Volume 601, May 2017
|Number of page(s)||23|
|Published online||19 April 2017|
A grid of MARCS model atmospheres for late-type stars
1 Institut d’Astronomie et d’Astrophysique, Université Libre de Bruxelles (ULB), CP 226, Boulevard du Triomphe, 1050 Bruxelles, Belgium
2 Laboratoire Univers et Particules de Montpellier, Université de Montpellier, CNRS, 34095 Montpellier, France
3 Department of Physics and Astronomy, Division of Astronomy and Space Physics, Uppsala University, Box 515, 751 20 Uppsala, Sweden
4 Niels Bohr Institute, Juliane Maries vej 30, 2100 Copenhagen, Denmark
5 Centre for Star and Planet Formation, Geological Museum, Øster Voldgade 5, 1350 Copenhagen, Denmark
Received: 13 February 2015
Accepted: 23 July 2016
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 ≤ Teff(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 (Teff>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 Teff, C/O, [Fe/H] and [s/Fe]. The Geneva U−B1 and B2−V1 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 Teff = 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 (Teff, V−K) relationship (for a fixed C/O value). Hence, the use of a (Teff, 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.
Key words: stars: atmospheres / stars: fundamental parameters / stars: late-type / stars: AGB and post-AGB / stars: abundances / stars: general
Based on observations carried out at the European Southern Observatory (ESO, La Silla, Chile; program 58.E-0942), on the Swiss 70 cm telescope (La Silla, Chile) and on the Mercator telescope (La Palma, Spain).
The MARCS S star model atmospheres will be archived on the MARCS website: http://marcs.astro.uu.se
Full Tables 2 and 3 are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (126.96.36.199) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/601/A10
© ESO, 2017
Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.
Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.
Initial download of the metrics may take a while.