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J/A+A/601/A10    A grid of MARCS model atmospheres for S stars  (Van Eck+, 2017)

A grid of MARCS model atmospheres for late-type stars II. S stars and their properties. Van Eck S., Neyskens P., Jorissen A., Plez B., Edvardsson B., Eriksson K., Gustafsson B., Jorgensen U.G., Nordlund A. <Astron. Astrophys. 601, A10 (2017)> =2017A&A...601A..10V (SIMBAD/NED BibCode)
ADC_Keywords: Models, atmosphere; Stars, S; Stars, late-type; Photometry, Geneva; Photometry, UBVRIJKLMNH; Abundances, peculiar Keywords: stars: atmospheres - stars: fundamental parameters - stars: late-type - stars: AGB and post-AGB - stars: abundances - stars: general 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≤Teff(K)≤4000, 0.50≤C/O<0.99, 0≤logg≤5, [Fe/H]=0., -0.5dex, 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 (1980MNRAS.190..441P) that ZrO bands in warm S stars (Teff > 3200K) 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-B_1 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=3200K, a change of C/O from 0.5 to 0.99 has a strong impact on V-K (2mag). Conversely, a range of 2 mag in V-K corresponds to a 200K 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. Description: Johnson and Geneva (G) photometric indices and band indices measured on the observed Henize S stars. File Summary:
FileName Lrecl Records Explanations
ReadMe 80 . This file table1.dat 27 12 Boundaries of the continuum and band windows used in the computation of the band indices table2.dat 100 135 Geneva and Johnson photometric indices and band indices for observed Henize S stars table3.dat 165 3523 Geneva and Johnson photometric indices and band indices computed on the grid of synthetic spectra of S stars
Byte-by-byte Description of file: table1.dat
Bytes Format Units Label Explanations
1- 3 A3 --- Band Band main contributor 5- 9 F5.1 nm LBi Initial wavelength of the band interval (1) 11- 15 F5.1 nm LBf Final wavelength band interval (1) 17- 21 F5.1 nm LCi Initial wavelength of the continuum interval(1) 23- 27 F5.1 nm LCf Final wavelength of the continuum interval (1)
Note (1): This paper, Equation (1).
Byte-by-byte Description of file: table2.dat
Bytes Format Units Label Explanations
1- 9 A9 --- Hen4- Hen 4-NNN: S stars from an unpublished list of Henize (1) 12- 16 F5.2 mag Vmag0 Dereddened Geneva V magnitude (2) 19- 23 F5.2 mag AV ? Total extinction at the V filter 26- 30 F5.2 mag (U-B)0 Dereddened Geneva (U-B) color index (2) 33- 37 F5.2 mag (B-V)0 Dereddened Geneva (B-V) color index (2) 40- 44 F5.2 mag (V-K)0 ? Dereddened (V-K) color index (2) 47- 51 F5.2 mag (J-K)0 ? Dereddened (J-K) color index (2) 54- 58 F5.2 mag (H-K)0 ? Dereddened (H-K) color index (2) 61- 65 F5.2 mag (K-L)0 ? Dereddened (K-L) color index (2) 68- 72 F5.2 mag (U-B1)0 Dereddened Geneva (U-B_1) color index (2) 75- 79 F5.2 mag (B2-V1)0 Dereddened Geneva (B2-V1) color index (2) 82- 86 F5.2 --- BTiO Titanium oxyde band strength index (2) 89- 93 F5.2 --- BZrO Zirconium oxyde band strength index (2) 96-100 F5.2 --- BNa Sodium lines band strength index (2)
Note (1): Description in 1960AJ......65..491H. N=166 among (Nos 1-236), cross-id. with CSS in 1984PW&SO...3....1S. (Nos 1-205), cross-id. with CSS in 2000A&AS..145...51V. Note (2): This paper, for references and formulae.
Byte-by-byte Description of file: table3.dat
Bytes Format Units Label Explanations
1- 4 I4 K Teff Effective temperature 6- 9 F4.2 [cm/s2] logg Logarithm (base 10) surface gravity (cgs) 12- 16 F5.2 [Sun] [Fe/H] Metallicity (1) 18- 23 F6.3 [Sun] [C/Fe] Abundance ratio [C/Fe] (1) 27- 30 F4.2 --- C/O Carbon to oxygen abundance ratio 34- 37 F4.2 [Sun] [s/Fe] Abundance ratio [s/Fe] (1)(2) 43- 48 F6.2 mag U-B (U-B) color index 51- 56 F6.2 mag B-V (B-V) color index 59- 64 F6.2 mag V-R (V-R) color index 67- 72 F6.2 mag R-I (R-I) color index 75- 80 F6.2 mag V-K (V-K) color index 83- 88 F6.2 mag J-K (J-K) color index 92- 97 F6.2 mag H-K (H-K) color index 100-105 F6.2 mag K-L (K-L) color index 108-113 F6.2 mag BV_V Bolometric correction in the V band 118-123 F6.2 mag BV_K Bolometric correction in the K band 126-131 F6.2 mag U-B1 (U-B_1) color index (Geneva) 134-139 F6.2 mag B2-V1 (B2-V1) color index (Geneva) 145-149 F5.2 --- BTiO Titanium oxyde band strength index (2) 153-157 F5.2 --- BZrO Zirconium oxyde band strength index (2) 161-165 F5.2 --- BNa Sodium lines band strength index (2)
Note (1): Log scale, H=12, normalised to the Sun Note (2): [s/Fe] is a global s-process enrichment weighted for each element by its fractionnal s-process abundance (Kappeler et al., 1989RPPh...52..945K, Arlandini et al., 1999ApJ...525..886A, Grevesse & Sauval, 1998SSRv...85..161G).
Acknowledgements: Sophie Van Eck, svaneck(at) References: Gustafsson et al., Paper I 2008A&A...486..951G
(End) Sophie Van Eck [ULB, Belgium], Patricia Vannier [CDS] 13-Feb-2017
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