Determining the Luminosity of Third dredge -up via S stars: The promise of Gaia

Slides presented at the pitch session Abstract: S stars are late-type giants whose spectra show distinctive molecular bands with as most noticeable characteristic the appearance of ZrO bands. They can be segregated in two classes: namely intrinsic and extrinsic. The intrinsic S stars on the Asymptotic Giant Branch (AGB), producing the s-process elements internally which are then transported to the stellar surface via third dredge-up (TDU). The intrinsic S stars are the first ones on the AGB to have undergone a third dredge-up. Understanding the AGB nucleosynthesis and the TDU episodes in function of evolutionary mass and metallicity involves several challenges. One such challenge is that on the AGB, the standard stellar evolution models produced TDUs for masses greater than 1.5Msun. However, there is a growing series of observations pointing at dredge-up occurring in lower mass stars (< 1.5 Msun). Here, for the first time thanks to a study of all intrinsic S stars in Gaia, we are able to test systematically the lower mass limit of TDU. The parameter study of intrinsic S stars along with their luminosity derived from the Gaia DR2 parallaxes has helped us marking the onset of the third dredge-up in the Hertzsprung-Russell (HR) diagram. The accurate parallaxes from Gaia DR2 combined with the well-constrained Teff also helps further constraining the masses and surface gravities of these stars. The positions of the intrinsic S stars in the HR diagram show that the TDU occurs in stars with masses as low as 1 Msun and [Fe/H] in the range of -0.2 to -0.5. Comparison with new, dedicated stellar evolution and nucleosynthesis models show that with some adjustments they are able to reproduce the s-process enhancements of these objects. Moreover, this study is a part of a larger investigation on S stars with Gaia DR2 parallaxes