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Doan, L., Ramstedt, S., Vlemmings, W., Mohamed, S., Höfner, S., De Beck, E., . . . Wittkowski, M. (2020). The extended molecular envelope of the asymptotic giant branch star π1 Gruis as seen by ALMA: II. The spiral-outflow observed at high-angular resolution. Astronomy and Astrophysics, 633, Article ID A13.
Open this publication in new window or tab >>The extended molecular envelope of the asymptotic giant branch star π1 Gruis as seen by ALMA: II. The spiral-outflow observed at high-angular resolution
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2020 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 633, article id A13Article in journal (Refereed) Published
Abstract [en]

Context. This study follows up the previous analysis of lower-angular resolution data in which the kinematics and structure of the circumstellar envelope (CSE) around the S-type asymptotic giant branch (AGB) star π1 Gruis were investigated. The AGB star has a known companion (at a separation of 400 AU) which cannot explain the strong deviations from spherical symmetry of the CSE. Recently, hydrodynamic simulations of mass transfer in closer binary systems have successfully reproduced the spiral-shaped CSEs found around a handful of sources. There is growing evidence for an even closer, undetected companion complicating the case of π1 Gruis further.

Aims. The improved spatial resolution allows for the investigation of the complex circumstellar morphology and the search for imprints on the CSE of the third component.

Methods. We have observed the 12CO J=3-2 line emission from π1 Gruis using both the compact and extended array of AtacamaLarge Millimeter/submillimeter Array (ALMA). The interferometric data has furthermore been combined with data from the ALMA total power (TP) array. The imaged brightness distribution has been used to constrain a non-local, non-LTE 3D radiative transfer model of the CSE.

Results. The high-angular resolution ALMA data have revealed the first example of a source on the AGB where both a faster bipolar outflow and a spiral pattern along the orbital plane can be seen in the gas envelope. The spiral can be traced in the low- to intermediate-velocity (13–25 km s-1) equatorial torus. The largest spiral-arm separation is ≈5”.5 and consistent with a companion with an orbital period of 330 yrs and a separation of less than 70 AU. The kinematics of the bipolar outflow is consistent with it being created during a mass-loss eruption where the mass-loss rate from the system increased by at least a factor of 5 during 10-15 yrs.

Conclusions. The spiral pattern is the result of an undetected companion. The bipolar outflow is the result of a rather recent mass-loss eruption event.

Place, publisher, year, edition, pages
EDP Sciences, 2020
Keywords
stars: AGB and post-AGB, stars: low-mass, radio lines: general, binaries: general, stars: individual: π1 Gru
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-391308 (URN)10.1051/0004-6361/201935245 (DOI)000504267100002 ()
Funder
Australian Research CouncilSwedish Research Council, 2017-00648EU, European Research Council, 614264
Available from: 2019-08-21 Created: 2019-08-21 Last updated: 2020-03-11Bibliographically approved
Bladh, S., Liljegren, S., Höfner, S., Aringer, B. & Marigo, P. (2019). An extensive grid of DARWIN models for M-type AGB stars: I. Mass-loss rates and other properties of dust-driven winds. Astronomy and Astrophysics, 626, Article ID A100.
Open this publication in new window or tab >>An extensive grid of DARWIN models for M-type AGB stars: I. Mass-loss rates and other properties of dust-driven winds
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2019 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 626, article id A100Article in journal (Refereed) Published
Abstract [en]

Context: The stellar winds of asymptotic giant branch (AGB) stars are commonly attributed to radiation pressure on dust grains, formed in the wake of shock waves that arise in the stellar atmospheres. The mass loss due to these outflows is substantial, and modelling the dynamical properties of the winds is essential both for studies of individual stars and for understanding the evolution of stellar populations with low to intermediate mass.

Aims: The purpose of this work is to present an extensive grid of dynamical atmosphere and wind models for M-type AGB stars, covering a wide range of relevant stellar parameters.

Methods: We used the DARWIN code, which includes frequency-dependent radiation-hydrodynamics and a time-dependent description of dust condensation and evaporation, to simulate the dynamical atmosphere. The wind-driving mechanism is photon scattering on submicron-sized Mg2SiO4 grains. The grid consists of similar to 4000 models, with luminosities from L-* = 890 L-circle dot to L-* = 40 000 L-circle dot and effective temperatures from 2200 to 3400 K. For the first time different current stellar masses are explored with M-type DARWIN models, ranging from 0.75 M-circle dot to 3 M-circle dot. The modelling results are radial atmospheric structures, dynamical properties such as mass-loss rates and wind velocities, and dust properties (e.g. grain sizes, dust-to-gas ratios, and degree of condensed Si).

Results: We find that the mass-loss rates of the models correlate strongly with luminosity. They also correlate with the ratio L-*/M-* : increasing L-*/M-* by an order of magnitude increases the mass-loss rates by about three orders of magnitude, which may naturally create a superwind regime in evolution models. There is, however, no discernible trend of mass-loss rate with effective temperature, in contrast to what is found for C-type AGB stars. We also find that the mass-loss rates level off at luminosities higher than similar to 14 000 L-circle dot, and consequently at pulsation periods longer than similar to 800 days. The final grain radii range from 0.25 to 0.6 mu m. The amount of condensed Si is typically between 10 and 40%, with gas-to-dust mass ratios between 500 and 4000.

National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-348124 (URN)10.1051/0004-6361/201935366 (DOI)000472465400001 ()
Funder
Swedish Research CouncilEU, European Research Council, 6 15 604
Note

Title in dissertation list of papers: An extensive grid of DARWIN models for M-type AGB stars I. Mass loss and the properties of wind and dust

Available from: 2018-04-10 Created: 2018-04-10 Last updated: 2019-08-05Bibliographically approved
Pastorelli, G., Marigo, P., Girardi, L., Chen, Y., Rubele, S., Trabucchi, M., . . . Cioni, M.-R. L. (2019). Constraining the thermally pulsing asymptotic giant branch phase with resolved stellar populations in the Small Magellanic Cloud. Monthly notices of the Royal Astronomical Society, 485(4), 5666-5692
Open this publication in new window or tab >>Constraining the thermally pulsing asymptotic giant branch phase with resolved stellar populations in the Small Magellanic Cloud
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2019 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 485, no 4, p. 5666-5692Article in journal (Refereed) Published
Abstract [en]

The thermally pulsing asymptotic giant branch (TP-AGB) experienced by low-and intermediate-mass stars is one of the most uncertain phases of stellar evolution and the models need to be calibrated with the aid of observations. To this purpose, we couple high-quality observations of resolved stars in the Small Magellanic Cloud (SMC) with detailed stellar population synthesis simulations computed with the TRILEGAL code. The strength of our approach relies on the detailed spatially resolved star formation history of the SMC, derived from the deep near-infrared photometry of the VISTA survey of the Magellanic Clouds, as well as on the capability to quickly and accurately explore a wide variety of parameters and effects with the COLIBRI code for the TP-AGB evolution. Adopting a well-characterized set of observations - star counts and luminosity functions - we set up a calibration cycle along which we iteratively change a few key parameters of the TP-AGB models until we eventually reach a good fit to the observations. Our work leads to identify two best-fitting models that mainly differ in the efficiencies of the third dredge-up and mass-loss in TP-AGB stars with initial masses larger than about 3 M-circle dot. On the basis of these calibrated models, we provide a full characterization of the TP-AGB stellar population in the SMC in terms of stellar parameters (initial masses, C/O ratios, carbon excess, mass-loss rates). Extensive tables of isochrones including these improved models are publicly available.

Place, publisher, year, edition, pages
OXFORD UNIV PRESS, 2019
Keywords
stars: AGB and post-AGB, stars: evolution, stars: carbon, stars: mass-loss, Magellanic Clouds
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-390640 (URN)10.1093/mnras/stz725 (DOI)000474880400090 ()
Funder
EU, European Research Council, 615604EU, Horizon 2020, 682115
Available from: 2019-08-21 Created: 2019-08-21 Last updated: 2019-08-21Bibliographically approved
Höfner, S. & Freytag, B. (2019). Exploring the origin of clumpy dust clouds around cool giants: A global 3D RHD model of a dust-forming M-type AGB star. Astronomy and Astrophysics, 623, Article ID A158.
Open this publication in new window or tab >>Exploring the origin of clumpy dust clouds around cool giants: A global 3D RHD model of a dust-forming M-type AGB star
2019 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 623, article id A158Article in journal (Refereed) Published
Abstract [en]

Context: Dust grains forming in the extended atmospheres of AGB stars are critical for the heavy mass loss of these cool luminous giants, as they provide radiative acceleration for the stellar winds. Characteristic mid-IR spectral features indicate that the grains consist mainly of silicates and corundum. The latter species seems to form in a narrow zone within about 2 stellar radii, preceding the condensation of silicate dust, which triggers the outflow. Recent high-angular-resolution observations show clumpy, variable dust clouds at these distances.

Aims: We explore possible causes for the formation of inhomogeneous dust layers, using 3D dynamical simulations.

Methods: We modeled the outer convective envelope and the dust-forming atmosphere of an M-type AGB star with the CO5BOLD radiation-hydrodynamics code. The simulations account for frequency-dependent gas opacities, and include a time-dependent description of grain growth and evaporation for corundum (Al2O3) and olivine-type silicates (Mg2SiO4).

Results: In the inner, gravitationally bound, and corundum-dominated layers of the circumstellar envelope, a patchy distribution of the dust emerges naturally, due to atmospheric shock waves that are generated by large-scale convective flows and pulsations. The formation of silicate dust at somewhat larger distances probably indicates the outer limit of the gravitationally bound layers. The current models do not describe wind acceleration, but the cloud formation mechanism should also work for stars with outflows. Timescales of atmospheric dynamics and grain growth are similar to observed values. In spherical averages of dust densities, more easily comparable to unresolved observations and 1D models, the variable 3D morphology manifests itself as cycle-to-cycle variations.

Conclusions: Grain growth in the wake of large-scale non-spherical shock waves, generated by convection and pulsations, is a likely mechanism for producing the observed clumpy dust clouds, and for explaining their physical and dynamical properties.

Place, publisher, year, edition, pages
EDP SCIENCES S A, 2019
Keywords
convection, shock waves, stars: AGB and post-AGB, stars: atmospheres, stars: oscillations, circumstellar matter
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-381580 (URN)10.1051/0004-6361/201834799 (DOI)000462266300004 ()
Funder
Swedish Research CouncilSwedish National Infrastructure for Computing (SNIC), snic2017-1-41Swedish National Infrastructure for Computing (SNIC), snic2018-3-74
Available from: 2019-04-12 Created: 2019-04-12 Last updated: 2019-04-12Bibliographically approved
Liljegren, S., Höfner, S., Freytag, B. & Bladh, S. (2018). Atmospheres and wind properties of non-spherical AGB stars. Astronomy and Astrophysics, 619, Article ID A47.
Open this publication in new window or tab >>Atmospheres and wind properties of non-spherical AGB stars
2018 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 619, article id A47Article in journal (Refereed) Published
Abstract [en]

Context. The wind-driving mechanism of asymptotic giant branch (AGB) stars is commonly attributed to a two-step process: first, gas in the stellar atmosphere is levitated by shockwaves caused by stellar pulsation, then accelerated outwards by radiative pressure on newly formed dust, inducing a wind. Dynamical modelling of such winds usually assumes a spherically symmetric star.

Aims. We explore the potential consequences of complex stellar surface structures, as predicted by three-dimensional (3D) star-in-a-box modelling of M-type AGB stars, on the resulting wind properties with the aim to improve the current wind models.

Methods. Two different modelling approaches are used; the CO5BOLD 3D star-in-a-box code to simulate the convective, pulsating interior and lower atmosphere of the star, and the DARWIN one-dimensional (1D) code to describe the dynamical atmosphere where the wind is accelerated. The gas dynamics of the inner atmosphere region at distances of R ∼ 1−2 R, which both modelling approaches simulate, are compared. Dynamical properties and luminosity variations derived from CO5BOLD interior models are used as input for the inner boundary in DARWIN wind models in order to emulate the effects of giant convection cells and pulsation, and explore their influence on the dynamical properties.

Results. The CO5BOLD models are inherently anisotropic, with non-uniform shock fronts and varying luminosity amplitudes, in contrast to the spherically symmetrical DARWIN wind models. DARWIN wind models with CO5BOLD-derived inner boundary conditions produced wind velocities and mass-loss rates comparable to the standard DARWIN models, however the winds show large density variations on time-scales of 10–20 yr.

Conclusions. The method outlined in this paper derives pulsation properties from the 3D star-in-a-box CO5BOLD models, to be used in the DARWIN models. If the current grid of CO5BOLD models is extended, it will be possible to construct extensive DARWIN grids with inner boundary conditions derived from 3D interior modelling of convection and pulsation, and avoid the free parameters of the current approach.

Keywords
stars: AGB and post-AGB, stars: atmospheres, stars: winds outflows, stars: oscillations, shock waves
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy with specialization in Astrophysics
Identifiers
urn:nbn:se:uu:diva-348123 (URN)10.1051/0004-6361/201833203 (DOI)000449278400001 ()
Funder
Swedish Research CouncilSwedish National Infrastructure for Computing (SNIC)
Available from: 2018-04-10 Created: 2018-04-10 Last updated: 2019-06-26Bibliographically approved
Höfner, S. & Olofsson, H. (2018). Mass loss of stars on the asymptotic giant branch Mechanisms, models and measurements. The Astronomy and Astrophysics Review, 26(1)
Open this publication in new window or tab >>Mass loss of stars on the asymptotic giant branch Mechanisms, models and measurements
2018 (English)In: The Astronomy and Astrophysics Review, ISSN 0935-4956, E-ISSN 1432-0754, Vol. 26, no 1Article, review/survey (Refereed) Published
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.

Keywords
Stars: AGB and post-AGB, Stars: atmospheres, Stars: evolution, Stars: mass loss, Stars: winds, outflows, Circumstellar matter
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-347107 (URN)10.1007/s00159-017-0106-5 (DOI)000419588900001 ()
Available from: 2018-03-26 Created: 2018-03-26 Last updated: 2018-03-26Bibliographically approved
Wittkowski, M., Rau, G., Chiavassa, A., Höfner, S., Scholz, M., Wood, P. R., . . . Paumard, T. (2018). VLTI-GRAVITY measurements of cool evolved stars I. Variable photosphere and extended atmosphere of the Mira star R Peg. Astronomy and Astrophysics, 613, Article ID L7.
Open this publication in new window or tab >>VLTI-GRAVITY measurements of cool evolved stars I. Variable photosphere and extended atmosphere of the Mira star R Peg
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2018 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 613, article id L7Article in journal (Refereed) Published
Abstract [en]

Context. Dynamic model atmospheres of Mira stars predict variabilities in the photospheric radius and in atmospheric molecular layers which are not yet strongly constrained by observations. Aims. Here we measure the variability of the oxygen-rich Mira star R Peg in near-continuum and molecular bands. Methods. We used near-infrared K-band spectro-interferometry with a spectral resolution of about 4000 obtained at four epochs between post-maximum and minimum visual phases employing the newly available GRAVITY beam combiner at the Very Large Telescope Interferometer (VLTI). Results. Our observations show a continuum radius that is anti-correlated with the visual lightcurve. Uniform disc (UD) angular diameters at a near-continuum wavelength of 2.25 mu m are steadily increasing with values of 8.7 +/- 0.1 mas, 9.4 +/- 0.1 mas, 9.8 +/- 0.1 mas, and 9.9 +/- 0.1 mas at visual phases of 0.15, 0.36, 0,45, 0.53, respectively. UD diameters at a bandpass around 2.05 mu m, dominated by water vapour, follow the near-continuum variability at larger UD diameters between 10.7 mas and 11.7 mas. UD diameters at the CO 2-0 bandhead, instead, are correlated with the visual lightcurve and anti-correlated with the near-continuum UD diameters, with values between 12.3 mas and 11.7 mas. Conclusions. The observed anti-correlation between continuum radius and visual lightcurve is consistent with an earlier study of the oxygen-rich Mira S Lac, and with recent 1D CODEX dynamic model atmosphere predictions. The amplitude of the variation is comparable to the earlier observations of S Lac, and smaller than predicted by CODEX models. The wavelength-dependent visibility variations at our epochs can be reproduced by a set of CODEX models at model phases between 0.3 and 0.6. The anti-correlation of water vapour and CO contributions at our epochs suggests that these molecules undergo different processes in the extended atmosphere along the stellar cycle. The newly available GRAVITY instrument is suited to conducting longer time series observations, which are needed to provide strong constraints on the model-predicted intra-and inter-cycle variability.

Place, publisher, year, edition, pages
EDP Sciences, 2018
Keywords
techniques: interferometric, stars: AGB and post-AGB, stars: atmospheres, stars: mass-loss, stars: variables: general, stars: individual: R Peg
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-357379 (URN)10.1051/0004-6361/201833029 (DOI)000433879500001 ()
Available from: 2018-08-24 Created: 2018-08-24 Last updated: 2018-08-31Bibliographically approved
Wittkowski, M., Hofmann, K.-H. -., Höfner, S., Le Bouquin, J. B., Nowotny, W., Paladini, C., . . . Weigelt, G. (2017). Aperture synthesis imaging of the carbon AGB star R Sculptoris Detection of a complex structure and a dominating spot on the stellar disk. Astronomy and Astrophysics, 601, Article ID A3.
Open this publication in new window or tab >>Aperture synthesis imaging of the carbon AGB star R Sculptoris Detection of a complex structure and a dominating spot on the stellar disk
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2017 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 601, article id A3Article in journal (Refereed) Published
Abstract [en]

Aims. We present near-infrared interferometry of the carbon-rich asymptotic giant branch (AGB) star R Sculptoris (R Scl). Methods. We employ medium spectral resolution K-band interferometry obtained with the instrument AMBER at the Very Large Telescope Interferometer (VLTI) and H-band low spectral resolution interferometric imaging observations obtained with the VLTI instrument PIONIER. We compare our data to a recent grid of dynamic atmosphere and wind models. We compare derived fundamental parameters to stellar evolution models. Results. The visibility data indicate a broadly circular resolved stellar disk with a complex substructure. The observed AMBER squared visibility values show drops at the positions of CO and CN bands, indicating that these lines form in extended layers above the photosphere. The AMBER visibility values are best fit by a model without a wind. The PIONIER data are consistent with the same model. We obtain a Rosseland angular diameter of 8.9 +/- 0.3 mas, corresponding to a Rosseland radius of 355 +/- 55 R-Theta, an effective temperature of 2640 +/- 80 K, and a luminosity of log L/L-Theta = 3.74 +/- 0.18. These parameters match evolutionary tracks of initial mass 1.5 +/- 0.5 M-Theta and current mass 1.3 +/- 0.7 M-Theta. The reconstructed PIONIER images exhibit a complex structure within the stellar disk including a dominant bright spot located at the western part of the stellar disk. The spot has an H- band peak intensity of 40% to 60% above the average intensity of the limb-darkening-corrected stellar disk. The contrast between the minimum and maximum intensity on the stellar disk is about 1:2.5. Conclusions. Our observations are broadly consistent with predictions by dynamic atmosphere and wind models, although models with wind appear to have a circumstellar envelope that is too extended compared to our observations. The detected complex structure within the stellar disk is most likely caused by giant convection cells, resulting in large-scale shock fronts, and their effects on clumpy molecule and dust formation seen against the photosphere at distances of 2-3 stellar radii.

Place, publisher, year, edition, pages
EDP Sciences, 2017
Keywords
techniques: interferometric, stars: AGB and post-AGB, stars: atmospheres, stars: fundamental parameters, stars: mass-loss, stars: individual: R Scl
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-327053 (URN)10.1051/0004-6361/201630214 (DOI)000402313500003 ()
Available from: 2017-08-03 Created: 2017-08-03 Last updated: 2018-12-18
Freytag, B., Liljegren, S. & Höfner, S. (2017). Global 3D radiation-hydrodynamics models of AGB stars: Effects of convection and radial pulsations on atmospheric structures. Astronomy and Astrophysics, 600, Article ID A137.
Open this publication in new window or tab >>Global 3D radiation-hydrodynamics models of AGB stars: Effects of convection and radial pulsations on atmospheric structures
2017 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 600, article id A137Article in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
EDP SCIENCES S A, 2017
Keywords
convection, shock waves, methods: numerical, stars: AGB and post-AGB, stars: atmospheres, stars: oscillations
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-324338 (URN)10.1051/0004-6361/201629594 (DOI)000400754000072 ()
Funder
Swedish Research CouncilSwedish National Infrastructure for Computing (SNIC), p2013234
Available from: 2017-06-15 Created: 2017-06-15 Last updated: 2018-04-10Bibliographically approved
Aronson, E., Bladh, S. & Höfner, S. (2017). Modelling polarized light from dust shells surrounding asymptotic giant branch stars. Astronomy and Astrophysics, 603, Article ID A116.
Open this publication in new window or tab >>Modelling polarized light from dust shells surrounding asymptotic giant branch stars
2017 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 603, article id A116Article in journal (Refereed) Published
Abstract [en]

Context. Winds of asymptotic giant branch (AGB) stars are commonly assumed to be driven by radiative acceleration of dust grains. For M-type AGB stars, the nature of the wind-driving dust species has been a matter of intense debate. A proposed source of the radiation pressure triggering the outflows is photon scattering on Fe-free silicate grains. This wind-driving mechanism requires grain radii of about 0.1-1 micron in order to make the dust particles efficient at scattering radiation around the stellar flux maximum. Grain size is therefore an important parameter for understanding the physics behind the winds of M-type AGB stars. Aims. We seek to investigate the diagnostic potential of scattered polarized light for determining dust grain sizes. Methods. We have developed a new tool for computing synthetic images of scattered light in dust and gas shells around AGB stars, which can be applied to detailed models of dynamical atmospheres and dust-driven winds. Results. We present maps of polarized light using dynamical models computed with the DARWIN code. The synthetic images clearly show that the intensity of the polarized light, the position of the inner edge of the dust shell, and the size of the dust grains near the inner edge are all changing with the luminosity phase. Non-spherical structures in the dust shells can also have an impact on the polarized light. We simulate this effect by combining different pulsation phases into a single 3D structure before computing synthetic images. An asymmetry of the circumstellar envelope can create a net polarization, which can be used as diagnostics for the grain size. The ratio between the size of the scattering particles and the observed wavelength determines at what wavelengths net polarization switches direction. If observed, this can be used to constrain average particle sizes.

Keywords
techniques: polarimetric, stars: AGB and post-AGB, stars: late-type, stars: mass-loss, stars: winds, outflows, circumstellar matter
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-360711 (URN)10.1051/0004-6361/201730495 (DOI)000406619100099 ()
Funder
Swedish Research CouncilEU, European Research Council, 615604
Available from: 2018-09-17 Created: 2018-09-17 Last updated: 2018-09-17Bibliographically approved
Organisations
Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0003-2356-643x

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