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 intrinsic S stars are on the AGB and have undergone third dredge-up events. 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. These atmospheric parameters are also entangled within each other. Nevertheless, high-resolution spectroscopic data of S stars combined with the Gaia Data Release 2 (GDR2) parallaxes and with the MARCS model atmospheres for S-type stars were used to derive effective temperatures, surface gravities, and luminosities. These parameters not only allow to locate the intrinsic and extrinsic S stars in the Hertzsprung-Russell (HR) diagram but also allow the accurate abundance analysis of the s-process elements.

The Yale–Potsdam Stellar Isochrones (YaPSI) cover the low and intermediate stellar mass regime (0.15 to 5.0 M⊙) for a wide range of solar-scaled chemical compositions (metallicity from −0.5 to +0.3; helium mass fraction from 0.25 to 0.37, assigned independently of each other). The tracks are finely spaced in mass, to allow for accurate interpolation. The models feature state-of-the-art input physics relevant to low-mass stars modeling (surface boundary conditions, equation of state), thus updating the faint end of the Yonsei-Yale (YY) isochrones. Utility codes, such as an isochrone interpolator in age, metallicity and helium content, are also provided. The YaPSI isochrones are in good agreement with the empirical mass–luminosity and mass–radius relations available to date, and provide satisfactory fitting of the color-magnitude diagrams of well-studied open clusters.

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.