Dynamic model atmospheres of AGB stars

Höfner, Susanne; Gautschy-Loidl, R.; Aringer, B.; Jørgensen, U. G.

doi:10.1051/0004-6361:20021757

Cited by 182 publications

(302 citation statements)

References 13 publications

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“…At the same time these observations provide constraints for existing model atmospheres: e.g. COMARCS ), Höfner et al (2003), PHOENIX (Hauschildt et al 1999), ATLAS (e.g. Kurucz 1993), and CODEX (Ireland et al 2008.…”

Section: Introductionmentioning

confidence: 82%

Catching the fish – Constraining stellar parameters for TX Piscium using spectro-interferometric observations

Klotz

Paladini

Hron

et al. 2013

A&A

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Context. Stellar parameter determination is a challenging task when dealing with galactic giant stars. The combination of different investigation techniques has proven to be a promising approach. Aims. We analyse archive spectra obtained with the Short Wavelength Spectrometer (SWS) onboard ISO, and new interferometric observations from the Very Large Telescope MID-infrared Interferometric instrument (VLTI/MIDI) of a very well studied carbon-rich giant: TX Psc. The aim of this work is to determine stellar parameters using spectroscopy and interferometry. Methods. The observations are used to constrain the model atmosphere, and eventually the stellar evolutionary model in the region where the tracks map the beginning of the carbon star sequence. Two different approaches are used to determine stellar parameters: (i) the "classic" interferometric approach where the effective temperature is fixed by using the angular diameter in the N-band (from interferometry) and the apparent bolometric magnitude; (ii) parameters are obtained by fitting a grid of state-of-the-art hydrostatic models to spectroscopic and interferometric observations. Results. We find good agreement between the parameters of the two methods. The effective temperature and luminosity clearly place TX Psc in the carbon-rich AGB star domain in the H-R-diagram. Current evolutionary tracks suggest that TX Psc became a C-star just recently, which means that the star is still in a "quiet" phase compared to the subsequent strong-wind regime. This agrees with the C/O ratio being only slightly greater than one.

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Section: Introductionmentioning

confidence: 82%

Catching the fish – Constraining stellar parameters for TX Piscium using spectro-interferometric observations

Klotz

Paladini

Hron

et al. 2013

A&A

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“…We consider strictly ballistic motions of the gas above the photosphere and identify the location of cluster formation and dust condensation in the inner wind. Therefore, our model provides a simplified description compared to more sophisticated A6, page 4 of 15 hydrodynamic models of the inner wind, which take into account the dust radiative feedback on the gas (e.g., Winters et al 2000;Höfner et al 2003;Woitke 2006). While such models aim to reproduce the mass loss rates and optical properties of AGB winds, they do not seem to explain the non-equilibrium chemical processes that trigger the formation of molecules, dust clusters and the growth of dust grains from the gas phase.…”

Section: Preshockmentioning

confidence: 99%

Dust formation in the oxygen-rich AGB star IK Tauri

Gobrecht

Cherchneff

Sarangi

et al. 2015

A&A

189

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Aims. We model the synthesis of molecules and dust in the inner wind of the oxygen-rich Mira-type star IK Tau by considering the effects of periodic shocks induced by the stellar pulsation on the gas and by following the non-equilibrium chemistry in the shocked gas layers between 1 R and 10 R . We consider a very complete set of molecules and dust clusters, and combine the nucleation phase of dust formation with the condensation of these clusters into dust grains. We also test the impact of increasing the local gas density. Our derived molecular abundances and dust properties are compared to the most recent observational data. Methods. A semi-analytical formalism based on parameterised fluid equations is used to describe the gas density, velocity, and temperature in the inner wind. The chemistry is described by using a chemical kinetic network of reactions and the condensation mechanism is described by a Brownian formalism. A set of stiff, ordinary, coupled differential equations is solved, and molecular abundances, dust cluster abundances, grain size distributions and dust masses are derived. Results. The shocks drive an active non-equilibrium chemistry in the dust formation zone of IK Tau where the collision destruction of CO in the post-shock gas triggers the formation of C-bearing species such as HCN and CS. Most of the modelled molecular abundances agree well with the latest values derived from Herschel data, except for SO 2 and NH 3 , whose formation may not occur in the inner wind. Clusters of alumina, Al 2 O 3 , are produced within 2 R and lead to a population of alumina grains close to the stellar surface. Clusters of silicates (Mg 2 SiO 4 ) form at larger radii (r > 3 R ), where their nucleation is triggered by the formation of HSiO and H 2 SiO. They efficiently condense and reach their final grain size distribution between ∼6 R and 8 R with a major population of medium size grains peaking at ∼200 Å. This two dust-shell configuration agrees with recent interferometric observations. The derived dust-to-gas mass ratio for IK Tau is in the range 1-6 × 10 −3 and agrees with values derived from observations of O-rich Mira-type stars. Conclusions. Our results confirm the importance of periodic shocks in chemically shaping the inner wind of AGB stars and providing gas conditions conducive to the efficient synthesis of molecules and dust by non-equilibrium processes. They indicate that the wind acceleration will possibly develop in the radius range 4-8 R in IK Tau.

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“…For our line profile modelling we used dynamic model atmospheres as described in detail by Höfner et al (2003;DMA3), Gautschy-Loidl et al (2004;DMA4), N05 or Papers I and II. These models represent the scenario of pulsation-enhanced dustdriven winds (cf.…”

Section: General Remarksmentioning

confidence: 99%

“…We refer to Höfner et al (2003) for more details about the numerical methods and the atmospheric models. Fig.…”

Section: Models For Atmospheres and Windsmentioning

confidence: 99%

Line formation in AGB atmospheres including velocity effects

Nowotny¹,

Höfner

Aringer³

2010

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Context. Towards the end of the evolutionary stage of the asymptotic giant branch (AGB) the atmospheres of evolved red giants are considerably influenced by radial pulsations of the stellar interiors and developing stellar winds. The resulting complex velocity fields severely affect molecular line profiles (shapes, time-dependent shifts in wavelength, multiple components) observable in near-infrared spectra of long period variables. Time-series high-resolution spectroscopy allows us to probe the atmospheric kinematics and thereby study the mass loss process. Aims. With the help of model calculations the complex line formation process in AGB atmospheres was explored with the focus on velocity effects. Furthermore, we aimed for atmospheric models which are able to quantitatively reproduce line profile variations found in observed spectra of pulsating late-type giants. Methods. Models describing pulsation-enhanced dust-driven winds were used to compute synthetic spectra under the assumptions of chemical equilibrium and LTE. For this purpose, we used molecular data from line lists for the considered species and solved the radiative transfer in spherical geometry including the effects of velocity fields. Radial velocities (RV) derived from Doppler-shifted (components of) synthetic line profiles provide information on the gas velocities in the line-forming region of the spectral features. In addition, we made use of radial optical depth distributions to give estimates for the layers where lines are formed and to illustrate the effects of velocities in the line formation process. Results. Assuming uniform gas velocities for all depth points of an atmospheric model we estimated the conversion factor between gas velocities and measured RVs to p = u gas /RV ≈ 1.2−1.5. On the basis of dynamic model atmospheres and by applying our spectral synthesis codes we investigated in detail the finding that various molecular features in AGB spectra originate at different geometrical depths of the very extended atmospheres of these stars. We show that the models are able to quantitatively reproduce the characteristic line profile variations of lines sampling the deep photosphere (CO Δv = 3, CN) of Mira variables and the corresponding discontinuous, S-shaped RV curve. The global velocity fields (traced by different features) of typical long-period variables are also realistically reproduced. Possible reasons for discrepancies concerning other modelling results (e.g. CO Δv = 2 lines) are outlined. In addition, we present a model showing variations of CO Δv = 3 line profiles comparable to observed spectra of semiregular variables and discuss that the non-occurence of line doubling in these objects may be due to a density effect. Conclusions. The results of our line profile modelling are another indication that the dynamic models studied here are approaching a realistic representation of the outer layers of AGB stars with or without mass loss.

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Dynamic model atmospheres of AGB stars

Cited by 182 publications

References 13 publications

Catching the fish – Constraining stellar parameters for TX Piscium using spectro-interferometric observations

Catching the fish – Constraining stellar parameters for TX Piscium using spectro-interferometric observations

Dust formation in the oxygen-rich AGB star IK Tauri

Line formation in AGB atmospheres including velocity effects

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