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 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 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:
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FileName Lrecl Records Explanations
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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
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Byte-by-byte Description of file: table1.dat
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Bytes Format Units Label Explanations
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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)
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Note (1): This paper, Equation (1).
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Byte-by-byte Description of file: table2.dat
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Bytes Format Units Label Explanations
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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)
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Note (1): Description in 1960AJ......65..491H. N=166 among (Nos 1-236),
cross-id. with CSS in 1984PW&SO...3....1S 1984PW&SO...3....1S.
(Nos 1-205), cross-id. with CSS in 2000A&AS..145...51V 2000A&AS..145...51V.
Note (2): This paper, for references and formulae.
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Byte-by-byte Description of file: table3.dat
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Bytes Format Units Label Explanations
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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)
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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 1989RPPh...52..945K,
Arlandini et al., 1999ApJ...525..886A 1999ApJ...525..886A, Grevesse & Sauval, 1998SSRv...85..161G 1998SSRv...85..161G).
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Acknowledgements:
Sophie Van Eck, svaneck(at)astro.ulb.ac.be
References:
Gustafsson et al., Paper I 2008A&A...486..951G 2008A&A...486..951G
(End) Sophie Van Eck [ULB, Belgium], Patricia Vannier [CDS] 13-Feb-2017