Atmospheric dynamics in carbon-rich Miras

Nowotny, W.; Aringer, B.; Höfner, Susanne; Gautschy-Loidl, R.; Windsteig, W.

doi:10.1051/0004-6361:20042461

Cited by 46 publications

(56 citation statements)

References 49 publications

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“…As illustrated in Fig. 5, dynamical model atmospheres can in principle produce similar line components (see also Nowotny et al 2005). However, none of the nine configurations of the dynamical models tested here (Table 4) gave a good fit to the entire observed high-resolution spectra.…”

Section: Discussionmentioning

confidence: 65%

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Abundance analysis for long-period variables

Lebzelter

Nowotny

Hinkle

et al. 2014

A&A

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

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Section: Discussionmentioning

confidence: 65%

“…This approach has been applied to AGB stars in various papers in the past (e.g. Hinkle et al 1982;Wallerstein 1985;Nowotny et al 2005Nowotny et al , 2010. All modelling results for this figure were calculated with a carbon isotopic ratio of 7 and C/O = 0.25.…”

Section: Investigation Of Different Line-forming Regionsmentioning

confidence: 99%

Abundance analysis for long-period variables

Lebzelter

Nowotny

Hinkle

et al. 2014

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“…In this way, our modelling approach bridges the gap between (i) the Australia-Heidelberg-type models for dust-free pulsating atmospheres without wind; and (ii) the Berlin-type models for dustdriven winds of highly dust-enshrouded objects (see Paper II and Sect. 3 of Nowotny et al 2005a for a more detailed description and references). Our models can reproduce spectra of C-rich AGB stars, for which the respective contributions by the photosphere and the dusty envelope are of varying relevance.…”

Section: Discussionmentioning

confidence: 99%

“…Model R was adopted from Höfner et al (2003), while models T and S were taken from Gautschy-Loidl et al (2004). The latter model was also used in various other studies (Nowotny et al 2005a(Nowotny et al ,b, 2010(Nowotny et al , 2011aPaladini et al 2009). Computed specifically for this study were the last two models, C1 and C2.…”

Section: Combined Atmosphere and Wind Modelsmentioning

confidence: 99%

Synthetic photometry for carbon-rich giants

Nowotny¹,

Aringer²,

Höfner

et al. 2013

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

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“…The dust-driven wind formation scenario in C-rich stars is to a certain amount satisfactorily solved (e.g. Gail & Sedlmayr 1987;Höfner et al 2003;Gautschy-Loidl et al 2004;Nowotny et al 2005;Woitke 2006b;Wachter et al 2008). Radiation pressure on carbon grains is efficient enough to initiate the wind.…”

Section: The Dust and Molecular Shellsmentioning

confidence: 99%

Dust and molecular shells in asymptotic giant branch stars

Zhao-Geisler

Quirrenbach

Köhler

et al. 2012

A&A

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Context. Asymptotic giant branch (AGB) stars are one of the largest distributors of dust into the interstellar medium. However, the wind formation mechanism and dust condensation sequence leading to the observed high mass-loss rates have not yet been constrained well observationally, in particular for oxygen-rich AGB stars. Aims. The immediate objective in this work is to identify molecules and dust species which are present in the layers above the photosphere, and which have emission and absorption features in the mid-infrared (IR), causing the diameter to vary across the N-band, and are potentially relevant for the wind formation. Methods. Mid-IR (8-13 μm) interferometric data of four oxygen-rich AGB stars (R Aql, R Aqr, R Hya, and W Hya) and one carbonrich AGB star (V Hya) were obtained with MIDI/VLTI between April 2007 and September 2009. The spectrally dispersed visibility data are analyzed by fitting a circular fully limb-darkened disk (FDD). Results. The FDD diameter as function of wavelength is similar for all oxygen-rich stars. The apparent size is almost constant between 8 and 10 μm and gradually increases at wavelengths longer than 10 μm. The apparent FDD diameter in the carbon-rich star V Hya essentially decreases from 8 to 12 μm. The FDD diameters are about 2.2 times larger than the photospheric diameters estimated from K-band observations found in the literature. The silicate dust shells of R Aql, R Hya and W Hya are located fairly far away from the star, while the silicate dust shell of R Aqr and the amorphous carbon (AMC) and SiC dust shell of V Hya are found to be closer to the star at around 8 photospheric radii. Phase-to-phase variations of the diameters of the oxygen-rich stars could be measured and are on the order of 15% but with large uncertainties. Conclusions. From a comparison of the diameter trend with the trends in RR Sco and S Ori it can be concluded that in oxygen-rich stars the overall larger diameter originates from a warm molecular layer of H 2 O, and the gradual increase longward of 10 μm can be most likely attributed to the contribution of a close Al 2 O 3 dust shell. The chromatic trend of the Gaussian FWHM in V Hya can be explained with the presence of AMC and SiC dust. The observations suggest that the formation of amorphous Al 2 O 3 in oxygenrich stars occurs mainly around or after visual minimum. However, no firm conclusions can be drawn concerning the mass-loss mechanism. Future modeling with hydrostatic and self-consistent dynamical stellar atmospheric models will be required for a more certain understanding.

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Atmospheric dynamics in carbon-rich Miras

Cited by 46 publications

References 49 publications

Abundance analysis for long-period variables

Abundance analysis for long-period variables

Synthetic photometry for carbon-rich giants

Dust and molecular shells in asymptotic giant branch stars

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