Astronomy and Astrophysics in the Gaia sky Proceedings IAU Symposium No. 330, 2017 A. Recio-Blanco, P. de Laverny, A.G.A. Brown & T. Prusti, eds. c  International Astronomical Union 2018 doi:10.1017/S1743921317005610 The TGAS HR diagram of S-type stars Shreeya Shetye 1 , 2 †, Sophie Van Eck 1 , Alain Jorissen 1 , Hans Van Winckel 2 and Lionel Siess 1 1 Institut d’Astronomie et d’Astrophysique, Universit´ e Libre de Bruxelles, CP 226, Boulevard du Triomphe, B-1050 Bruxelles,Belgium email: Shreeya.Shetye@ulb.ac.be 2 Instituut voor Sterrenkunde (IvS), KU Leuven, Celestijnenlaan 200D, B-3001 Leuven, Belgium Abstract. S-type stars are late-type giants enhanced with s-process elements originating either from nucleosynthesis during the Asymptotic Giant Branch (AGB) or from a pollution by a binary companion. The former are called intrinsic S stars, and the latter extrinsic S stars. The atmospheric parameters of S stars are more numerous than those of M-type giants (C/O ratio and s-process abundances affect the thermal structure and spectral synthesis), and hence they are more difficult to derive. Nevertheless, high-resolution spectroscopic data of S stars combined with the TGAS (Tycho-Gaia Astrometric solution) parallaxes were used to derive effective temperatures, surface gravities, and luminosities. These parameters allow to locate the intrinsic and extrinsic S stars in the Hertzsprung-Russell diagram. Keywords. S stars, AGB stars, TGAS, HR diagram 1. Introduction S stars are late-type giants showing ZrO molecular bands along with TiO bands as the most characteristic distinctive spectral features (Merrill 1922). The C/O ratio of S stars ranges from 0.5 to 1 suggesting that they are transition objects between M-type giants (C/O ∼ 0.5) and carbon stars (C/O > 1) on the Asymptotic Giant Branch (AGB) (Iben and Renzini 1983). Their spectra show signatures of overabundances in s-process elements (Smith and Lambert 1990). The evolutionary status of S stars as AGB stars was challenged when Tc lines (an s-process element with no stable long-lived isotope) were reported as missing in some S stars (Merrill 1952; Smith and Lambert 1986; Jorissen et al. 1993). This puzzle regarding the evolutionary status of S stars was solved when it was perceived that the Tc-poor S stars belong to binary systems (Smith and Lambert 1986; Jorissen et al. 1993). S stars may therefore be classified into two different classes: Tc-rich as intrinsic S stars that are genuine thermally-pulsing AGB (TP-AGB) stars and Tc-poor as extrinsic S stars that owe their s-process element overabundances to a mass transfer from a former AGB companion which is now a white dwarf. They are the cooler analogues of barium stars. The thermal structure of the atmospheres of S stars depends on effective temperature (T eff ), surface gravity (log g), [Fe/H], C/O as well as [s/Fe] (s-process element abun- dances). The abundance analysis of S stars requires a reliable determination of all these stellar atmosphere parameters. † This work has made use of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. 345 https://www.cambridge.org/core/terms. https://doi.org/10.1017/S1743921317005610 Downloaded from https://www.cambridge.org/core. IP address: 181.215.217.69, on 22 Apr 2020 at 04:37:02, subject to the Cambridge Core terms of use, available at