. Conclusions. Our models confirm that metallic iron and Fe-bearing silicates are not viable as wind-drivers due to their near-infrared optical properties and resulting large condensation distances. TiO2 is also excluded as a wind-driver due to the low abundance of Ti. Other species, such a SiO2 and Al2O3, are less clear-cut cases due to uncertainties in the optical and chemical data and further work is needed. A strong candidate is Mg2SiO4 with grain sizes of 0.1-1 mu m, where scattering contributes significantly to the radiative acceleration, as suggested by earlier theoretical work and supported by recent observations.

  • 4.
    Bladh, Sara
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Höfner, Susanne
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Aringer, B.
    Dust in AGB Stars: Transparent or Opaque?2011In: Why Galaxies Care about AGB Stars II: Shining Examples and Common Inhabitants, 2011, Vol. 445Conference paper (Refereed)
    Abstract [en]

    The optical properties of the dust particles that drive the winds of cool giant stars affect the stellar spectra in two ways: (1) indirectly, through their influence on the dynamical structure of the atmosphere/envelope and the resulting molecular features, and (2) directly, by changes of the spectral energy distribution due to absorption and scattering on dust grains. The qualitative differences in the energy distributions of C-type and M-type AGB stars in the visual and near-infrared regions suggest that the dust particles in oxygen rich atmospheres are relatively transparent to radiation. By using detailed dynamical models of gas and radiation combined with a simple description for the dust opacity (which can be adjusted to mimic different wavelength dependences and condensation temperatures) and also by adjusting the fraction of the opacity that is treated as true absorption, we investigate which dust properties produce synthetic photometry consistent with observations. The goal of this study is to narrow down the possible dust species that may be driving the winds in M-type AGB stars.

  • 5.
    Bladh, Sara
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
    Höfner, Susanne
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
    Aringer, Bernhard
    Univ Vienna, Dept Astrophys, A-1010 Vienna, Austria..
    Eriksson, Kjell
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
    Exploring Mass-Loss in M-type AGB Stars2015In: WHY GALAXIES CARE ABOUT AGB STARS III: A CLOSER LOOK IN SPACE AND TIME, ASTRONOMICAL SOC PACIFIC , 2015, Vol. 497, p. 345-350Conference paper (Refereed)
    Abstract [en]

    Stellar winds observed in asymptotic giant branch (AGB) stars are usually attributed to a combination of stellar pulsations and radiation pressure on dust. Strong candidates for wind-driving dust species in M-type AGB stars are magnesium silicates (Mg2SiO4 and MgSiO3). Such grains can form close to the stellar surface; they consist of abundant materials and, if they grow to sizes comparable to the wavelength of the stellar flux maximum, they experience strong acceleration by photon scattering. Here we present results from an extensive set of time-dependent wind models for M-type AGB stars with a detailed description for the growth of Mg2SiO4 grains. We show that these models reproduce observed mass-loss rates and wind velocities, as well as visual and near-IR photometry. However, the current models do not show the characteristic silicate features at 10 and 18 mu m, due to a rapidly falling temperature of Mg2SiO4 grains in the wind. Including a small amount of Fe in the grains further out in the circumstellar envelope will increase the grain temperature and result in pronounced silicate features, without significantly affecting the photometry in the visual and near-IR.

  • 6.
    Bladh, Sara
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Höfner, Susanne
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Nowotny, W.
    Aringer, B.
    Eriksson, Kjell
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Exploring wind-driving dust species in cool luminous giants II. Constraints from photometry of M-type AGB stars2013In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 553, p. A20-Article in journal (Refereed)
    Abstract [en]

    Context. The heavy mass loss observed in evolved asymptotic giant branch (AGB) stars is usually attributed to a two-stage process: atmospheric levitation by pulsation-induced shock waves, followed by radiative acceleration of newly formed dust grains. The dust transfers momentum to the surrounding gas through collisions and thereby triggers a general outflow. Radiation-hydrodynamical models of M-type AGB stars suggest that these winds can be driven by photon scattering - in contrast to absorption - on Fe-free silicate grains of sizes 0.1-1 mu m. Aims. In this paper we study photometric constraints for wind-driving dust species in M-type AGB stars, as part of an ongoing effort to identify likely candidates among the grain materials observed in circumstellar envelopes. Methods. To investigate the scenario of stellar winds driven by photon scattering on dust, and to explore how different optical and chemical properties of wind-driving dust species affect photometry we focus on two sets of dynamical models atmospheres: (i) models using a detailed description for the growth of Mg2SiO4 grains, taking into account both scattering and absorption cross-sections when calculating the radiative acceleration; and (ii) models using a parameterized dust description, constructed to represent different chemical and optical dust properties. By comparing synthetic photometry from these two sets of models to observations of M-type AGB stars we can provide constraints on the properties of wind-driving dust species. Results. Photometry from wind models with a detailed description for the growth of Mg2SiO4 grains reproduces well both the values and the time-dependent behavior of observations of M-type AGB stars, providing further support for the scenario of winds driven by photon scattering on dust. The photometry from the models with a parameterized dust description suggests that wind-drivers need to have a low absorption cross-section in the visual and near-IR to reproduce the time-dependent behavior, i. e. small variations in (J-K) and spanning a larger range in (V-K). This places constraints on the optical and chemical properties of the wind-driving dust species. Conclusions. To reproduce the observed photometric variations in (V-K) and (J-K) both detailed and parameterized models suggest that the wind-driving dust materials have to be quite transparent in the visual and near-IR. Consequently, strong candidates for outflows driven by photon scattering on dust grains are Mg2SiO4, MgSiO3, and potentially SiO2.

  • 7.
    Bladh, Sara
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Astronomy and Space Physics, Theoretical Astrophysics.
    Susanne, Höfner
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Aringer, Bernhard
    Eriksson, Kjell
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Exploring wind-driving dust species in cool luminous giants III: Wind models for M-type AGB stars2015In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 575, article id A105Article in journal (Refereed)
  • 8.
    Doan, Lam
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Ramstedt, Sofia
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Vlemmings, W. H. T.
    Department of Earth and Space Sciences, Chalmers University of Technology.
    Höfner, Susanne
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    De Beck, E.
    Department of Earth and Space Sciences, Chalmers University of Technology.
    Kerschbaum, F.
    Department of Astrophysics, University of Vienna.
    Lindqvist, M.
    Department of Earth and Space Sciences, Chalmers University of Technology.
    Maercker, M.
    Department of Earth and Space Sciences, Chalmers University of Technology.
    Mohamed, S.
    South African Astronomical Observatory; Astronomy Department, University of Cape Town; National Institute for Theoretical Physics, Private Bag X1, 7602 Matieland.
    Paladini, C.
    Institut d’Astronomie et d’Astrophysique, Université Libre de Bruxelles.
    Wittkowski, M.
    European Southern Observatory, Germany.
    The extended molecular envelope of the asymptotic giant branchstar π1 Gruis as seen by ALMA: I. Large-scale kinematic structure and CO excitation properties2017In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 605, article id A28Article in journal (Refereed)
    Abstract [en]

    Context. The S-type asymptotic giant branch (AGB) star pi(1) Gru has a known companion at a separation of 2 ''.7 (approximate to 400 AU). Previous observations of the circumstellar envelope (CSE) show strong deviations from spherical symmetry. The envelope structure, including an equatorial torus and a fast bipolar outflow, is rarely seen in the AGB phase and is particularly unexpected in such a wide binary system. Therefore a second, closer companion has been suggested, but the evidence is not conclusive.

    Aims. The aim is to make a 3D model of the CSE and to constrain the density and temperature distribution using new spatially resolved observations of the CO rotational lines.

    Methods. We have observed the J = 3-2 line emission from (CO)-C-12 and (CO)-C-13 using the compact arrays of the Atacama Large Millimeter/submillimeter Array (ALMA). The new ALMA data, together with previously published (CO)-C-12 J = 2-1 data from the Submillimeter Array (SMA), and the (CO)-C-12 J = 5-4 and J = 9-8 lines observed with Herschel/Heterodyne Instrument for the Far-Infrared (HIFI), is modeled with the 3D non-LTE radiative transfer code SHAPEMOL.

    Results. The data analysis clearly confirms the torus-bipolar structure. The 3D model of the CSE that satisfactorily reproduces the data consists of three kinematic components: a radially expanding torus with velocity slowly increasing from 8 to 13 km s(-1) along the equator plane; a radially expanding component at the center with a constant velocity of 14 km s(-1); and a fast, bipolar outflow with velocity proportionally increasing from 14 km s(-1) at the base up to 100 km s(-1) at the tip, following a linear radial dependence. The results are used to estimate an average mass-loss rate during the creation of the torus of 7.7 x 10(-7) M-circle dot yr(-1). The total mass and linear momentum of the fast outflow are estimated at 7.3 x 10(-4) M-circle dot and 9.6 x 10(37) g cm s(-1), respectively. The momentum of the outflow is in excess (by a factor of about 20) of what could be generated by radiation pressure alone, in agreement with recent findings for more evolved sources. The best-fit model also suggests a (CO)-C-12/(CO)-C-13 abundance ratio of 50. Possible shaping scenarios for the gas envelope are discussed.

  • 9.
    Eriksson, Kjell
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Nowotny, W.
    Univ Vienna, Dept Astrophys, A-1010 Vienna, Austria..
    Höfner, Susanne
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Aringer, B.
    Univ Vienna, Dept Astrophys, A-1010 Vienna, Austria..
    A New Library of Synthetic Spectra and Photometry for Evolved C Stars2015In: WHY GALAXIES CARE ABOUT AGB STARS III: A CLOSER LOOK IN SPACE AND TIME, ASTRONOMICAL SOC PACIFIC , 2015, Vol. 497, p. 111-112Conference paper (Refereed)
    Abstract [en]

    We present a library of synthetic spectra and photometry calculated on the basis of a grid of atmosphere and wind models for carbon-rich, pulsating AGB stars. The spectra cover the range between 0.35 and 25 mu m with a spectral resolution of R=200. The corresponding photometric variations during several pulsation periods were computed for standard broadband filters in the visual and near-infrared.

  • 10.
    Eriksson, Kjell
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Nowotny, W.
    Höfner, Susanne
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Aringer, B.
    Wachter, A.
    Synthetic photometry for carbon-rich giants IV: An extensive grid of dynamic atmosphere and wind models2014In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 566, p. A95-Article in journal (Refereed)
    Abstract [en]

    Context. The evolution and spectral properties of stars on the asymptotic giant branch (AGB) are significantly affected by mass loss through dusty stellar winds. Dynamic atmosphere and wind models are an essential tool for studying these evolved stars, both individually and as members of stellar populations, to understand their contribution to the integrated light and chemical evolution of galaxies. Aims. This paper is part of a series with the purpose of testing state-of-the-art atmosphere and wind models of C-type AGB stars against observations, and making them available to the community for use in various theoretical and observational studies. Methods. We have computed low-resolution spectra and photometry (in the wavelength range 0.35-25 mu m) for a grid of 540 dynamic models with stellar parameters typical of solar-metallicity C-rich AGB stars and with a range of pulsation amplitudes. The models cover the dynamic atmosphere and dusty outflow (if present), assuming spherical symmetry, and taking opacities of gas-phase species and dust grains consistently into account. To characterize the time-dependent dynamic and photometric behaviour of the models in a concise way we defined a number of classes for models with and without winds. Results. Comparisons with observed data in general show a quite satisfactory agreement for example regarding mass-loss rates vs. (J - K) colours or K magnitudes vs. (J - K) colours. Some exceptions from the good overall agreement, however, are found and attributed to the range of input parameters (e.g. relatively high carbon excesses) or intrinsic model assumptions (e.g. small particle limit for grain opacities). Conclusions. While current results indicate that some changes in model assumptions and parameter ranges should be made in the future to bring certain synthetic observables into better agreement with observations, it seems unlikely that these pending improvements will significantly affect the mass-loss rates of the models.

  • 11.
    Eriksson, Kjell
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Wachter, A.
    Höfner, Susanne
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Aringer, B.
    Nowotny, W.
    Synthetic Colors for Dynamic C-Star Models2011In: Why Galaxies Care about AGB Stars II: Shining Examples and Common Inhabitants, 2011, Vol. 445Conference paper (Refereed)
    Abstract [en]

    Dust is formed in the outer atmospheres and winds of AGB stars and severely affects their observed spectra and colours. Synthetic JHK colours from a grid of time-dependent C-star models are presented and compared to observed colours. The combined effects of pulsation and dust are significant, especially for the coolest and most luminous models. We are now extending the calculations with respect to effective temperature, luminosity, stellar mass, and pulsation amplitude, as well as the carbon excess.

  • 12.
    Eriksson, Kjell
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Wachter, A.
    Höfner, Susanne
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Aringer, B.
    Nowotny, W.
    Synthetic Colors for Dynamic C-Star Models2011In: Why Galaxies Care about AGB Stars II: Shining Examples and Common Inhabitants, 2011, Vol. 445Conference paper (Refereed)
    Abstract [en]

    Dust is formed in the outer atmospheres and winds of AGB stars and severely affects their observed spectra and colours. Synthetic JHK colours from a grid of time-dependent C-star models are presented and compared to observed colours. The combined effects of pulsation and dust are significant, especially for the coolest and most luminous models. We are now extending the calculations with respect to effective temperature, luminosity, stellar mass, and pulsation amplitude, as well as the carbon excess.

  • 13.
    Freytag, Bernd
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Astronomy and Space Physics.
    Höfner, Susanne
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Astronomy and Space Physics.
    Three-dimensional simulations of the atmosphere of an AGB star2008In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 483, no 2, p. 571-583Article in journal (Refereed)
    Abstract [en]

    Context. Winds of asymptotic giant branch stars are assumed to be driven by radiation pressure on dust. Previously, this process has been modeled with detailed time-dependent simulations of atmospheres and winds assuming spherically symmetric flows. In such models kinetic energy is injected by a variable inner boundary ("piston") simulating the effects of stellar pulsation. However, the dynamical processes in these atmospheres - convection and pulsations - are actually three-dimensional. Aims. We present and analyze first 3D radiation hydrodynamics simulations of the convective interior and the atmosphere of a typical AGB star. In particular, we check whether the piston description in the 1D wind models is compatible with the 3D results. Methods. We used two different RHD codes, one (CO5BOLD) to produce 3D models of the outer convective envelope and the inner atmosphere of an AGB star, the other to describe the atmosphere and the wind acceleration region, including dust formation and non-grey radiative transfer, but assuming spherically symmetric flows. From the movements of stellar surface layers in the 3D models, we derived a description for the variable inner boundary in the 1D models. Results. The 3D models show large convection cells and pulsations that give rise to roughly spherically expanding shock waves in the atmosphere, levitating material into regions which are cool enough to allow for dust formation. The atmospheric velocity fields have amplitudes and time scales close to the values that are necessary to start dust formation in the 1D wind models. Conclusions. The convection cells in the 3D simulations are so large that the associated shock fronts appear almost spherical, justifying the assumption of spherical symmetry and the use of a piston boundary condition in the context of wind models. Nevertheless, certain non-radial structures exist in the dust shell developing in the 3D simulations which should be detectable with current interferometric techniques.

  • 14.
    Freytag, Bernd
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Liljegren, Sofie
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Höfner, Susanne
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Global 3D radiation-hydrodynamics models of AGB stars: Effects of convection and radial pulsations on atmospheric structures2017In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 600, article id A137Article in journal (Refereed)
    Abstract [en]

    Context. Observations of asymptotic giant branch (AGB) stars with increasing spatial resolution reveal new layers of complexity of atmospheric processes on a variety of scales.

    Aims. To analyze the physical mechanisms that cause asymmetries and surface structures in observed images, we use detailed 3D dynamical simulations of AGB stars; these simulations self-consistently describe convection and pulsations.

    Methods. We used the CO5BOLD radiation-hydrodynamics code to produce an exploratory grid of global "star-in-a-box" models of the outer convective envelope and the inner atmosphere of AGB stars to study convection, pulsations, and shock waves and their dependence on stellar and numerical parameters.

    Results. The model dynamics are governed by the interaction of long-lasting giant convection cells, short-lived surface granules, and strong, radial, fundamental-mode pulsations. Radial pulsations and shorter wavelength, traveling, acoustic waves induce shocks on various scales in the atmosphere. Convection, waves, and shocks all contribute to the dynamical pressure and, thus, to an increase of the stellar radius and to a levitation of material into layers where dust can form. Consequently, the resulting relation of pulsation period and stellar radius is shifted toward larger radii compared to that of non-linear 1D models. The dependence of pulsation period on luminosity agrees well with observed relations. The interaction of the pulsation mode with the non-stationary convective flow causes occasional amplitude changes and phase shifts. The regularity of the pulsations decreases with decreasing gravity as the relative size of convection cells increases. The model stars do not have a well-defined surface. Instead, the light is emitted from a very extended inhomogeneous atmosphere with a complex dynamic pattern of high-contrast features.

    Conclusions. Our models self-consistently describe convection, convectively generated acoustic noise, fundamental-mode radial pulsations, and atmospheric shocks of various scales, which give rise to complex changing structures in the atmospheres of AGB stars.

  • 15. Gautschy-Loidl, Rita
    et al.
    Höfner, Susanne
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Astronomy and Space Physics.
    Jørgensen, Uffe G.
    Hron, Josef
    Dynamic model atmospheres of AGB stars: IV. A comparison of synthetic carbon star spectra with observations2004In: Astronomy and Astrophysics, ISSN 0004-6361, Vol. 422, p. 289-306Article in journal (Refereed)
    Abstract [en]

    We have calculated synthetic opacity sampling spectra for carbon-rich Asymptotic Giant Branch (AGB) stars based on dynamic model atmospheres which couple time-dependent dynamics and frequency-dependent radiative transfer, as presented in the third paper of this series. We include the molecules CO, CH, CN, C2, CS, HCN, C2H2 and C3 in our calculations, both when computing the atmospheric structures, and the synthetic spectra. A comparison of the synthetic spectra with various observed colours and spectra in the wavelength range between 0.5 and 25 μm,of TX Psc, WZ Cas, V460 Cyg, T Lyr and S Cep is presented. We obtain good agreement between observations gathered at different phases and synthetic spectra of one single hydrodynamical model for each star in the wavelength region between 0.5 and 5 μm. At longer wavelengths our models showing mass loss offer a first self-consistent qualitative explanation of why a strong feature around 14 μm, which is predicted by all hydrostatic models as well as dynamical models showing no mass loss, is missing in observed AGB carbon star spectra.

  • 16.
    Hofner, Susanne
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Wind Acceleration in AGB Stars: Solid Ground and Loose Ends2015In: Why Galaxies Care About AGB Stars III: A Closer Look In Space And Time, ASTRONOMICAL SOC PACIFIC , 2015, Vol. 497, p. 333-343Conference paper (Refereed)
    Abstract [en]

    The winds of cool luminous AGB stars are commonly assumed to be driven by radiative acceleration of dust grains which form in the extended atmospheres produced by pulsation-induced shock waves. The dust particles gain momentum by absorption or scattering of stellar photons, and they drag along the surrounding gas particles through collisions, triggering an outflow. This scenario, here referred to as Pulsation Enhanced Dust-DRiven Outflow (PEDDRO), has passed a range of critical observational tests as models have developed from empirical and qualitative to increasingly self-consistent and quantitative. A reliable theory of mass loss is an essential piece in the bigger picture of stellar and galactic chemical evolution, and central for determining the contribution of AGB stars to the dust budget of galaxies. In this review, I discuss the current understanding of wind acceleration and indicate areas where further efforts by theorists and observers are needed.

  • 17. Hron, J.
    et al.
    Aringer, B.
    Gautschy-Loidl, R.
    Höfner, Susanne
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Astronomy and Space Physics.
    Jørgensen, U.G.
    Synthetic spectra for pulsating red giants: status, limitations and applications2002In: Observed HR Diagrams and Stellar Evolution, ASP Conference Proceedings, Vol. 274, 2002Conference paper (Other scientific)
  • 18.
    Höfner, Susanne
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Dust Formation and Winds around Evolved Stars The Good, the Bad and the Ugly Cases2009In: COSMIC DUST - NEAR AND FAR / [ed] Henning T, Grun E, Steinacker J, 2009, Vol. 414, p. 3-21Conference paper (Refereed)
    Abstract [en]

    Cool luminous giants, in particular asymptotic giant branch stars, are among the most important sources of cosmic dust Their extended dynamical atmospheres are places where grains form and initiate outflows driven by radiation pressure, leading to considerable stellar mass loss and the enrichment of the interstellar medium with newly-produced elements This review summarizes the current understanding of dust formation and winds in such stars, sketching a system of criteria for identifying crucial types of dust grains in the range of possible condensates Starting with an overview of the specific conditions for dust formation in cool dynamic atmospheres, the role of grains as wind drivers, as well as their influence on observable properties of cool giants and the circum-stellar environment is discussed in some detail Regarding the literature, special attention is given to current developments, e g, the debate concerning the Fe-content and size of silicate grains in M-type AGB stars which are critical issues for the wind mechanism, or recent advances in spatially resolved observations and 3D modeling of giants and their dusty envelopes

  • 19.
    Höfner, Susanne
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Fresh light on stardust2012In: Nature, ISSN 0028-0836, E-ISSN 1476-4687, Vol. 484, no 7393, p. 172-173Article in journal (Other academic)
  • 20.
    Höfner, Susanne
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Astronomy and Space Physics. Department of Physics and Astronomy, Theoretical Astrophysics.
    Headwind: Modelling Mass Loss of AGB Stars, Against All Odds2007In: Why Galaxies Care About AGB Stars: Their Importance as Actors and Probes, 2007, p. p.145-155Conference paper (Other scientific)
    Abstract [en]

    The intricate interplay of atmospheric shock waves and a complex, variable radiation field with non-equilibrium dust formation presents a considerable challenge to self-consistent modelling of atmospheres and winds of AGB stars. Nevertheless it is clear that realistic models predicting mass loss rates and synthetic spectra are crucial for our understanding of this important phase of stellar evolution. While a number of questions are still open, significant progress has been achieved in recent years. In particular, self-consistent models for atmospheres and winds of C-stars have reached a level of sophistication which allows direct quantitative comparison with observations. In the case of stars with C/O < 1, however, recent work points to serious problems with the dust-driven wind scenario. This contribution analyzes the basic ingredients of this scenario with analytical estimates, focusing on dust formation, non-grey effects, and differences between C-rich and O-rich environments, as well as discussing the status of detailed dynamical wind models and current trends in this field.

  • 21.
    Höfner, Susanne
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Astronomy and Space Physics. Department of Physics and Astronomy, Theoretical Astrophysics.
    Solens lysande framtid2007In: Populär Astronomi, no 4, p. 16-18Article in journal (Other (popular scientific, debate etc.))
  • 22.
    Höfner, Susanne
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Starlight and Sandstorms: Mass Loss Mechanisms on the AGB2011In: Why Galaxies Care about AGB Stars II: Shining Examples and Common Inhabitants, 2011, Vol. 445Conference paper (Refereed)
    Abstract [en]

    There are strong observational indications that the dense slow winds of cool luminous AGB stars are driven by radiative pressure on dust grains which form in the extended atmospheres resulting from pulsation-induced shocks. For carbon stars, detailed models of outflows driven by amorphous carbon grains show good agreement with observations. Some still existing discrepancies may be due to a simplified treatment of cooling in shocks, drift of the grains relative to the gas, or effects of giant convection cells or dust-induced pattern formation. For stars with C/O \lt 1, recent models indicate that absorption by silicate dust is probably insufficient to drive their winds. A possible alternative is scattering by Fe-free silicate grains with radii of a few tenths of a micron. In this scenario one should expect less circumstellar reddening for M- and S-type AGB stars than for C-stars with comparable stellar parameters and mass loss rates.

  • 23.
    Höfner, Susanne
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Astronomy and Space Physics.
    Winds of Cool Giant Stars: Models and Observations2005In: Proc. of 13th Cambridge Workshop on Cool Stars, Stellar Systems and the Sun, 2005, p. 335-342Conference paper (Other scientific)
    Abstract [en]

    This review describes recent advances in models and observations of cool winds of asymptotic giant branch stars, focusing on infrared spectroscopy and interferometric methods, in combination with self-consistent time-dependent models of atmospheres and winds. The progress in these fields is leading to a more comprehensive picture of the complicated physical processes which cause the significant mass loss of these cool, evolved stars

  • 24.
    Höfner, Susanne
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Astronomy and Space Physics.
    Andersen, Anja C.
    Winds of M- and S-type AGB stars: An unorthodox suggestion for the driving mechanism2007In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 465, no 3, p. L39-L42Article in journal (Refereed)
    Abstract [en]

    Context: Current knowledge suggests that the dust-driven wind scenario provides a realistic framework for understanding mass loss from C-rich AGB stars. For M-type objects, however, recent detailed models demonstrate that radiation pressure on silicate grains is not sufficient to drive the observed winds, contrary to previous expectations. Aims: In this paper, we suggest an alternative mechanism for the mass loss of M-type AGB stars, involving the formation of both carbon and silicate grains due to non-equilibrium effects, and we study the viability of this scenario. Methods: We model the dynamical atmospheres and winds of AGB stars by solving the coupled system of frequency-dependent radiation hydrodynamics and time-dependent dust formation, using a parameterized description of non-equilibrium effects in the gas phase. This approach allows us to assess under which circumstances it is possible to drive winds with small amounts of carbon dust and to get silicate grains forming in these outflows at the same time. Results: The properties of the resulting wind models, such as mass-loss rates and outflow velocities, are well within the observed limits for M-type AGB stars. Furthermore, according to our results, it is quite unlikely that significant amounts of silicate grains will condense in a wind driven by a force totally unrelated to dust formation, as the conditions in the upper atmosphere and wind acceleration region put strong constraints on grain growth. Conclusions: .The proposed scenario provides a natural explanation for the observed similarities in wind properties of M-type and C-type AGB stars and implies a smooth transition for stars with increasing carbon abundance, from solar-composition to C-rich AGB stars, possibly solving the longstanding problem of the driving mechanism for stars with a C/O close to one.

  • 25.
    Höfner, Susanne
    et al.
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Astronomy and Space Physics.
    Gautschy-Loidl, R.
    Aringer, B.
    Nowotny, W.
    Hron, J.
    Freytag, B.
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Astronomy and Space Physics.
    Dynamic Model Atmospheres of Cool Giants2005In: High Resolution Infrared Spectroscopy in Astronomy, 2005, p. 271-282Conference paper (Other scientific)
    Abstract [en]

    Cool giant stars are highly dynamical objects, and complex micro-physical processes play an important role in their extended atmospheres and winds. The interpretation of observations, and in particular of high-resolution IR spectra, requires realistic self-consistent model atmospheres. Current dynamical models include rather detailed micro-physics, and the resulting synthetic spectra compare reasonably well with observations. A transition from qualitative to quantitative modelling is taking place at present. We give an overview of existing dynamical model atmospheres for AGB stars, discussing recent advances and current trends in modelling. When comparing synthetic spectra and other observable properties resulting from dynamical models with observations we focus on the near- and mid-IR wavelength range.

  • 26.
    Höfner, Susanne
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Olofsson, Hans
    Chalmers Univ Technol, Dept Space Earth & Environm, Onsala Space Observ, Onsala.
    Mass loss of stars on the asymptotic giant branch Mechanisms, models and measurements2018In: The Astronomy and Astrophysics Review, ISSN 0935-4956, E-ISSN 1432-0754, Vol. 26, no 1Article, review/survey (Refereed)
    Abstract [en]

    As low-and intermediate-mass stars reach the asymptotic giant branch (AGB), they have developed into intriguing and complex objects that are major players in the cosmic gas/dust cycle. At this stage, their appearance and evolution are strongly affected by a range of dynamical processes. Large-scale convective flows bring newly-formed chemical elements to the stellar surface and, together with pulsations, they trigger shock waves in the extended stellar atmosphere. There, massive outflows of gas and dust have their origin, which enrich the interstellar medium and, eventually, lead to a transformation of the cool luminous giants into white dwarfs. Dust grains forming in the upper atmospheric layers play a critical role in the wind acceleration process, by scattering and absorbing stellar photons and transferring their outward-directed momentum to the surrounding gas through collisions. Recent progress in high-angular-resolution instrumentation, from the visual to the radio regime, is leading to valuable new insights into the complex dynamical atmospheres of AGB stars and their windforming regions. Observations are revealing asymmetries and inhomogeneities in the photospheric and dust-forming layers which vary on time-scales of months, as well as more long-lived large-scale structures in the circumstellar envelopes. High-angular-resolution observations indicate at what distances from the stars dust condensation occurs, and they give information on the chemical composition and sizes of dust grains in the close vicinity of cool giants. These are essential constraints for building realistic models of wind acceleration and developing a predictive theory of mass loss for AGB stars, which is a crucial ingredient of stellar and galactic chemical evolution models. At present, it is still not fully possible to model all these phenomena from first principles, and to predict the mass-loss rate based on fundamental stellar parameters only. However, much progress has been made in recent years, which is described in this review. We complement this by discussing how observations of emission from circumstellar molecules and dust can be used to estimate the characteristics of the mass loss along the AGB, and in different environments. We also briefly touch upon the issue of binarity.

  • 27.
    Lebzelter, T.
    et al.
    Univ Vienna, Dept Astrophys, A-1010 Vienna, Austria..
    Nowotny, W.
    Univ Vienna, Dept Astrophys, A-1010 Vienna, Austria..
    Hinkle, K. H.
    Natl Opt Astron Observ, Tucson, AZ 85726 USA..
    Höfner, Susanne
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Aringer, B.
    Univ Vienna, Dept Astrophys, A-1010 Vienna, Austria..
    Heiter, Ulrike
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Observational Astronomy.
    The Challenges of Abundance Analysis for Long-Period Variables2015In: WHY GALAXIES CARE ABOUT AGB STARS III: A CLOSER LOOK IN SPACE AND TIME, ASTRONOMICAL SOC PACIFIC , 2015, Vol. 497, p. 283-288Conference paper (Refereed)
    Abstract [en]

    The measurement of elemental abundances of Asymptotic Giant Branch (AGB) stars is a highly challenging task that so far has not been solved in a satisfactory way. We report here on our efforts to confront predictions from hydrostatic and dynamical model atmospheres with observational results based on high-resolution spectra of variable and non-variable giants in the globular cluster 47 Tuc. Our goal is to estimate the impact of atmospheric dynamics on the abundance determination.

  • 28. Lebzelter, T.
    et al.
    Nowotny, W.
    Hinkle, K. H.
    Höfner, Susanne
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
    Aringer, Bernhard
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
    Abundance analysis for long-period variables II. RGB and AGB stars in the globular duster 47 Tucanae2014In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 567, p. A143-Article in journal (Refereed)
    Abstract [en]

    Context Asymptotic giant branch (AGB) stars play a key role in the enrichment of galaxies with heavy elements. Due to their large amplitude variability, the measurement of elemental abundances is a highly challenging task that has not been solved in a satisfactory V ay yet. Aims. Following our previous work we use hydrostatic and dynamical model atmospheres to simulate observed high-resolution near-infrared spectra of 12 variable and non variable red giants in the globular cluster 47 Tuc. The 47 Tuc red giants are independently well characterized in important parameters (mass, metallicity, luminosity). The principal aim was to compare synthetic spectra based on the dynamical models with observational spectra of 47 Ric variables. Assuming that the abundances are unchanged on the upper giant branch in these low-mass stars, our goal is to estimate the impact of atmospheric dynamics on the abundance determination. Methods. To estimate abundances we measured the equivalent widths of selected features in observed spectra and compared the results with predictions from a set of hydrostatic and dynamical model atmospheres resembling 47 Tun AGB stars in their fundamental parameters. Our study includes lines of (CO)-C-12, (CO)-C-13. OH, and Na. Furthermore, we investigated the variations in line intensities over a pulsation cycle. Results. We present new measurements of the C/O and C-12/C-13 ratio for 5 non variable red giants in 47 Tuc. The equivalent widths measured for our 7 variable star, strongly differ from the non variable stars arid cannot be reproduced by either hydrostatic or dynainical model atmospheres. Nevertheless, the dynamical models fit the observed spectra of long-period variables much better than any hydrostatic model. For some spectral features. the variations in the line intensities predicted by dynamical models over a pulsation cycle give similar values as a sequence of hydrostatic models with varying temperature and constant surface gravity. Conclusions. Our study of the dynamical effects on abundance determination visible in these well characterized cluster stars prepares the ground for the long-term goal of deriving abundarices for variable AGB stars in general.

  • 29. Lebzelter, T.
    et al.
    Nowotny, W.
    Höfner, Susanne
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Lederer, M. T.
    Hinkle, K. H.
    Aringer, B.
    Abundance analysis for long period variables Velocity effects studied with O-rich dynamic model atmospheres2010In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 517, p. A6-Article in journal (Refereed)
    Abstract [en]

    Context. Measuring the surface abundances of AGB stars is an important tool for studying the effects of nucleosynthesis and mixing in the interior of low-to intermediate mass stars during their final evolutionary phases. The atmospheres of AGB stars can be strongly affected by stellar pulsation and the development of a stellar wind, though, and the abundance determination of these objects should therefore be based on dynamic model atmospheres. Aims. We investigate the effects of stellar pulsation and mass loss on the appearance of selected spectral features (line profiles, line intensities) and on the derived elemental abundances by performing a systematic comparison of hydrostatic and dynamic model atmospheres. Methods. High-resolution synthetic spectra in the near infrared range were calculated based on two dynamic model atmospheres (at various phases during the pulsation cycle) as well as a grid of hydrostatic COMARCS models with effective temperatures T-eff and surface gravities log g over an adequate range. Equivalent widths of a selection of atomic and molecular lines (Fe, OH, CO) were derived in both cases and compared with each other. Results. In the case of the dynamic models, the equivalent widths of all investigated features vary over the pulsation cycle. A consistent reproduction of the derived variations with a set of hydrostatic models is not possible, but several individual phases and spectral features can be reproduced well with the help of specific hydrostatic atmospheric models. In addition, we show that the variations in equivalent width that we found on the basis of the adopted state-of-the-art dynamic model atmospheres agree qualitatively with observational results for the Mira R Cas over its light cycle. Conclusions. The findings of our modelling form a starting point to deal with the problem of abundance determination in strongly dynamic AGB stars (i.e., long-period variables). Our results illustrate that some quantities such as the C/O ratio can probably still be determined to a reasonable accuracy, but the measurement of other quantities will be hampered by the dynamics. The qualitative agreement with observations of R Cas opens promising possibilities for a forthcoming quantitative comparison of our synthetic spectra with observed ones of AGB variables in the globular cluster 47 Tuc.

  • 30. Lederer, Michael
    et al.
    Aringer, Bernhard
    Höfner, Susanne
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Astronomy and Space Physics. Department of Physics and Astronomy, Theoretical Astrophysics.
    Kerschbaum, Franz
    Water Opacity in M Stars2007In: Why Galaxies Care About AGB Stars: Their Importance as Actors and Probes, 2007, p. 127-128Conference paper (Other scientific)
    Abstract [en]

    We compare the recently published BT2 water line list tep{2006MNRAS.368.1087B} with other available lists -- NASA Ames tep{partridge:4618} and SCAN tep{2001A&A...372..249J} -- and examine the impact on hydrostatic as well as dynamic models for cool M-type stars. Emerging spectra are compared to ISO-SWS observations of semiregular and Mira variables.

  • 31. Lederer, M.T.
    et al.
    Lebzelter, T.
    Aringer, B.
    Nowotny, W.
    Hron, J.
    Uttenthaler, S.
    Höfner, Susanne
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Astronomy and Space Physics.
    Towards abundance determination from dynamic atmospheres2006In: Memorie della Societa Astronomica Italiana, v.77, 2006, p. 1008-1013Conference paper (Other scientific)
    Abstract [en]

    We summarize our efforts to determine element abundances (e.g. for the elements C and O) and isotopic ratios (e. g. isotope[12]{C}/isotope[13]{C}) along the Asymptotic Giant Branch (AGB). For the modelling of AGB star spectra for stars on the lower part of the AGB, hydrostatic model atmospheres have been shown to reproduce observations quite well. Stars climbing up the AGB get more and more dynamic. For these strongly pulsating stars dynamic model atmospheres have to be used in order to reproduce the observed spectra which are dominated by dynamical effects. Both the developments of hydrostatic and dynamic model atmospheres are considered, referring to the problems encountered in the synthesis of cool star spectra. We emphasize the importance of AGB stars in globular clusters, which provide a method to get a homogenous sample of stars well-defined in mass, metallicity, and luminosity. Finally, we point out the relevance of our investigations for stellar evolutionary models.

  • 32.
    Liljegren, Sofie
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Höfner, Susanne
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Eriksson, Kjell
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Winds of AGB Stars - The Role of Stellar Pulsation2015In: WHY GALAXIES CARE ABOUT AGB STARS III: A CLOSER LOOK IN SPACE AND TIME, ASTRONOMICAL SOC PACIFIC , 2015, Vol. 497, p. 127-128Conference paper (Refereed)
    Abstract [en]

    Changing the stellar pulsation properties has large impact on the behavior of the atmosphere of C-type AGB stars. This relationship is examined.

  • 33.
    Liljegren, Soofie
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Höfner, Susanne
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Eriksson, Kjell
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Nowotny, W.
    University of Vienna, Department of Astrophysics.
    Pulsation-induced atmospheric dynamics in M-type AGB stars: Effects on wind properties, photometric variations and near-IR CO line profiles2017In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 606, article id A6Article in journal (Refereed)
    Abstract [en]

    Context. Wind-driving in asymptotic giant branch (AGB) stars is commonly attributed to a two-step process. First, matter in the stellar atmosphere is levitated by shock waves, induced by stellar pulsation, and second, this matter is accelerated by radiation pressure on dust, resulting in a wind. In dynamical atmosphere and wind models the effects of the stellar pulsation are often simulated by a simplistic prescription at the inner boundary.

    Aims. We test a sample of dynamical models for M-type AGB stars, for which we kept the stellar parameters fixed to values characteristic of a typical Mira variable but varied the inner boundary condition. The aim was to evaluate the effect on the resulting atmosphere structure and wind properties. The results of the models are compared to observed mass-loss rates and wind velocities, photometry, and radial velocity curves, and to results from 1D radial pulsation models. The goal is to find boundary conditions which give realistic atmosphere and wind properties.

    Methods. Dynamical atmosphere models are calculated, using the DARWIN code for different combinations of photospheric velocities and luminosity variations. The inner boundary is changed by introducing an offset between maximum expansion of the stellar surface and the luminosity and/or by using an asymmetric shape for the luminosity variation. Ninety-nine different combinations of theses two changes are tested.

    Results. The model atmospheres are very sensitive to the inner boundary. Models that resulted in realistic wind velocities and mass-loss rates, when compared to observations, also produced realistic photometric variations. For the models to also reproduce the characteristic radial velocity curve present in Mira stars (derived from CO Delta v = 3 lines), an overall phase shift of 0.2 between the maxima of the luminosity and radial variation had to be introduced. This is a larger phase shift than is found by 1D radial pulsation models.

    Conclusions. We find that a group of models with different boundary conditions (29 models, including the model with standard boundary conditions) results in realistic velocities and mass-loss rates, and in photometric variations. To achieve the correct line splitting time variation a phase shift is needed.

  • 34.
    Liljegren, Soofie
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Höfner, Susanne
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Nowotny, W.
    Univ Vienna, Dept Astrophys, Turkenschanzstr 17, A-1180 Vienna, Austria..
    Eriksson, Kjell
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Dust-driven winds of AGB stars: The critical interplay of atmospheric shocks and luminosity variations2016In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 589, article id A130Article in journal (Refereed)
    Abstract [en]

    Context. Winds of AGB stars are thought to be driven by a combination of pulsation-induced shock waves and radiation pressure on dust. In dynamic atmosphere and wind models, the stellar pulsation is often simulated by prescribing a simple sinusoidal variation in velocity and luminosity at the inner boundary of the model atmosphere.

    Aims. We experiment with different forms of the luminosity variation in order to assess the effects on the wind velocity and mass-loss rate, when progressing from the simple sinusoidal recipe towards more realistic descriptions. This will also give an indication of how robust the wind properties derived from the dynamic atmosphere models are.

    Methods. Using state-of-the-art dynamical models of C-rich AGB stars, a range of different asymmetric shapes of the luminosity variation and a range of phase shifts of the luminosity variation relative to the radial variation are tested. These tests are performed on two stellar atmosphere models. The first model has dust condensation and, as a consequence, a stellar wind is triggered, while the second model lacks both dust and wind.

    Results. The first model with dust and stellar wind is very sensitive to moderate changes in the luminosity variation. There is a complex relationship between the luminosity minimum, and dust condensation: changing the phase corresponding to minimum luminosity can either increase or decrease mass-loss rate and wind velocity. The luminosity maximum dominates the radiative pressure on the dust, which in turn, is important for driving the wind. An earlier occurrence of the maximum, with respect to the propagation of the pulsation-induced shock wave, then increases the wind velocity, while a later occurrence leads to a decrease. These effects of changed luminosity variation are coupled with the dust formation. In contrast there is very little change to the structure of the model without dust.

    Conclusions. Changing the luminosity variation, both by introducing a phase shift and by modifying the shape, influences wind velocity and the mass-loss rate. To improve wind models it would probably be desirable to extract boundary conditions from 3D dynamical interior models or stellar pulsation models.

  • 35. Loidl, R.
    et al.
    Hron, J.
    Jorgensen, U.G.
    Höfner, Susanne
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Astronomy and Space Physics.
    Probing the outer atmosphere of carbon stars - C2H2, HCN and C3 features in the SWS range2000In: ISO beyond the peaks: The 2nd ISO workshop on analytical spectroscopy, ESA-SP 456, 2000Conference paper (Other scientific)
    Abstract [en]

    We have obtained ISO-SWS spectra of a number of carbon-rich AGB stars in the wavelength range 2.4 - 44 μm with a resolution of about 400. We compare these spectra with results of hydrostatic and dynamic model atmospheres. Of special interest are the features which are formed far out in the atmosphere like the C2H2, HCN and C3 features. For these outer regions of the atmosphere deviations from hydrostatic structures are to be expected.

  • 36. Lugaro, Maria
    et al.
    Höfner, Susanne
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Astronomy and Space Physics. Department of Physics and Astronomy, Theoretical Astrophysics.
    What can presolar grains tell us about AGB stars2007In: Highlights of Astronomy, Vol.14, XXVIth General Assembly of the IAU, 2007, p. 345-348Conference paper (Other scientific)
    Abstract [en]

    The vast majority of presolar grains recovered to date show the signature of an origin in asymptotic giant branch (AGB) stars. In AGB stars, the compositions of elements lighter than silicon and heavier than iron are largely affected by proton- and neutron-capture processes, respectively, while the compositions of the elements in between also carry the signature of the initial composition of the star. Dust is produced and observed around AGB stars and the strong mass loss experienced by these stars is believed to be driven by radiation pressure on dust grains. We briefly review the main developments that have occurred in the past few years in the study of AGB stars in relation to dust and presolar grains. From the nucleosynthesis point of view these include: more stringent constraints on the main neutron source nucleus, 13C, for the slow neutron capture process (the s-process); the possibility of presolar grains coming from massive AGB stars; and the unique opportunity to infer the "isotopic'' evolution of the Galaxy by combining presolar grain data and AGB model predictions. Concerning the formation of grains in AGB stars, considerable progress has been achieved in modelling. In particular, self-consistent models for atmospheres and winds of C-stars have reached a level of sophistication which allows direct quantitative comparison with observations. In the case of stars with C/O < 1, however, recent work points to serious problems with the dust-driven wind scenario. A current trend in atmosphere and wind modelling is to investigate the possible effects of inhomogenieties (e.g., due to giant convection cells) with 2D/3D models.

  • 37. Mattsson, L.
    et al.
    Höfner, Susanne
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Relaxing the Small Particle Approximation for Dust-Grain Opacities in Carbon-star Wind Models2011In: Why Galaxies Care about AGB Stars II: Shining Examples and Common Inhabitants, 2011, Vol. 445Conference paper (Refereed)
    Abstract [en]

    We have computed wind models with time-dependent dust formation and grain-size dependent opacities, where (1) the problem is simplified by assuming a fixed dust-grain size, and where (2) the radiation pressure efficiency is approximated using grain sizes based on various means of the actual grain size distribution. It is shown that in critical cases, the effect of grain sizes can be significant. For well-developed winds, however, the effects on the mass-loss rate and the wind speed are small.

  • 38.
    Mattsson, Lars
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
    Höfner, Susanne
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Dust-driven mass loss from carbon stars as a function of stellar parameters: II. Effects of grain size on wind properties2011In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 533, p. A42-Article in journal (Refereed)
    Abstract [en]

    Context. It is well established that the winds of carbon-rich AGB stars (carbon stars) can be driven by radiation pressure on grains of amorphous carbon and collisional transfer of momentum to the gas. This has been demonstrated convincingly by different numerical wind models that include time-dependent dust formation. To simplify the treatment of dust opacities, radiative cross sections are usually computed using the assumption that the dust grains are small compared to wavelengths around the stellar flux maximum. Considering the typical grain sizes that result from these models, however, the applicability of this small-particle limit (SPL) seems questionable.

    Aims. We explore grain size effects on wind properties of carbon stars, using a generalized description of radiative cross sections valid for particles of arbitrary sizes. The purpose of the study is to investigate under which circumstances the SPL may give acceptable results, and to quantify the possible errors that may occur when the SPL does not hold.

    Methods. The time-dependent description of grain growth in our detailed radiation-hydrodynamical models gives information about dust particle radii in every layer at every instant of time. Theses grain radii are used for computing opacities and determining the radiative acceleration of the dust-gas mixture. From the large number of models presented in the first paper of this series (based on SPL dust opacities) we selected two samples, i.e., a group of models with strong, well-developed outflows that are probably representative of the majority of wind-forming models, and another group, close to thresholds in stellar parameter space for dust-driven winds, which are referred to as critical cases.

    Results. We show that in the critical cases the effect of the generalized description of dust opacities can be significant, resulting in more intense mass loss and higher wind velocities compared to models using SPL opacities. For well-developed winds, however, grain size effects on mass-loss rates and wind velocities are found to be small. Both groups of models tend towards lower degrees of dust condensation compared to corresponding SPL models, owing to a self-regulating feedback between grain growth and radiative acceleration. Consequently, the "dust-loss rates" are lower in the models with the generalized treatment of grain opacities.

    Conclusions. We conclude that our previous results on mass-loss rates obtained with SPL opacities are reliable within a wide region of stellar parameter space, except for critical cases close to thresholds of dust-driven outflows where SPL models will tend to underestimate the mass-loss rates and wind velocities.

  • 39.
    Mattsson, Lars
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Astronomy and Space Physics, Theoretical Astrophysics.
    Höfner, Susanne
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Astronomy and Space Physics, Theoretical Astrophysics.
    Herwig, Falk
    Mass loss evolution and the formation of detached shells around TP-AGB stars2007In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 470, no 1, p. 339-352Article in journal (Refereed)
    Abstract [en]

    Context: The origin of the so called “detached shells” around AGB stars is not fully understood, but two common hypotheses state that these shells form either through the interaction of distinct wind phases or an eruptive mass loss associated with a He-shell flash. We present a model of the formation of detached shells around thermal pulse asymptotic giant branch (TP-AGB) stars, based on detailed modelling of mass loss and stellar evolution, leading to a combination of eruptive mass loss and wind interaction. Aims: The purpose of this paper is first of all to connect stellar evolution with wind and mass loss evolution and demonstrate its consistency with observations, but also to show how thin detached shells around TP-AGB stars can be formed. Previous attempts to link mass loss evolution with the formation of detached shells were based on approximate prescriptions for the mass loss and have not included detailed modelling of the wind formation as we do here. Methods: Using stellar parameters sampled from an evolutionary track for a 2 ~M_ȯ star, we have computed the time evolution of the atmospheric layers and wind acceleration region during a typical thermal pulse with detailed radiation hydrodynamical models including dust formation. Based on these results, we simulate the subsequent circumstellar envelope (CSE) evolution using a spherical hydrodynamic model. Results: We find that existing simple mass loss prescriptions all suggest different mass loss evolutions and that they differ from our detailed wind modelling. The most important factor for the formation of a detached shell is the wind velocity evolution which has a strong impact on the wind interaction and the resulting pile-up of matter. Our CSE model shows that a thin shell structure may be formed as a consequence of a rather short phase of intense mass loss in combination with a significant variation in the wind velocity, as obtained by our wind models. This situation can only be obtained for a limited range of amplitudes for the piston boundary used in the dynamic atmosphere models. Conclusions: The combined mass loss eruption and wind interaction scenario for the formation of detached shells around AGB stars (suggested by previous work) is confirmed by the present modelling. Changes in mass loss rate and wind velocity due to a He-shell flash are adequate for creating distinct wind phases and a “snow plow effect” that is necessary to form a geometrically thin detached shell. The derived properties of the shell (i.e. radius, thickness and density) are more or less consistent with existing observational constraints.

  • 40.
    Mattsson, Lars
    et al.
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Astronomy and Space Physics. Department of Physics and Astronomy, Theoretical Astrophysics.
    Höfner, Susanne
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Astronomy and Space Physics. Department of Physics and Astronomy, Theoretical Astrophysics.
    Wahlin, Rurik
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Astronomy and Space Physics. Department of Physics and Astronomy, Theoretical Astrophysics.
    Herwig, Falk
    On the Connection between Mass Loss and Evolution of C-rich AGB Stars2007In: Why Galaxies Care About AGB Stars: Their Importance as Actors and Probes, 2007, p. 239-243Conference paper (Other scientific)
    Abstract [en]

    The mass-loss properties of carbon rich AGB stars are not very well constrained at present. A variety of empirical or theoretical formulae with different parameterisations are available in the literature and the agreement between them is anything but good. These simple mass-loss prescriptions are nonetheless used in many models of stellar evolution without much consideration of their applicability in various cases. We present here an on-going project aiming at a better description of the mass loss, that could be used to improve stellar evolution models -- especially the evolution during the TP-AGB phase. As a first step, we have considered the mass-loss evolution during a He-shell flash. Using stellar parameters sampled from a stellar evolutionary track, we have computed the time evolution of the atmospheric layers and wind acceleration region during a flash event with detailed frequency-dependent radiation-hydrodynamical models including dust formation. We find that existing simple mass-loss prescriptions imply mass-loss evolutions different than our model. Based on these results, we have also simulated the subsequent long-term dynamical evolution of the circumstellar envelope (CSE), including the formation of a detached shell. The second step of the project deals with the dependence of mass loss on the basic stellar parameters. At the moment we are computing a large grid of wind models for C-rich AGB stars. Preliminary results show that simple parameterisations are difficult to obtain in certain regions of the parameter space considered, due to strong non-linearities in the wind mechanism.

  • 41.
    Mattsson, Lars
    et al.
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Astronomy and Space Physics. Department of Physics and Astronomy, Theoretical Astrophysics.
    Wahlin, Rurik
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Astronomy and Space Physics. Department of Physics and Astronomy, Theoretical Astrophysics.
    Höfner, Susanne
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Astronomy and Space Physics. Department of Physics and Astronomy, Theoretical Astrophysics.
    Thresholds for the Dust Driven Mass Loss from C-rich AGB Stars2007In: Stellar Populations as Building Blocks of Galaxies: Proceedings of IAU Symposium #241, 2007, p. 37-38Conference paper (Other scientific)
    Abstract [en]

    It is well established that mass loss from AGB stars due to dust driven winds cannot be arbitrarily low. We model the mass loss from carbon rich AGB stars using detailed frequency-dependent radiation hydrodynamics including dust formation. We present a study of the thresholds for the mass loss rate as a function of stellar parameters based on a subset of a larger grid of such models and compare these results to previous observational and theoretical work. Furthermore, we demonstrate the impact of the pulsation mechanism and dust formation for the creation of a stellar wind and how it affects these thresholds and briefly discuss the consequences for stellar evolution.

  • 42. Nowotny, W.
    et al.
    Aringer, B.
    Höfner, Susanne
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Eriksson, Kjell
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Synthetic photometry for carbon-rich giants III. Tracing the sequence of mass-losing galactic C-type Miras2013In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 552, p. A20-Article in journal (Refereed)
    Abstract [en]

    Late-type giant stars in the evolutionary stage of the asymptotic giant branch increasingly lose mass via comparatively slow but dense stellar winds. Not only do these evolved red giants contribute in this way to the enrichment of the surrounding interstellar medium, but the outflows also have a substantial influence on the spectro-photometric appearance of such objects. In the case of carbon-rich atmospheric chemistries, the developing cool circumstellar envelopes contain dust grains mainly composed of amorphous carbon. With increasing mass-loss rates, this leads to more and more pronounced circumstellar reddening. With the help of model calculations we aim at reproducing the observational photometric findings for a large sample of well-characterised galactic C-type Mira variables losing mass at different rates. We used dynamic model atmospheres, describing the outer layers of C-rich Miras, which are severly affected by dynamic effects. Based on the resulting structures and under the assumptions of chemical equilibrium as well as LTE, we computed synthetic spectra and synthetic broad-band photometry (Johnson-Cousins-Glass BVRIJHKL'M). A set of five representative models with different stellar parameters describes a sequence from less to more evolved objects with steadily increasing mass-loss rates. This allowed us to study the significant influence of circumstellar dust on the spectral energy distributions and the (amplitudes of) lightcurves in different filters. We tested the photometric properties (mean NIR magnitudes, colours, and amplitudes) and other characteristics of the models (mass-loss rates, periods, and bolometric corrections) by comparing these with the corresponding observational data adopted from the literature. Using different kinds of diagrams we illustrate where the models are located in a supposed evolutionary sequence defined by observed C-type Mira samples. Based on comparisons of galactic targets with empirical relations derived for C stars in the Large Magellanic Cloud we discuss the relevance of metallicity and excess carbon (C-O) for the development of dust-driven winds. Having investigated the dynamic model atmospheres from different (mainly photometric) perspectives, we conclude that our modelling approach (meaning the combination of numerical method and a suitable choice of model parameters) is able to describe C-rich long-period variables over a wide range of mass-loss rates, i.e., from moderately pulsating objects without any dusty wind to highly dust-enshrouded Carbon Miras. Thus, we can trace the observed sequence of C-type Miras, which is mainly determined by the mass loss.

  • 43. Nowotny, W.
    et al.
    Aringer, B.
    Höfner, Susanne
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Lederer, M. T.
    Photometric Modeling of Mass-losing C-type Miras2011In: Why Galaxies Care about AGB Stars II: Shining Examples and Common Inhabitants, 2011, Vol. 445Conference paper (Refereed)
    Abstract [en]

    Based on dynamic model atmospheres for Mira variables with carbon-rich atmospheric chemistry we have calculated synthetic spectra and photometry for standard broadband filters from the visual to the near-infrared. The modeling allows us to study in detail the influence of two effects characteristic of such objects: (i) the pulsation of the stellar interior, and (ii) the dusty circumstellar envelope resulting from an outflow containing grains of amorphous carbon. A comparison with observational data shows that the models used describe the outer layers of pulsating, mass-losing AGB stars reasonably well.

  • 44. Nowotny, W.
    et al.
    Aringer, B.
    Höfner, Susanne
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Lederer, M. T.
    Synthetic photometry for carbon-rich giants: II. The effects of pulsation and circumstellar dust2011In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 529, p. A129-Article in journal (Refereed)
    Abstract [en]

    Context. Red giant stars approaching the end of the evolutionary phase of the asymptotic giant branch (AGB) are, inter alia, characterised by (i) pulsations of the stellar interiors; and (ii) the development of dusty stellar winds. Therefore, such very evolved objects cannot be adequately described with hydrostatic dust-free model atmospheres.

    Aims. By using self-consistent dynamic model atmospheres which simulate pulsation-enhanced dust-driven winds we studied in detail the influence of the above mentioned two effects on the spectral appearance of long period variables with carbon-rich atmospheric chemistry. While the pulsations lead to large-amplitude photometric variability, the dusty envelopes (resulting from the outflows which contain dust particles composed of amorphous carbon) cause pronounced circumstellar reddening.

    Methods. Based on one selected dynamical model which is representative of C-type Mira variables with intermediate mass loss rates, we calculated synthetic spectra and photometry for standard broad-band filters (Johnson-Cousins-Glass system) from the visual to the near-infrared. The synthetic photometry was subsequently compared with observational results.

    Results. Our modelling allows to investigate in detail the substantial effect of circumstellar dust on the resultant photometry. The pronounced absorption of amorphous carbon dust grains (increasing towards shorter wavelengths; Q(abs)/a proportional to lambda(-beta) with beta approximate to 1), leads to colour indices which are significantly redder than the corresponding ones based on hydrostatic dust-free models. Only if we account for this circumstellar reddening we get synthetic colours that are comparable to observations of evolved AGB stars. The photometric variations of the dynamical model were compared to observed lightcurves of the C-type Mira RU Vir which appears to be quite similar to the model (although the model is not a dedicated fit). We found good agreement concerning the principal behaviour of the BVRIJHKL lightcurves and also quantitatively fitting details (e. g. magnitude ranges, the amplitude decrease from visual to NIR, absolute magnitudes). The analysed model is able to reproduce the variations of RU Vir and other Miras in (J - H) vs. (H - K) diagrams throughout the light cycle (ranges, loops). Contrasting the model photometry with observational data for a variety of galactic C-rich giants in such colour-colour diagrams proved that the chosen atmospheric model fits well into a sequence of objects with increasing mass loss rates, i. e., redder colour indices.

    Conclusions. The comparison of our synthetic photometry with observational results provides a further indication that the applied dynamic model atmospheres represent the outer layers of pulsating and mass-losing C-rich AGB stars reasonably well.

  • 45. Nowotny, W.
    et al.
    Höfner, Susanne
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Astronomy and Space Physics.
    Lebzelter, T.
    Aringer, B.
    Hron, J.
    Modelling NIR molecular lines for Miras2006In: Memorie della Societa Astronomica Italiana, v.77, 2006, p. 180-181Conference paper (Other scientific)
    Abstract [en]

    The atmospheric structure of Mira variables is considerably influenced by pulsation. Molecular absorption lines in the near-infrared (NIR), especially second overtone CO lines, show therefore a characteristic behaviour in time-series of high-resolution spectra. We computed synthetic CO line profiles based on a new dynamic model atmosphere and derived radial velocities (RVs) from the Doppler shifted lines. For the first time, we could quantitatively reproduce observations of the very typical, discontinuous RV curves.

  • 46. Nowotny, Walter
    et al.
    Aringer, Bernhard
    Höfner, Susanne
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Astronomy and Space Physics. Department of Physics and Astronomy, Theoretical Astrophysics.
    Tracing AGB Mass Loss by (Synthetic) High-Resolution IR Spectroscopy2007In: Why Galaxies Care About AGB Stars: Their Importance as Actors and Probes, 2007, p. 325-326Conference paper (Other scientific)
    Abstract [en]

    We report on recent advances in line profile modelling of different near-infrared (NIR) molecular absorption features based on dynamic model atmospheres for pulsating and mass-losing AGB stars (Mira variables).

  • 47.
    Nowotny, Walter
    et al.
    Institut für Astronomie, Universität Wien.
    Aringer, Bernhard
    Institut für Astronomie, Universität Wien.
    Höfner, Susanne
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Astronomy and Space Physics, Theoretical Astrophysics.
    Hron, Josef
    Institut für Astronomie, Universität Wien.
    Kerschbaum, Franz
    Institut für Astronomie, Universität Wien.
    A multi-method approach to the outer layers of AGB stars2007In: Astronomische Nachrichten, ISSN 0004-6337, Vol. 328, no 7, p. 645-660Article in journal (Other academic)
  • 48. Olofsson, H.
    et al.
    Ramstedt, S.
    Sacuto, Stéphane
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Maercker, M.
    Höfner, Susanne
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Kerschbaum, F.
    A Possible Solution to the Mass-Loss Problem in M-type AGB Stars2011In: Why Galaxies Care about AGB Stars II: Shining Examples and Common Inhabitants, 2011, Vol. 445Conference paper (Refereed)
    Abstract [en]

    Mass loss is a fundamental, observationally well-established feature of AGB stars, but many aspects of this process are not yet understood. To date, self-consistent dynamical models of dust-driven winds reproducing the observed mass-loss rates seem only possible for M-type stars if the grains in the close circumstellar environment grow to larger sizes than previously assumed. In order to study the grain-size distribution where the mass loss is initiated, high-spatial-resolution interferometry observations are necessary. We have observed two M-type stars using the VLTI/MIDI instrument to constrain the dust-grain sizes through modeling the 10 μm silicate feature. Here we present preliminary results.

  • 49. Ruiz-Velasco, A. E.
    et al.
    Wittkowski, M.
    Driebe, T.
    Wachter, A.
    Höfner, Susanne
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Circumstellar Envelopes of OH/IR Stars2011In: Why Galaxies Care about AGB Stars II: Shining Examples and Common Inhabitants, 2011, Vol. 445Conference paper (Refereed)
    Abstract [en]

    We present VLTI observations of the tip-AGB star IRAS 17020-5254 carried out with the AMBER and MIDI instruments using the 1.8-m Auxiliary Telescopes (ATs) with three different baselines. This star is an evolved oxygen-rich single star of the OH/IR type. We fit the resulting near-infrared visibilities with a model consisting of a Gaussian component plus a central uniform disk, obtaining apparent diameters of the dust shell, the central star, and their flux ratios. We also show the uniform disk diameter as a function of wavelength from both the AMBER and MIDI observations. The mid-infrared diameter is about two to three times larger than the near-infrared one. We plan to compare these data to hydrodynamic atmosphere and wind models.

  • 50.
    Sacuto, Stéphane
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Observational Astronomy.
    Ramstedt, Sofia
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Observational Astronomy.
    Höfner, Susanne
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Olofsson, H.
    Bladh, Sara
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Eriksson, Kjell
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Aringer, B.
    Klotz, D.
    Maercker, M.
    The wind of the M-type AGB star RT Virginis probed by VLTI/MIDI2013In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 551, p. A72-Article in journal (Refereed)
    Abstract [en]

    Aims. We study the circumstellar environment of the M-type AGB star RT Vir using mid-infrared high spatial resolution observations from the ESO-VLTI focal instrument MIDI. The aim of this study is to provide observational constraints on theoretical prediction that the winds of M-type AGB objects can be driven by photon scattering on iron-free silicate grains located in the close environment (about 2 to 3 stellar radii) of the star. Methods. We interpreted spectro-interferometric data, first using wavelength-dependent geometric models. We then used a self-consistent dynamic model atmosphere containing a time-dependent description of grain growth for pure forsterite dust particles to reproduce the photometric, spectrometric, and interferometric measurements of RT Vir. Since the hydrodynamic computation needs stellar parameters as input, a considerable effort was first made to determine these parameters. Results. MIDI differential phases reveal the presence of an asymmetry in the stellar vicinity. Results from the geometrical modeling give us clues to the presence of aluminum and silicate dust in the close circumstellar environment (<5 stellar radii). Comparison between spectro-interferometric data and a self-consistent dust-driven wind model reveals that silicate dust has to be present in the region between 2 to 3 stellar radii to reproduce the 59 and 63 m baseline visibility measurements around 9.8 mu m. This gives additional observational evidence in favor of winds driven by photon scattering on iron-free silicate grains located in the close vicinity of an M-type star. However, other sources of opacity are clearly missing to reproduce the 10-13 mu m visibility measurements for all baselines. Conclusions. This study is a first attempt to understand the wind mechanism of M-type AGB stars by comparing photometric, spectrometric, and interferometric measurements with state-of-the-art, self-consistent dust-driven wind models. The agreement of the dynamic model atmosphere with interferometric measurements in the 8-10 mu m spectral region gives additional observational evidence that the winds of M-type stars can be driven by photon scattering on iron-free silicate grains. Finally, a larger statistical study and progress in advanced self-consistent 3D modeling are still required to solve the remaining problems.

12 1 - 50 of 55
CiteExportLink to result list
Permanent link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf