A grid of MARCS model atmospheres for late-type stars

Abstract

S-type stars are late-type giants whose atmospheres are enriched in carbon and s-process elements because of either extrinsic pollution by a binary companion or intrinsic nucleosynthesis and dredge-up on the thermally-pulsing asymptotic giant branch. A grid of MARCS model atmospheres has been computed for S stars, covering the range 2700 ≤ T eff (K) ≤ 4000, 0.50 ≤ C/O ≤ 0.99, 0 ≤ log g ≤ 5, [Fe/H] = 0., −0.5 dex, and [s/Fe] = 0, 1, and 2 dex (where the latter quantity refers to the global overabundance of s-process elements). The MARCS models make use of a new ZrO line list. Synthetic spectra computed from these models are used to derive photometric indices in the Johnson and Geneva systems, as well as TiO and ZrO band strengths. A method is proposed to select the model best matching any given S star, a non-trivial operation since the grid contains more than 3500 models covering a five-dimensional parameter space. The method is based on the comparison between observed and synthetic photometric indices and spectral band strengths, and has been applied on a vast subsample of the Henize sample of S stars. Our results confirm the old claim by Piccirillo (1980, MNRAS, 190, 441) that ZrO bands in warm S stars (T eff > 3200 K) are not caused by the C/O ratio being close to unity, as traditionally believed, but rather by some Zr overabundance. The TiO and ZrO band strengths, combined with V − K and J − K photometric indices, are used to select T eff , C/O, [Fe/H] and [s/Fe]. The Geneva U − B 1 and B 2 − V 1 indices (or any equivalent) are good at selecting the gravity. The defining spectral features of dwarf S stars are outlined, but none is found among the Henize S stars. More generally, it is found that, at T eff = 3200 K, a change of C/O from 0.5 to 0.99 has a strong impact on V − K (2 mag). Conversely, a range of 2 mag in V − K corresponds to a 200 K shift along the (T eff , V − K) relationship (for a fixed C/O value). Hence, the use of a (T eff , V − K) calibration established for M stars will yield large errors for S stars, so that a specific calibration must be used, as provided in the present paper. Using the atmospheric parameters derived by our method for the sample of Henize S stars, we show that the extrinsic-intrinsic dichotomy among S stars reveals itself very clearly as a bimodal distribution in the effective temperatures. Moreover, the increase of s-process element abundances with increasing C/O ratios and decreasing temperatures is apparent among intrinsic stars, confirming theoretical expectations.

Observational evidence of third dredge-up occurrence in S-type stars with initial masses around 1 M⊙

“…The atmospheric parameters {T eff , log g, [Fe/H], C/O, [s/Fe]}, where [s/Fe] 1 is the s-process enhancement with respect to the (metallicity-scaled) solar s-process contribution, were derived as in S18. In summary, this method performs a χ 2 adjustment between a grid of S-star MARCS synthetic spectra (Van Eck et al 2017) and observed HERMES spectra within carefully selected spectral regions, also considering synthetic and observed photometric color indices. Luminosity was calculated using the distances derived from the Gaia DR2 parallaxes, the reddening E B−V (retrieved from Gontcharov 2012) and the bolometric correction in the K-band as computed from the MARCS model atmospheres.…”

“…As a complement to Van Eck et al (2017), we give here details about the exact meaning of the [s/Fe] parameter in the MARCS grid of S stars, which provides models with [s/Fe] = 0, +1, and +2 dex. This parameter adjusts the abundances log X for elements from Ga to Bi in the following way: X s = log 10 (n X /n H ) × 10 [Fe/H] × 10 [s/Fe] × f X,s , X r = log 10 (n X /n H ) × 10 [Fe/H] × 10 [r/Fe] × (1 − f X,s ) , log X = log 10 (10 X s + 10 X r ).…”

Equilibrium chemistry down to 100 K

“…Helling & Lucas 2009;Witte et al 2009;Crossfield et al 2013;Hu & Seager 2014;Morley et al 2017) or by changing the carbon-to-oxygen ratio (e.g. Madhusudhan et al 2011;Madhusudhan 2012;Helling et al 2017;Van Eck et al 2017). Gaidos (2000) was one of the earliest works suggesting the importance of C/O in an exoplanet context.…”

“…Three major schools are: (i) use of law of mass action and molecular equilibrium constants based on Gibbs free energy data (Tsuji 1973;Stock 2008;Bilger et al 2013), (ii) use of law of mass action and molecular equilibrium constants based on partition functions (Allard & Hauschildt 1995;Gustafsson et al 2008;Barklem & Collet 2016;Van Eck et al 2017), and (iii) minimisation of the system Gibbs free energy (Lodders 2003;Ackerman & Marley 2001;Miller-Ricci et al 2009;Mbarek & Kempton 2016;Blecic et al 2016). Helling et al (2008) provide a summary of approaches used in brown dwarf and planetary atmospheres.…”

Self-consistent atmosphere modeling with cloud formation for low-mass stars and exoplanets

“…In this paper we present an extension of the Marcs code (Gustafsson 1971;Jørgensen et al 1992;Gustafsson et al 2008; Article number, page 1 of 17 arXiv:1708.06976v1 [astro-ph.SR] 23 Aug 2017 Van Eck et al 2017) that has so far been used extensively for modeling atmospheres of cool stars (Lambert et al 1986;Plez 1992;Aringer et al 1997), including abundance analysis (e.g. Blackwell et al (1995); Matrozis et al (2013); Nissen et al (2014); Hill et al (2016); Siqueira-Mello et al (2016)), H 2 O detections (Ryde et al 2002;Aringer et al 2002), microdiamonds in carbon stars (Andersen & Jørgensen 1995), and instrument calibration (Decin et al 2003;Decin & Eriksson 2007).…”