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  • 1. Arroyo-Torres, B.
    et al.
    Wittkowski, M.
    Chiavassa, A.
    Scholz, M.
    Freytag, Bernd
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Marcaide, J. M.
    Hauschildt, P. H.
    Wood, P. R.
    Abellan, F. J.
    What causes the large extensions of red supergiant atmospheres?: Comparisons of interferometric observations with 1D hydrostatic, 3D convection, and 1D pulsating model atmospheres2015In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 575, article id A50Article in journal (Refereed)
    Abstract [en]

    Aims. This research has two main goals. First, we present the atmospheric structure and the fundamental parameters of three red supergiants (RSGs), increasing the sample of RSGs observed by near-infrared spectro-interferometry. Additionally, we test possible mechanisms that may explain the large observed atmospheric extensions of RSGs. Methods. We carried out spectro-interferometric observations of the.RSGs V602 Car, EID 95687, and EID 183589 in the near-infrared K-band (1.92-2.47 mu m) with the VLTI/AMBER instrument at medium spectral resolution (R similar to 1500). To categorize and comprehend the extended atmospheres, we compared our observational results to predictions by available hydrostatic PHOENIX, available 3D convection, and new 1D self-excited pulsation models of RSGs. Results. Our near-infrared flux spectra of V602 Car, HD 95687, and HD 183589 are well reproduced by the PHOENIX model atmospheres. The continuum visibility values are consistent with a limb-darkened disk as predicted by the PHOENIX models, allowing us to determine the angular diameter and the fundamental parameters of our sources. Nonetheless, in the case of V602 Car and HD 95686, the PHOENIX model visibilities do not predict the large observed extensions of molecular layers, most remarkably in the CO bands. Likewise, the 3D convection models and the ID pulsation models with typical parameters of RSGs lead to compact atmospheric structures as well, which are similar to the structure of the hydrostatic PHOENIX models. They can also not explain the observed decreases in the visibilities and thus the large atmospheric molecular extensions. The full sample of our RSGs indicates increasing observed atmospheric extensions with increasing luminosity and decreasing surface gravity, and no correlation with effective temperature or variability amplitude. Conclusions. The location of our RSG sources in the Hertzsprung-Russell diagram is contirm.ed to be consistent with the red limits of recent evolutionary tracks. The observed extensions of the atmospheric layers of our sample of RSGs are comparable to those of Mira stars. This phenomenon is not predicted by any of the considered model atmospheres including as 311) convection and new 1D pulsation models of.RSGs. This confirms that neither convection nor pulsation alone can levitate the molecular atmospheres of.RSGs. Our observed correlation of atmospheric extension with luminosity supports a scenario of radiative acceleration on Doppler-shifted molecular lines.

  • 2.
    Arroyo-Torres, B.
    et al.
    Univ Valencia, Dept Astron & Astrofis, E-46100 Burjassot, Spain..
    Wittkowski, M.
    ESO, Garching, Germany..
    Marcaide, J. M.
    Univ Valencia, Dept Astron & Astrofis, E-46100 Burjassot, Spain.;Donostia Int Phys Ctr, Donostia San Sebastian, Spain..
    Abellan, F. J.
    Univ Valencia, Dept Astron & Astrofis, E-46100 Burjassot, Spain..
    Chiavassa, A.
    Univ Nice Sophia Antipolis, Lab Lagrange, CNRS, Observ Cote Azur, Sophia Anitpolis, France..
    Fabregat, J.
    Univ Valencia, Astron Observ, E-46003 Valencia, Spain..
    Freytag, Bernd
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Guirado, J. C.
    Univ Valencia, Dept Astron & Astrofis, E-46100 Burjassot, Spain.;Univ Valencia, Astron Observ, E-46003 Valencia, Spain..
    Hauschildt, P. H.
    Hamburger Sternwarte, Hamburg, Germany..
    Marti-Vidal, I.
    Chalmers, Onsala Space Observ, Gothenburg, Sweden..
    Quirrenbach, A.
    Heidelberg Univ, Zentrum Astron, Landessternwarte, D-69115 Heidelberg, Germany..
    Scholz, M.
    Heidelberg Univ, Zentrum Astron, Inst Theoret Astrophys, D-69115 Heidelberg, Germany.;Univ Sydney, Sch Phys, Sydney Inst Astron, Sydney, NSW 2006, Australia..
    Wood, P. R.
    Australian Natl Univ, Res Sch Astron & Astrophys, Canberra, ACT, Australia..
    VLTI/AMBER Studies of the Atmospheric Structure and Fundamental Parameters of Red Giant and Supergiant Stars2015In: WHY GALAXIES CARE ABOUT AGB STARS III: A CLOSER LOOK IN SPACE AND TIME, ASTRONOMICAL SOC PACIFIC , 2015, Vol. 497, p. 91-96Conference paper (Other academic)
    Abstract [en]

    We present recent near-IR interferometric studies of red giant and super giant stars, which are aimed at obtaining information on the structure of the atmospheric layers and constraining the fundamental parameters of these objects. The observed visibilities of six red supergiants (RSGs), and also of one of the five red giants observed, indicate large extensions of the molecular layers, as previously observed for Mira stars. These extensions are not predicted by hydrostatic PHOENIX model atmospheres, hydrodynamical (RED) simulations of stellar convection, or self-excited pulsation models. All these models based on parameters of RSGs lead to atmospheric structures that are too compact compared to our observations. We discuss how alternative processes might explain the atmospheric extensions for these objects. As the continuum appears to be largely free of contamination by molecular layers, we can estimate reliable Rosseland angular radii for our stars. Together with distances and bolometric fluxes, we estimate the effective temperatures and luminosities of our targets, locate them in the HR diagram, and compare their positions to recent evolutionary tracks.

  • 3. Bonifacio, P.
    et al.
    Caffau, E.
    Ludwig, H. -G
    Steffen, M.
    Castelli, F.
    Gallagher, A. J.
    Kučinskas, A.
    Prakapavičius, D.
    Cayrel, R.
    Freytag, Bernd
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Plez, B.
    Homeier, D.
    Using the CIFIST grid of CO5BOLD 3D model atmospheres to study the effects of stellar granulation on photometric colours: I. Grids of 3D corrections in the UBVRI, 2MASS, HIPPARCOS, Gaia, and SDSS systems2018In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 611, no 9, article id A68Article in journal (Refereed)
    Abstract [en]

    Context. The atmospheres of cool stars are temporally and spatially inhomogeneous due to the effects of convection. The influence of this inhomogeneity, referred to as granulation, on colours has never been investigated over a large range of effective temperatures and gravities.

    Aim. We aim to study, in a quantitative way, the impact of granulation on colours.

    Methods. We use the CIFIST (Cosmological Impact of the FIrst Stars) grid of CO5BOLD (COnservative COde for the COmputation of COmpressible COnvection in a BOx of L Dimensions, L = 2, 3) hydrodynamical models to compute emerging fluxes. These in turn are used to compute theoretical colours in the UBV RI, 2MASS, HIPPARCOS, Gaia and SDSS systems. Every CO5BOLD model has a corresponding one dimensional (1D) plane-parallel LHD (Lagrangian HydroDynamics) model computed for the same atmospheric parameters, which we used to define a “3D correction” that can be applied to colours computed from fluxes computed from any 1D model atmosphere code. As an example, we illustrate these corrections applied to colours computed from ATLAS models.

    Results. The 3D corrections on colours are generally small, of the order of a few hundredths of a magnitude, yet they are far from negligible. We find that ignoring granulation effects can lead to underestimation of Teff by up to 200 K and overestimation of gravity by up to 0.5 dex, when using colours as diagnostics. We have identified a major shortcoming in how scattering is treated in the current version of the CIFIST grid, which could lead to offsets of the order 0.01 mag, especially for colours involving blue and UV bands. We have investigated the Gaia and HIPPARCOS photometric systems and found that the (GHp), (BPRP) diagram is immune to the effects of granulation. In addition, we point to the potential of the RVS photometry as a metallicity diagnostic.

    Conclusions. Our investigation shows that the effects of granulation should not be neglected if one wants to use colours as diagnostics of the stellar parameters of F, G, K stars. A limitation is that scattering is treated as true absorption in our current computations, thus our 3D corrections are likely an upper limit to the true effect. We are already computing the next generation of the CIFIST grid, using an approximate treatment of scattering.

  • 4. Bonifacio, P.
    et al.
    Caffau, E.
    Ludwig, H. -G
    Steffen, M.
    Castelli, F.
    Gallagher, A. J.
    Prakapavičius, D.
    Kučinskas, A.
    Cayrel, R.
    Freytag, Bernd
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Plez, B.
    Homeier, D.
    Using CO5BOLD models to predict the effects of granulation on colours .2017In: MEMORIE della Società Astronomica Italiana, ISSN 0037-8720, E-ISSN 1824-016X, Vol. 88Article in journal (Other academic)
    Abstract [en]

    Abstract.In order to investigate the effects of granulation on fluxes and colours, we computedthe emerging fluxes from the models in theCO5BOLDgrid with metallicities [M/H]=0.0,–1.0,–2.0 and –3.0. These fluxes have been used to compute colours in different photometric systems.We explain here how our computations have been performed and provide some results.Key words.Convection – Hydrodynamics - Stars: atmosphere

  • 5. Caffau, E.
    et al.
    Ludwig, H. -G.
    Steffen, M.
    Ayres, T. R.
    Bonifacio, P.
    Cayrel, R.
    Freytag, Bernd
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy. Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Plez, B.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
    The photospheric solar oxygen project - I. Abundance analysis of atomic lines and influence of atmospheric models2008In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 488, no 3, p. 1031-1046Article in journal (Refereed)
    Abstract [en]

    Context. The solar oxygen abundance has undergone a major downward revision in the past decade, the most noticeable one being the update including 3D hydrodynamical simulations to model the solar photosphere. Up to now, such an analysis has only been carried out by one group using one radiation-hydrodynamics code. Aims. We investigate the photospheric oxygen abundance considering lines from atomic transitions. We also consider the relationship between the solar model used and the resulting solar oxygen abundance, to understand whether the downward abundance revision is specifically related to 3D hydrodynamical effects. Methods. We performed a new determination of the solar photospheric oxygen abundance by analysing different high-resolution high signal-to-noise ratio atlases of the solar flux and disc-centre intensity, making use of the latest generation of CO5BOLD 3D solar model atmospheres. Results. We find 8.73 <= log (N-O/N-H) + 12 <= 8.79. The lower and upper values represent extreme assumptions on the role of collisional excitation and ionisation by neutral hydrogen for the NLTE level populations of neutral oxygen. The error of our analysis is +/- (0.04 +/- 0.03) dex, the last being related to NLTE corrections, the first error to any other effect. The 3D "granulation effects" do not play a decisive role in lowering the oxygen abundance. Conclusions. Our recommended value is log (N-O/N-H) = 8.76 +/- 0.07, considering our present ignorance of the role of collisions with hydrogen atoms on the NLTE level populations of oxygen. The reasons for lower O abundances in the past are identified as (1) the lower equivalent widths adopted and (2) the choice of neglecting collisions with hydrogen atoms in the statistical equilibrium calculations for oxygen.

  • 6.
    Calvo, F.
    et al.
    Ist Ric Solari Locarno IRSOL, Via Patocchi 57 Prato Pernice, CH-6605 Locarno, Switzerland.;Univ Geneva, Observ Geneva, Ch Maillettes 51, CH-1290 Sauverny, Switzerland..
    Steiner, O.
    Ist Ric Solari Locarno IRSOL, Via Patocchi 57 Prato Pernice, CH-6605 Locarno, Switzerland.;Kiepenheuer Inst Sonnenphys, Schoneckstr 6, D-79104 Freiburg, Germany..
    Freytag, Bernd
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Non-magnetic photospheric bright points in 3D simulations of the solar atmosphere2016In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 596, article id A43Article in journal (Refereed)
    Abstract [en]

    Context. Small-scale bright features in the photosphere of the Sun, such as faculae or G-band bright points, appear in connection with small-scale magnetic flux concentrations. Aims. Here we report on a new class of photospheric bright points that are free of magnetic fields. So far, these are visible in numerical simulations only. We explore conditions required for their observational detection. Methods. Numerical radiation (magneto-) hydrodynamic simulations of the near-surface layers of the Sun were carried out. The magnetic field-free simulations show tiny bright points, reminiscent of magnetic bright points, only smaller. A simple toy model for these non-magnetic bright points (nMBPs) was established that serves as a base for the development of an algorithm for their automatic detection. Basic physical properties of 357 detected nMBPs were extracted and statistically evaluated. We produced synthetic intensity maps that mimic observations with various solar telescopes to obtain hints on their detectability. Results. The nMBPs of the simulations show a mean bolometric intensity contrast with respect to their intergranular surroundings of approximately 20%, a size of 60-80 km, and the isosurface of optical depth unity is at their location depressed by 80-100 km. They are caused by swirling downdrafts that provide, by means of the centripetal force, the necessary pressure gradient for the formation of a funnel of reduced mass density that reaches from the subsurface layers into the photosphere. Similar, frequently occurring funnels that do not reach into the photosphere, do not produce bright points. Conclusions. Non-magnetic bright points are the observable manifestation of vertically extending vortices (vortex tubes) in the photosphere. The resolving power of 4-m-class telescopes, such as the DKIST, is needed for an unambiguous detection of them.

  • 7.
    Chiavassa, A.
    et al.
    Univ Nice Sophia Antipolis, Observ Cote Azur, Lab Lagrange, F-06189 Nice, France..
    Freytag, Bernd
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    3D Hydrodynamical Simulations of Evolved Stars and Observations of Stellar Surfaces2015In: WHY GALAXIES CARE ABOUT AGB STARS III: A CLOSER LOOK IN SPACE AND TIME, ASTRONOMICAL SOC PACIFIC , 2015, Vol. 497, p. 11-21Conference paper (Other academic)
    Abstract [en]

    Evolved stars are among the largest and brightest stars and they are ideal targets for the new generation of sensitive, high resolution instrumentation that provides spectrophotometric, interferometric, astrometric, and imaging observables. The interpretation of the complex stellar surface images requires numerical simulations of stellar convection that take into account multi-dimensional time-dependent radiation hydrodynamics with realistic input physics. We show how the evolved star simulations are obtained using the radiative hydrodynamics code (COBOLD)-B-5 and how the accurate observables are computed with the post-processing radiative transfer code OPTIM3D. The synergy between observations and theoretical work is supported by a proper and quantitative analysis using these simulations, and by strong constraints from the observational side.

  • 8.
    Chiavassa, A.
    et al.
    Univ Cote Azur, Observ Cote Azur, Lab Lagrange, CNRS, Bd Observ,CS 34229, F-06304 Nice 4, France..
    Freytag, Bernd
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
    Pathways For Observing Stellar Surfaces Using 3D Hydrodynamical Simulations Of Evolved Stars2015In: Physics Of Evolved Stars: A Conference Dedicated To The Memory Of Olivier Chesneau, 2015, p. 237-242Conference paper (Other academic)
    Abstract [en]

    Evolved stars are among the largest and brightest stars and they are ideal targets for the new generation of sensitive, high resolution instrumentation that provides spectrophotometric, interferometric, astrometric, and imaging observables. The interpretation of the complex stellar surface images requires numerical simulations of stellar convection that take into account multi-dimensional time-dependent radiation hydrodynamics with realistic input physics. We show how the evolved star simulations are obtained using the radiative hydrodynamics code CO5BOLD and how the accurate observables are computed with the post-processing radiative transfer code OPTIM3D. The synergy between observations and theoretical work is supported by a proper and quantitative analysis using these simulations, and by strong constraints from the observational side.

  • 9.
    Chiavassa, A.
    et al.
    Univ Cote dAzur, Observ Cote dAzur, CNRS, Lagrange, CS 34229, Nice, France.
    Freytag, Bernd
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Schultheis, M.
    Univ Cote dAzur, Observ Cote dAzur, CNRS, Lagrange, CS 34229, Nice, France.
    Heading Gaia to measure atmospheric dynamics in AGB stars2018In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 617, article id L1Article in journal (Refereed)
    Abstract [en]

    Context. Asymptotic giant branch (AGB) stars are characterised by complex stellar surface dynamics that affect the measurements and amplify the uncertainties on stellar parameters. The uncertainties in observed absolute magnitudes have been found to originate mainly from uncertainties in the parallaxes. The resulting motion of the stellar photocentre could have adverse effects on the parallax determination with Gaia. Aims. We explore the impact of the convection-related surface structure in AGBs on the photocentric variability. We quantify these effects to characterise the observed parallax errors and estimate fundamental stellar parameters and dynamical properties. Methods. We use three-dimensional (3D) radiative hydrodynamics simulations of convection with CO5BOLD and the post-processing radiative transfer code OPTIM3D to compute intensity maps in the Gaia G band [325-1030 nm]. From those maps, we calculate the intensity-weighted mean of all emitting points tiling the visible stellar surface (i.e. the photocentre) and evaluate its motion as a function of time. We extract the parallax error from Gaia data-release 2 (DR2) for a sample of semi-regular variables in the solar neighbourhood and compare it to the synthetic predictions of photocentre displacements. Results. AGB stars show a complex surface morphology characterised by the presence of few large-scale long-lived convective cells accompanied by short-lived and small-scale structures. As a consequence, the position of the photocentre displays temporal excursions between 0.077 and 0.198 AU (approximate to 5 to approximate to 11% of the corresponding stellar radius), depending on the simulation considered. We show that the convection-related variability accounts for a substantial part of the Gaia DR2 parallax error of our sample of semi-regular variables. Finally, we present evidence for a correlation between the mean photocentre displacement and the stellar fundamental parameters: surface gravity and pulsation. We suggest that parallax variations could be exploited quantitatively using appropriate radiation-hydrodynamics (RHD) simulations corresponding to the observed star.

  • 10. Cukanovaite, E.
    et al.
    Tremblay, P. -E
    Freytag, Bernd
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Ludwig, H. -G
    Bergeron, P.
    Pure-helium 3D model atmospheres of white dwarfs2018In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 481, no 2, p. 1522-1537Article in journal (Refereed)
    Abstract [en]

    We present the first grid of 3D simulations for the pure-helium atmospheres of DB white dwarfs. The simulations were computed with the co(5) BOLD radiation-hydrodynamics code and cover effective temperatures and surface gravities between 12 000 K less than or similar to T-eff less than or similar to 34 000 K and 7.5 <= log g (cgs units) <= 9.0, respectively. In this introductory work, synthetic spectra calculated from the 3D simulations are compared to appropriate 1 D model spectra under a differential approach. This results in the derivation of 3D corrections for the spectroscopically derived atmospheric parameters of DB stars with respect to the 1D ML2/alpha = 1.25 mixing-length parametrization. No significant T-eff corrections are found for the V777 Her instability strip region, and therefore no 3D revision is expected for the empirical blue and red edges of the strip. However, large log g corrections are found in the range 12 000 K < T-eff < 23 000 K for all log g values covered by the 3D grid. These corrections indicate that 1D model atmospheres overpredict log g, reminiscent of the results found from 3D simulations of pure-hydrogen white dwarfs. The next step will be to compute 3D simulations with mixed helium and hydrogen atmospheres to comprehend the full implications for the stellar parameters of DB and DBA white dwarfs.

  • 11.
    Cukanovaite, E.
    et al.
    Univ Warwick, Dept Phys, Coventry CV4 7AL, W Midlands, England.
    Tremblay, P-E
    Univ Warwick, Dept Phys, Coventry CV4 7AL, W Midlands, England.
    Freytag, Bernd
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Ludwig, H-G
    Heidelberg Univ, Zentrum Astron, Landessternwarte, Konigstuhl 12, D-69117 Heidelberg, Germany.
    Fontaine, G.
    Univ Montreal, Dept Phys, CP 6128, Montreal, PQ H3C 3J7, Canada.
    Brassard, P.
    Univ Montreal, Dept Phys, CP 6128, Montreal, PQ H3C 3J7, Canada.
    Toloza, O.
    Univ Warwick, Dept Phys, Coventry CV4 7AL, W Midlands, England.
    Koester, D.
    Univ Kiel, Inst Theoret Phys & Astrophys, D-24098 Kiel, Germany.
    Calibration of the mixing-length theory for structures of helium-dominated atmosphere white dwarfs2019In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 490, no 1, p. 1010-1025Article in journal (Refereed)
    Abstract [en]

    We perform a calibration of the mixing-length parameter at the bottom boundary of the convection zone for helium-dominated atmospheres of white dwarfs. This calibration is based on a grid of 3D DB (pure-helium) and DBA (helium-dominated with traces of hydrogen) model atmospheres computed with the CO5BOLD radiation-hydrodynamics code, and a grid of 1D DB and DBA envelope structures. The 3D models span a parameter space of hydrogen-to-helium abundances in the range -10.0 <= log (H/He) <= -2.0, surface gravities in the range 7.5 <= log g <= 9.0, and effective temperatures in the range 12 000K less than or similar to T-eff less than or similar to 34 000 K. The 1D envelopes cover a similar atmospheric parameter range, but are also calculated with different values of the mixing-length parameter, namely 0.4 <= ML2/alpha <= 1.4. The calibration is performed based on two definitions of the bottom boundary of the convection zone: the Schwarzschild and the zero convective flux boundaries. Thus, our calibration is relevant for applications involving the bulk properties of the convection zone including its total mass, which excludes the spectroscopic technique. Overall, the calibrated ML2/alpha is smaller than what is commonly used in evolutionary models and theoretical determinations of the blue edge of the instability strip for pulsating DB and DBA stars. With calibrated ML2/alpha we are able to deduce more accurate convection zone sizes needed for studies of planetary debris mixing and dredge-up of carbon from the core. We highlight this by calculating examples of metal-rich 3D DBAZ models and finding their convection zone masses. Mixing-length calibration represents the first step of in-depth investigations of convective overshoot in white dwarfs with helium-dominated atmospheres.

  • 12.
    Cunningham, Tim
    et al.
    Univ Warwick, Dept Phys, Coventry CV4 7AL, W Midlands, England.
    Tremblay, Pier-Emmanuel
    Univ Warwick, Dept Phys, Coventry CV4 7AL, W Midlands, England.
    Freytag, Bernd
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
    Ludwig, Hans-Guenter
    Heidelberg Univ, Zentrum Astron, Landessternwarte, Konigstuhl 12, D-69117 Heidelberg, Germany.
    Koester, Detlev
    Univ Kiel, Inst Theoret Phys & Astrophys, D-24098 Kiel, Germany.
    Convective overshoot and macroscopic diffusion in pure-hydrogen-atmosphere white dwarfs2019In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 488, no 2, p. 2503-2522Article in journal (Refereed)
    Abstract [en]

    We present a theoretical description of macroscopic diffusion caused by convective overshoot in pure-hydrogen DA white dwarfs using 3D, closed-bottom, radiation hydrodynamics (COBOLD)-B-5 simulations. We rely on a new grid of deep 3D white dwarf models in the temperature range 11 400 <= T-eff <= 18 000 K where tracer particles and a tracer density are used to derive macroscopic diffusion coefficients driven by convective overshoot. These diffusion coefficients are compared to microscopic diffusion coefficients from 1D structures. We find that the mass of the fully mixed region is likely to increase by up to 2.5 orders of magnitude while inferred accretion rates increase by a more moderate order of magnitude. We present evidence that an increase in settling time of up to 2 orders of magnitude is to be expected, which is of significance for time-variability studies of polluted white dwarfs. Our grid also provides the most robust constraint on the onset of convective instabilities in DA white dwarfs to be in the effective temperature range from 18 000 to 18 250 K.

  • 13.
    Freytag, Bernd
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Boundary conditions in CO5BOLD2017In: MEMORIE della Società Astronomica Italiana, ISSN 0037-8720, E-ISSN 1824-016X, Vol. 88, p. 12-21Article in journal (Other academic)
    Abstract [en]

    The declaration of boundary conditions is a crucial step in the setup of a CO5BOLD simulation (and many others) due to the physical nature of the problem, that is reflected in the mathematical description by partial differential equations, discrete versions of which are integrated by the numerical solver(s). While parameters controlling the flux of energy through the computational box are most important for all simulations of convective flows, the detailed specifications describing the behavior of energy, gas and dust densities, velocities, and magnetic fields at or just beyond the boundaries influence the flow, dynamics, and stratification within the box. Recent refinements of the treatment of boundary conditions in CO5BOLD resulted in reliably working implementations of open and closed versions for top, bottom, and ``inner'' boundaries even under conditions with strong velocity fields (waves, shocks, or downdrafts). They are implemented and available in the current version of CO5BOLD - but have to be activated properly with parameters adapted to the type of the star under consideration (by defining for instance the depth of the damping layers for the closed-bottom boundary or by specifying the damping constants for the open-bottom boundary).

  • 14.
    Freytag, Bernd
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Studying the Generation of Shock Waves in AGB Stars with 3-Dimensional Radiation-Hydrodynamics Simulations2015In: WHY GALAXIES CARE ABOUT AGB STARS III: A CLOSER LOOK IN SPACE AND TIME, ASTRONOMICAL SOC PACIFIC , 2015, Vol. 497, p. 23-29Conference paper (Other academic)
    Abstract [en]

    In the Sun, low-amplitude small-scale acoustic waves are just detectable in the photosphere and start to become dynamically relevant only in the lower chromosphere. The generation of these waves by non-stationary convective flows can be studied in detail by local 3-D radiation-hydrodynamics simulations. Using this technique for global models of AGB stars reveals roughly similar phenomena but on a larger scale and with much higher amplitude. Convection cells spanning a significant fraction of the entire surface produce strong waves that cause a network of smaller shocks in the inner photosphere and occasional global shocks, travelling outward in large arcs. Material falling back interacts with the surface convection cells. A new generation of 3-D RHD simulations of these layers with CO5BOLD is presented and analyzed with particular attention given to acoustic waves and shock fronts.

  • 15.
    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.

  • 16.
    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.

  • 17. Gallagher, A. J.
    et al.
    Steffen, M.
    Caffau, E.
    Bonifacio, P.
    Ludwig, H. -G
    Freytag, Bernd
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Enhanced methods for computing spectra from CO5BOLD models using Linfor3D. Molecular bands in metal-poor stars2017Manuscript (preprint) (Other academic)
    Abstract [en]

    Molecular features such as the G-band, CN-band and NH-band are important diagnostics for measuring a star's carbon and nitrogen abundances, especially in metal-poor stars where atomic lines are no longer visible in stellar spectra. Unlike atomic transitions, molecular features tend to form in bands, which cover large wavelength regions in a spectrum. While it is a trivial matter to compute carbon and nitrogen molecular bands under the assumption of 1D, it is extremely time consuming in 3D. In this contribution to the 2016 CO5BOLD workshop we review the improvements made to the 3D spectral synthesis code Linfor3D, and discuss the new challenges found when computing molecular features in 3D.

  • 18. Herwig, F.
    et al.
    Freytag, Bernd
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Hueckstaedt, R. M.
    Timmes, F. X.
    Hydrodynamic Simulations of He Shell Flash Convection2006In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 642, p. 1057-1074Article in journal (Refereed)
  • 19.
    Höfner, Susanne
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Freytag, Bernd
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Exploring the origin of clumpy dust clouds around cool giants: A global 3D RHD model of a dust-forming M-type AGB star2019In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 623, article id A158Article in journal (Refereed)
    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.

  • 20.
    Kochukhov, Oleg
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Astronomy and Space Physics. Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Astronomy and Space Physics, Observational Astronomy.
    Freytag, Bernd
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Astronomy and Space Physics. Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Astronomy and Space Physics, Observational Astronomy. Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Piskunov, Nikolai
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Astronomy and Space Physics. Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Astronomy and Space Physics, Observational Astronomy.
    Steffen, M.
    3-D Hydrodynamic Simulation of Convection in A Stars2007In: IAUS 239, Convection in Astrophysics, 2007, Vol. 239, p. 68-70Conference paper (Other academic)
  • 21.
    Kravchenko, K.
    et al.
    European Southern Observ, Karl Schwarzschild Str 2, D-85748 Garching, Germany;Univ Libre Bruxelles, Inst Astron & Astrophys, CP 226,Blvd Triomphe, B-1050 Brussels, Belgium.
    Chiavassa, A.
    Univ Cote dAzur, Observ Cote dAzur, CNRS, CS 34229, F-06304 Nice 4, France.
    Van Eck, S.
    Univ Libre Bruxelles, Inst Astron & Astrophys, CP 226,Blvd Triomphe, B-1050 Brussels, Belgium.
    Jorissen, A.
    Univ Libre Bruxelles, Inst Astron & Astrophys, CP 226,Blvd Triomphe, B-1050 Brussels, Belgium.
    Merle, T.
    Univ Libre Bruxelles, Inst Astron & Astrophys, CP 226,Blvd Triomphe, B-1050 Brussels, Belgium.
    Freytag, Bernd
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Plez, B.
    Univ Montpellier, Lab Univers & Particules Montpellier, CNRS, F-34095 Montpellier 05, France.
    Tomography of cool giant and supergiant star atmospheres II: Signature of convection in the atmosphere of the red supergiant star mu Cep2019In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 632, article id A28Article in journal (Refereed)
    Abstract [en]

    Context: Red supergiants are cool massive stars and are the largest and the most luminous stars in the Universe. They are characterized by irregular or semi-regular photometric variations, the physics of which is not clearly understood.

    Aims: The paper aims to derive the velocity field in the red supergiant star mu Cep and to relate it to the photometric variability with the help of the tomographic method.

    Methods: The tomographic method allows one to recover the line-of-sight velocity distribution over the stellar disk and within different optical-depth slices. This method was applied to a series of high-resolution spectra of mu Cep, and these results are compared to those obtained from 3D radiative-hydrodynamics CO5BOLD simulations of red supergiants. Fluctuations in the velocity field are compared with photometric and spectroscopic variations, the latter were derived from the TiO band strength and serve, at least partly, as a proxy of the variations in effective temperature.

    Results: The tomographic method reveals a phase shift between the velocity and spectroscopic and photometric variations. This phase shift results in a hysteresis loop in the temperature - velocity plane with a timescale of a few hundred days, which is similar to the photometric one. The similarity between the hysteresis loop timescale measured in mu Cep and the timescale of acoustic waves disturbing the convective pattern suggests that such waves play an important role in triggering the hysteresis loops.

  • 22.
    Kravchenko, K.
    et al.
    Univ Libre Bruxelles, Inst Astron & Astrophys, CP 226,Blvd Triomphe, B-1050 Brussels, Belgium..
    Van Eck, S.
    Univ Libre Bruxelles, Inst Astron & Astrophys, CP 226,Blvd Triomphe, B-1050 Brussels, Belgium..
    Chiavassa, A.
    Univ Cote dAzur, Observ Cote dAzur, CNRS, CS 34229, F-06304 Nice 4, France..
    Jorissen, A.
    Univ Libre Bruxelles, Inst Astron & Astrophys, CP 226,Blvd Triomphe, B-1050 Brussels, Belgium..
    Freytag, Bernd
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Plez, B.
    Univ Montpellier II, CNRS, Lab Univers & Particules Montpellier, F-34095 Montpellier 05, France..
    Tomography of cool giant and supergiant star atmospheres: I. Validation of the method2018In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 610, article id A29Article in journal (Refereed)
    Abstract [en]

    Context. Cool giant and supergiant star atmospheres are characterized by complex velocity fields originating from convection and pulsation processes which are not fully understood yet. The velocity fields impact the formation of spectral lines, which thus contain information on the dynamics of stellar atmospheres. Aims. The tomographic method allows to recover the distribution of the component of the velocity field projected on the line of sight at different optical depths in the stellar atmosphere. The computation of the contribution function to the line depression aims at correctly identifying the depth of formation of spectral lines in order to construct numerical masks probing spectral lines forming at different optical depths. Methods. The tomographic method is applied to one-dimensional (1D) model atmospheres and to a realistic three-dimensional (3D) radiative hydrodynamics simulation performed with CO5BOLD in order to compare their spectral line formation depths and velocity fields. Results. In 1D model atmospheres, each spectral line forms in a restricted range of optical depths. On the other hand, in 3D simulations, the line formation depths are spread in the atmosphere mainly because of temperature and density inhomogeneities. Comparison of cross-correlation function profiles obtained from 3D synthetic spectra with velocities from the 3D simulation shows that the tomographic method correctly recovers the distribution of the velocity component projected on the line of sight in the atmosphere.

  • 23.
    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.
    Freytag, Bernd
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Bladh, Sara
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Atmospheres and wind properties of non-spherical AGB stars2018In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 619, article id A47Article in journal (Refereed)
    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.

  • 24.
    Paladini, C.
    et al.
    Univ Libre Bruxelles, Inst Astron & Astrophys, CP 226, B-1050 Brussels, Belgium.;European Southern Observ, Alonso de Cordova 3107, Santiago, Chile., Chile..
    Baron, F.
    Georgia State Univ, Dept Phys & Astron, POB 5060, Atlanta, GA 30302 USA..
    Jorissen, A.
    Univ Libre Bruxelles, Inst Astron & Astrophys, CP 226, B-1050 Brussels, Belgium..
    Le Bouquin, J. -B
    Univ Grenoble Alpes, CNRS, IPAG, F-38000 Grenoble, France. .
    Freytag, Bernd
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Van Eck, S.
    Univ Libre Bruxelles, Inst Astron & Astrophys, CP 226, B-1050 Brussels, Belgium..
    Wittkowski, M.
    European Southern Observ, Karl Schwarzschild Str 2, D-85748 Garching, Germany..
    Hron, J.
    Univ Vienna, Dept Astrophys, Turkenschanzstr 17, A-1180 Vienna, Austria..
    Chiavassa, A.
    Univ Nice Sophia Antipolis, Lab Lagrange, Observ Cote Azur, UMR 7293,CNRS, BP 4229, F-06304 Nice 4, France..
    Berger, J. -P
    Siopis, C.
    Univ Libre Bruxelles, Inst Astron & Astrophys, CP 226, B-1050 Brussels, Belgium..
    Mayer, A.
    Univ Vienna, Dept Astrophys, Turkenschanzstr 17, A-1180 Vienna, Austria..
    Sadowski, G.
    Univ Libre Bruxelles, Inst Astron & Astrophys, CP 226, B-1050 Brussels, Belgium..
    Kravchenko, K.
    Univ Libre Bruxelles, Inst Astron & Astrophys, CP 226, B-1050 Brussels, Belgium..
    Shetye, S.
    Univ Libre Bruxelles, Inst Astron & Astrophys, CP 226, B-1050 Brussels, Belgium..
    Kerschbaum, F.
    Univ Vienna, Dept Astrophys, Turkenschanzstr 17, A-1180 Vienna, Austria..
    Kluska, J.
    Univ Exeter, Dept Phys & Astron, Stocker Rd, Exeter EX4 4QL, Devon, England..
    Ramstedt, Sofia
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Large granulation cells on the surface of the giant star π1 Gruis2018In: Nature, ISSN 0028-0836, E-ISSN 1476-4687, Vol. 553, no 7688, p. 310-+Article in journal (Refereed)
    Abstract [en]

    Convection plays a major part in many astrophysical processes, including energy transport, pulsation, dynamos and winds on evolved stars, in dust clouds and on brown dwarfs1,2. Most of our knowledge about stellar convection has come from studying the Sun: about two million convective cells with typical sizes of around 2,000 kilometres across are present on the surface of the Sun3—a phenomenon known as granulation. But on the surfaces of giant and supergiant stars there should be only a few large (several tens of thousands of times larger than those on the Sun) convective cells3, owing to low surface gravity. Deriving the characteristic properties of convection (such as granule size and contrast) for the most evolved giant and supergiant stars is challenging because their photospheres are obscured by dust, which partially masks the convective patterns4. These properties can be inferred from geometric model fitting5,6,7, but this indirect method does not provide information about the physical origin of the convective cells5,6,7. Here we report interferometric images of the surface of the evolved giant star π1 Gruis, of spectral type8,9 S5,7. Our images show a nearly circular, dust-free atmosphere, which is very compact and only weakly affected by molecular opacity. We find that the stellar surface has a complex convective pattern with an average intensity contrast of 12 per cent, which increases towards shorter wavelengths. We derive a characteristic horizontal granule size of about 1.2 × 1011 metres, which corresponds to 27 per cent of the diameter of the star. Our measurements fall along the scaling relations between granule size, effective temperature and surface gravity that are predicted by simulations of stellar surface convection10,11,12.

  • 25.
    Salhab, R. G.
    et al.
    Kiepenheuer Inst Sonnenphys, Schoneckstr 6, D-79104 Freiburg, Germany.
    Steiner, O.
    Kiepenheuer Inst Sonnenphys, Schoneckstr 6, D-79104 Freiburg, Germany;Ist Ric Solari Locarno IRSOL, Via Patocchi 57 Prato Pernice, CH-6605 Locarno, Switzerland.
    Berdyugina, S. V.
    Kiepenheuer Inst Sonnenphys, Schoneckstr 6, D-79104 Freiburg, Germany.
    Freytag, Bernd
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Rajaguru, S. P.
    Indian Inst Astrophys, Koramangala 2 Block, Bangalore 560034, Karnataka, India.
    Steffen, M.
    Leibniz Inst Astrophys AIP, Sternwarte 16, D-14482 Potsdam, Germany.
    Simulation of the small-scale magnetism in main-sequence stellar atmospheres2018In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 614, article id A78Article in journal (Refereed)
    Abstract [en]

    Context: Observations of the Sun tell us that its granular and subgranular small-scale magnetism has significant consequences for global quantities such as the total solar irradiance or convective blueshift of spectral lines.

    Aims: In this paper, properties of the small-scale magnetism of four cool stellar atmospheres, including the Sun, are investigated, and in particular its effects on the radiative intensity and flux.

    Methods: We carried out three-dimensional radiation magnetohydrodynamic simulations with the (COBOLD)-B-5 code in two different settings: with and without a magnetic field. These are thought to represent states of high and low small-scale magnetic activity of a stellar magnetic cycle.

    Results: We find that the presence of small-scale magnetism increases the bolometric intensity and flux in all investigated models. The surplus in radiative flux of the magnetic over the magnetic field-free atmosphere increases with increasing effective temperature, T-eff, from 0.47% for spectral type K8V to 1.05% for the solar model, but decreases for higher effective temperatures than solar. The degree of evacuation of the magnetic flux concentrations monotonically increases with T-eff as does their depression of the visible optical surface, that is the Wilson depression. Nevertheless, the strength of the field concentrations on this surface stays remarkably unchanged at approximate to 1560G throughout the considered range of spectral types. With respect to the surrounding gas pressure, the field strength is close to (thermal) equipartition for the Sun and spectral type F5V but is clearly sub-equipartition for K2V and more so for K8V. The magnetic flux concentrations appear most conspicuous for model K2V owing to their high brightness contrast.

    Conclusions: For mean magnetic flux densities of approximately 50 G, we expect the small-scale magnetism of stars in the spectral range from F5V to K8V to produce a positive contribution to their bolometric luminosity. The modulation seems to be most effective for early G-type stars.

  • 26. Schaffenberger, W.
    et al.
    Wedemeyer-Böhm, S.
    Steiner, O.
    Freytag, Bernd
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Holistic MHD-Simulation from the Convection Zone to the Chromosphere2006In: Solar MHD Theory and Observations: A High Spatial Resolution Perspective, 2006, Vol. 354Conference paper (Other academic)
  • 27. Steiner, Oskar
    et al.
    Salhab, Rene
    Freytag, Bernd
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Rajaguru, Paul
    Schaffenberger, Werner
    Steffen, Matthias
    Properties of small-scale magnetism of stellar atmospheres2014In: Nippon Tenmon Gakkai obun kenkyu hokoku, ISSN 0004-6264, Vol. 66, no SI1, p. S5-Article in journal (Refereed)
    Abstract [en]

    The magnetic field outside of sunspots is concentrated in the intergranular space, where it forms a delicate filigree of bright ribbons and dots as seen on broad band images of the Sun. We expect this small-scale magnetic field to exhibit a similar behavior in stellar atmospheres. In order to find out more about it, we perform numerical simulations of the surface layers of stellar atmospheres. Here, we report on preliminary results from simulations in the range between 4000 K and 6500 K effective temperature with an initial vertical, homogeneous magnetic field of 50 G strength. We find that the field strength of the strongest magnetic flux concentrations increases with decreasing effective temperature at the height level where the average Rosseland optical depth is one. On the other hand, at the same level, the field is less strong than the thermal equipartition value in the coolest model but assumes superequipartition in the models hotter than 5000 K. While the Wilson depression of the strongest field concentrations is about one pressure scale height in the coolest model, it is more than four times the pressure scale height in the hottest one. We also find that the relative contribution of the bright filigree to the bolometric, vertically directed radiative intensity is most significant for the T-eff = 5000 K model (0.6%-0.79%) and least significant for the hottest and coolest models (0.1%-0.46% and 0.14%-0.32%, respectively). This behavior suggests that the effect of the small-scale magnetic field on the photometric variability is more significant for K dwarf stars than for F-type and also M-type stars.

  • 28. Tremblay, P. -E
    et al.
    Fontaine, G.
    Univ Montreal, Dept Phys, Montreal, PQ H3C 3J7, Canada..
    Freytag, Bernd
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
    Steiner, O.
    Kiepenheuer Inst Sonnenphys, D-79104 Freiburg, Germany.;Ist Ric Solari Locarno, CH-6605 Locarno, Switzerland..
    Ludwig, H. -G
    Steffen, M.
    Leibniz Inst Astrophys Potsdam, D-14482 Potsdam, Germany..
    Wedemeyer, S.
    Univ Oslo, Inst Theoret Astrophys, NO-0315 Oslo, Norway..
    Brassard, P.
    Univ Montreal, Dept Phys, Montreal, PQ H3C 3J7, Canada..
    On The Evolution Of Magnetic White Dwarfs2015In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 812, no 1, article id 19Article in journal (Refereed)
    Abstract [en]

    We present the first radiation magnetohydrodynamic simulations of the atmosphere of white dwarf stars. We demonstrate that convective energy transfer is seriously impeded by magnetic fields when the plasma-beta parameter, the thermal-to-magnetic-pressure ratio, becomes smaller than unity. The critical field strength that inhibits convection in the photosphere of white dwarfs is in the range B = 1-50 kG, which is much smaller than the typical 1-1000MG field strengths observed in magnetic white dwarfs, implying that these objects have radiative atmospheres. We have employed evolutionary models to study the cooling process of high-field magnetic white dwarfs, where convection is entirely suppressed during the full evolution (B greater than or similar to 10 MG). We find that the inhibition of convection has no effect on cooling rates until the effective temperature (T-eff) reaches a value of around 5500 K. In this regime, the standard convective sequences start to deviate from the ones without convection due to the convective coupling between the outer layers and the degenerate reservoir of thermal energy. Since no magnetic white dwarfs are currently known at the low temperatures where this coupling significantly changes the evolution, the effects of magnetism on cooling rates are not expected to be observed. This result contrasts with a recent suggestion that magnetic white dwarfs with Teff less than or similar to 10,000 K cool significantly slower than non-magnetic degenerates.

  • 29. Tremblay, P. -E
    et al.
    Gianninas, A.
    Univ Oklahoma, Dept Phys & Astron, Norman, OK 73019 USA..
    Kilic, M.
    Univ Oklahoma, Dept Phys & Astron, Norman, OK 73019 USA..
    Ludwig, H. -G
    Steffen, M.
    Leibniz Inst Astrophys Potsdam, D-14482 Potsdam, Germany..
    Freytag, Bernd
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics. Uppsala Univ, Dept Phys & Astron, SE-75120 Uppsala, Sweden..
    Hermes, J. J.
    Univ Warwick, Dept Phys, Coventry CV4 7AL, W Midlands, England..
    3D Model Atmospheres for Extremely Low-Mass White Dwarfs2015In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 809, no 2, article id 148Article in journal (Refereed)
    Abstract [en]

    We present an extended grid of mean three-dimensional (3D) spectra for low-mass, pure-hydrogen atmosphere DA white dwarfs (WDs). We use CO5BOLD radiation-hydrodynamics 3D simulations covering T-eff = 6000-11,500 K and log g = 5-6.5 (g in cm s(-2)) to derive analytical functions to convert spectroscopically determined 1D temperatures and surface gravities to 3D atmospheric parameters. Along with the previously published 3D models, the 1D to 3D corrections are now available for essentially all known convective DA WDs (i.e., log g - 5-9). For low-mass WDs, the correction in temperature is relatively small (a few percent at the most), but the surface gravities measured from the 3D models are lower by as much as 0.35 dex. We revisit the spectroscopic analysis of the extremely low-mass (ELM) WDs, and demonstrate that the 3D models largely resolve the discrepancies seen in the radius and mass measurements for relatively cool ELM WDs in eclipsing double WD and WD + millisecond pulsar binary systems. We also use the 3D corrections to revise the boundaries of the ZZ Ceti instability strip, including the recently found ELM pulsators.

  • 30. Tremblay, P. -E
    et al.
    Leggett, S. K.
    Lodieu, N.
    Freytag, Bernd
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Bergeron, P.
    Kalirai, J. S.
    Ludwig, H. -G
    White Dwarfs in the UKIRT Infrared Deep Sky Survey Data Release 92014In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 788, no 2, p. 103-Article in journal (Refereed)
    Abstract [en]

    We have identified 8 to 10 new cool white dwarfs from the Large Area Survey (LAS) Data Release 9 of the United Kingdom InfraRed Telescope (UKIRT) Infrared Deep Sky Survey (UKIDSS). The data set was paired with the Sloan Digital Sky Survey to obtain proper motions and a broad ugrizYJHK wavelength coverage. Optical spectroscopic observations were secured at Gemini Observatory and confirm the degenerate status for eight of our targets. The final sample includes two additional white dwarf candidates with no spectroscopic observations. We rely on improved one-dimensional model atmospheres and new multi-dimensional simulations with CO5BOLD to review the stellar parameters of the published LAS white dwarf sample along with our additional discoveries. Most of the new objects possess very cool atmospheres with effective temperatures below 5000 K, including two pure-hydrogen remnants with a cooling age between 8.5 and 9.0 Gyr, and tangential velocities in the range 40 km s(-1) <= v(tan) <= 60 km s(-1). They are likely thick disk 10-11 Gyr old objects. In addition, we find a resolved double degenerate system with v(tan) similar to 155 km s(-1) and a cooling age between 3.0 and 5.0 Gyr. These white dwarfs could be disk remnants with a very high velocity or former halo G stars. We also compare the LAS sample with earlier studies of very cool degenerates and observe a similar deficit of helium-dominated atmospheres in the range 5000 < T-eff (K) < 6000. We review the possible explanations for the spectral evolution from helium-dominated toward hydrogen-rich atmospheres at low temperatures.

  • 31. Tremblay, P. -E
    et al.
    Ludwig, H. -G
    Freytag, Bernd
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Fontaine, G.
    Univ Montreal, Dept Phys, Montreal, PQ H3C 3J7, Canada..
    Steffen, M.
    Leibniz Inst Astrophys Potsdam, D-14482 Potsdam, Germany..
    Brassard, P.
    Univ Montreal, Dept Phys, Montreal, PQ H3C 3J7, Canada..
    Calibration of the Mixing-Length Free Parameter for White Dwarf Structures2015In: 19th European Workshop on White Dwarfs, 2015, Vol. 493, p. 89-94Conference paper (Other academic)
    Abstract [en]

    We present a comparison of our grid of 3D radiation-hydrodynamical simulations for 70 pure-hydrogen DA white dwarfs, in the surface gravity range 7.0 <= log g <= 9.0, with 1D envelope models based on the mixing-length theory (MLT) for convection. We perform a calibration of the mixing-length parameter for the lower part of the convection zone. The 3D simulations are often restricted to the upper convective layers, and in those cases, we rely on the asymptotic entropy value of the adiabatic 3D upfiows to calibrate 1D envelopes. Our results can be applied to 1D structure calculations, and in particular for pulsation and convective mixing studies. We demonstrate that while the 1D MLT only provides a bottom boundary of the convection zone based on the Schwarzschild criterion, the 3D stratifications are more complex. There is a large overshoot region below the convective layers that is likely critical for chemical diffusion applications.

  • 32. Tremblay, P. -E
    et al.
    Ludwig, H. -G
    Freytag, Bernd
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Fontaine, G.
    Steffen, M.
    Brassard, P.
    Calibration of the Mixing-Length Theory for Convective White Dwarf Envelopes2015In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 799, no 2, article id 142Article in journal (Refereed)
    Abstract [en]

    A calibration of the mixing-length parameter in the local mixing-length theory (MLT) is presented for the lower part of the convection zone in pure-hydrogen-atmosphere white dwarfs. The parameterization is performed from a comparison of three-dimensional (3D) CO5BOLD simulations with a grid of one-dimensional (1D) envelopes with a varying mixing-length parameter. In many instances, the 3D simulations are restricted to the upper part of the convection zone. The hydrodynamical calculations suggest, in those cases, that the entropy of the upflows does not change significantly from the bottom of the convection zone to regions immediately below the photosphere. We rely on this asymptotic entropy value, characteristic of the deep and adiabatically stratified layers, to calibrate 1D envelopes. The calibration encompasses the convective hydrogen-line (DA) white dwarfs in the effective temperature range 6000 <= T-eff (K) <= 15,000 and the surface gravity range 7.0 <= log g <= 9.0. It is established that the local MLT is unable to reproduce simultaneously the thermodynamical, flux, and dynamical properties of the 3D simulations. We therefore propose three different parameterizations for these quantities. The resulting calibration can be applied to structure and envelope calculations, in particular for pulsation, chemical diffusion, and convective mixing studies. On the other hand, convection has no effect on the white dwarf cooling rates until there is a convective coupling with the degenerate core below T-eff similar to 5000 K. In this regime, the 1D structures are insensitive to the MLT parameterization and converge to the mean 3D results, hence they remain fully appropriate for age determinations.

  • 33. Tremblay, P. -E
    et al.
    Ludwig, H. -G
    Freytag, Bernd
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Koester, D.
    Fontaine, G.
    Convective overshoot and metal accretion onto white dwarfs.2017In: MEMORIE della Società Astronomica Italiana, ISSN 0037-8720, E-ISSN 1824-016X, Vol. 88, p. 104-Article in journal (Other academic)
    Abstract [en]

    A large fraction of white dwarfs host evolved planetary systems and show evidence of accretion from planetary debris. The accretion-diffusion model is the preferred method to understand the metal pollution in these otherwise hydrogen- and helium-rich white dwarf atmospheres. In this scenario, the accreted material first settles on the atmosphere. If the outer stellar layers are unstable to convection, the metals are then rapidly mixed up within the convection zone. In the classical 1D approach, it is generally assumed that the convection zone has a sharp bottom boundary, below which microscopic diffusion is unhampered and slowly removes metals from the visible layers. More realistic 3D radiation-hydrodynamics simulations of white dwarfs with CO5BOLD demonstrate, however, that the bottom of the convection zone does not have a sharp boundary, and that velocities decay exponentially below the unstable convective layers with a velocity scale height of the order of one pressure scale height. This has a potentially dramatic effect on the inferred mass of accreted materiel, hence on the chemical composition and size of planetary debris around white dwarfs.

  • 34.
    Vasilyev, V.
    et al.
    Heidelberg Univ, Zentrum Astron, Landessternwarte, Konigstuhl 12, D-69117 Heidelberg, Germany.;Max Planck Inst Astron, Konigstuhl 17, D-69117 Heidelberg, Germany..
    Ludwig, H. -G
    Freytag, Bernd
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Lemasle, B.
    Heidelberg Univ, Zentrum Astron, Astron Recheninst, Monchhofstr 12-14, D-69120 Heidelberg, Germany..
    Marconi, M.
    INAF Osservatorio Astron Capodimonte, Via Moiariello 16, I-80131 Naples, Italy..
    Spectroscopic properties of a two-dimensional time-dependent Cepheid model2017In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 606, article id A140Article in journal (Refereed)
    Abstract [en]

    Context. Standard spectroscopic analyses of Cepheid variables are based on hydrostatic one-dimensional model atmospheres, with convection treated using various formulations of mixing-length theory. Aims. This paper aims to carry out an investigation of the validity of the quasi-static approximation in the context of pulsating stars. We check the adequacy of a two-dimensional time-dependent model of a Cepheid-like variable with focus on its spectroscopic properties. Methods. With the radiation-hydrodynamics code CO5BOLD, we construct a two-dimensional time-dependent envelope model of a Cepheid with T-eff = 5600 K, log g = 2:0, solar metallicity, and a 2.8-day pulsation period. Subsequently, we perform extensive spectral syntheses of a set of artificial iron lines in local thermodynamic equilibrium. The set of lines allows us to systematically study effects of line strength, ionization stage, and excitation potential. Results. We evaluate the microturbulent velocity, line asymmetry, projection factor, and Doppler shifts. The microturbulent velocity, averaged over all lines, depends on the pulsational phase and varies between 1.5 and 2.7 km s(-1). The derived projection factor lies between 1.23 and 1.27, which agrees with observational results. The mean Doppler shift is non-zero and negative, 1 km s(-1), after averaging over several full periods and lines. This residual line-of-sight velocity (related to the "K-term") is primarily caused by horizontal inhomogeneities, and consequently we interpret it as the familiar convective blueshift ubiquitously present in non-pulsating late-type stars. Limited statistics prevent firm conclusions on the line asymmetries. Conclusions. Our two-dimensional model provides a reasonably accurate representation of the spectroscopic properties of a shortperiod Cepheid-like variable star. Some properties are primarily controlled by convective inhomogeneities rather than by the Cepheid-defining pulsations. Extended multi-dimensional modelling offers new insight into the nature of pulsating stars.

  • 35.
    Vasilyev, V.
    et al.
    Heidelberg Univ, Zentrum Astron, Konigstuhl 12, D-69117 Heidelberg, Germany.;Max Planck Inst Astron, Konigstuhl 17, D-69117 Heidelberg, Germany..
    Ludwig, H. -G
    Freytag, Bernd
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Lemasle, B.
    Heidelberg Univ, Zentrum Astron, Astronom Rechen Inst, Mnchhofstr 12-14, D-69120 Heidelberg, Germany..
    Marconi, M.
    Osserv Astron Capodimonte, INAF, Via Moiariello 16, I-80131 Naples, Italy..
    Spectroscopic properties of a two-dimensional time-dependent Cepheid model II. Determination of stellar parameters and abundances2018In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 611, article id A19Article in journal (Refereed)
    Abstract [en]

    Context. Standard spectroscopic analyses of variable stars are based on hydrostatic 1D model atmospheres. This quasi-static approach has not been theoretically validated. Aims. We aim at investigating the validity of the quasi-static approximation for Cepheid variables. We focus on the spectroscopic determination of the effective temperature T-eff, surface gravity log g, microturbulent velocity xi(t), and a generic metal abundance log A; here taken as iron. Methods. We calculated a grid of 1D hydrostatic plane-parallel models covering the ranges in effective temperature and gravity that are encountered during the evolution of a 2D time-dependent envelope model of a Cepheid computed with the radiation-hydrodynamics code CO5BOLD. We performed 1D spectral syntheses for artificial iron lines in local thermodynamic equilibrium by varying the microturbulent velocity and abundance. We fit the resulting equivalent widths to corresponding values obtained from our dynamical model for 150 instances in time, covering six pulsational cycles. In addition, we considered 99 instances during the initial non-pulsating stage of the temporal evolution of the 2D model. In the most general case, we treated T-eff, log g, xi(t), and log A as free parameters, and in two more limited cases, we fixed T-eff and log g by independent constraints. We argue analytically that our approach of fitting equivalent widths is closely related to current standard procedures focusing on line-by-line abundances. Results. For the four-parametric case, the stellar parameters are typically underestimated and exhibit a bias in the iron abundance of approximate to-0.2 dex. To avoid biases of this type, it is favorable to restrict the spectroscopic analysis to photometric phases phi(ph) approximate to 0.3...0.65 using additional information to fix the effective temperature and surface gravity. Conclusions. Hydrostatic 1D model atmospheres can provide unbiased estimates of stellar parameters and abundances of Cepheid variables for particular phases of their pulsations. We identified convective inhomogeneities as the main driver behind potential biases. To obtain a complete view on the effects when determining stellar parameters with 1D models, multidimensional Cepheid atmosphere models are necessary for variables of longer period than investigated here.

  • 36. Wedemeyer, S.
    et al.
    Freytag, Bernd
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Steffen, M.
    Ludwig, H. -G
    Holweger, H.
    Numerical simulation of the three-dimensional structure and dynamics of the non-magnetic solar chromosphere2004In: Astronomy and Astrophysics, Vol. 414, p. 1121-1137Article in journal (Refereed)
  • 37.
    Wedemeyer, Sven
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Astronomy and Space Physics.
    Freytag, Bernd
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Steffen, Matthias
    Ludwig, Hans-Günter
    Holweger, Hartmut
    3-D hydrodynamic simulations of the solar chromosphere2003In: Astronomical Notes - Astronomische Nachrichten, ISSN 0004-6337, E-ISSN 1521-3994, Vol. 324, p. 410-411Article in journal (Other academic)
  • 38. Wedemeyer-Böhm, S.
    et al.
    Kamp, I.
    Bruls, J.
    Freytag, Bernd
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Carbon monoxide in the solar atmosphere. I. Numerical method and two-dimensional models2005In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 438, p. 1043-1057Article in journal (Refereed)
  • 39. Wedemeyer-Böhm, S.
    et al.
    Schaffenberger, W.
    Steiner, O.
    Steffen, M.
    Freytag, Bernd
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Kamp, I.
    Simulations of Magnetohydrodynamics and CO Formation from the Convection Zone to the Chromosphere2005In: Chromospheric and Coronal Magnetic Fields, 2005, Vol. 596, p. 16-1Conference paper (Refereed)
  • 40.
    Wittkowski, M.
    et al.
    ESO, Karl Schwarzschild Str 2, D-85748 Garching, Germany..
    Chiavassa, A.
    Univ Nice Sophia Antipolis, CNRS, Observ Cote Azur, Lab Lagrange,UMR 7293, BP 4229, F-06304 Nice 4, France..
    Freytag, Bernd
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Scholz, M.
    Univ Heidelberg ZAH, Zentrum Astron, Inst Theoret Astrophys, Albert Ueberle Str 2, D-69120 Heidelberg, Germany.;Univ Sydney, Sch Phys, Sydney Inst Astron, Sydney, NSW 2006, Australia..
    Höfner, Susanne
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
    Karovicova, I.
    Univ Heidelberg ZAH, Zentrum Astron, Inst Theoret Astrophys, Albert Ueberle Str 2, D-69120 Heidelberg, Germany..
    Whitelock, P. A.
    S African Astron Observ, POB 9, ZA-7935 Observatory, South Africa.;Univ Cape Town, Dept Astron, Astron Cosmol & Grav Ctr, ZA-7701 Rondebosch, South Africa..
    Near-infrared spectro-interferometry of Mira variables and comparisons to 1D dynamic model atmospheres and 3D convection simulations2016In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 587, article id A12Article in journal (Refereed)
    Abstract [en]

    Aims:

    We aim at comparing spectro-interferometric observations of Mira variable asymptotic giant branch (AGB) stars with the latest 1D dynamic model atmospheres based on self-excited pulsation models (CODEX models) and with 3D dynamic model atmospheres including pulsation and convection (CO5BOLD models) to better understand the processes that extend the molecular atmosphere to radii where dust can form.

    Methods: We obtained a total of 20 near-infrared K-band spectro-interferometric snapshot observations of the Mira variables o Cet, R Leo, R Aqr, X Hya, W Vel, and R Cnc with a spectral resolution of about 1500. We compared observed flux and visibility spectra with predictions by CODEX 1D dynamic model atmospheres and with azimuthally averaged intensities based on CO5BOLD 3D dynamic model atmospheres.

    Results: Our visibility data confirm the presence of spatially extended molecular atmospheres located above the continuum radii with large-scale inhomogeneities or clumps that contribute a few percent of the total flux. The detailed structure of the inhomogeneities or clumps show a variability on time scales of 3 months and above. Both modeling attempts provided satisfactory fits to our data. In particular, they are both consistent with the observed decrease in the visibility function at molecular bands of water vapor and CO, indicating a spatially extended molecular atmosphere. Observational variability phases are mostly consistent with those of the best-fit CODEX models, except for near-maximum phases, where data are better described by near-minimum models. Rosseland angular diameters derived from the model fits are broadly consistent between those based on the 1D and the 3D models and with earlier observations. We derived fundamental parameters including absolute radii, effective temperatures, and luminosities for our sources.

    Conclusions: Our results provide a first observational support for theoretical results that shocks induced by convection and pulsation in the 3D CO5BOLD models of AGB stars are roughly spherically expanding and of similar nature to those of self-excited pulsations in 1D CODEX models. Unlike for red supergiants, the pulsation- and shock-induced dynamics can levitate the molecular atmospheres of Mira variables to extensions that are consistent with observations.

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