Shreeya Shetye
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, in my poster I will present how 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. Furthermore, I will present interesting insights into AGB nucleosynthesis that we obtained through the analysis of S-type stars.
Of particular interest in this thesis are the Asymptotic Giant Branch (AGB) stars.
AGB stars are low- to intermediate-mass stars in the late stages of stellar evolution.
Due to their huge mass-loss and sheer number, these stars are major contributors of heavy (s-process) elements in the interstellar medium. AGB stars are ideal test beds for understanding the mixing processes that take place in the stellar interiors.
Despite its importance, the AGB is one of the least understood phases of stellar evolution. Constraints on the AGB models from systematic observational studies is therefore mandatory. Such investigations will shed light not only on the future of our Sun, but also on the heavy element production and thus on the chemical evolution of our Galaxy.
Among evolved stars, S-type stars are especially focused in this poster, because they come in two flavours: the intrinsic ones (genuine AGB stars) and the extrinsic ones (binaries which have been polluted in the past by an AGB, now an extinct white dwarf). We recently found a third type of S stars called 'bitrinsic S stars' using the methodology presented in this poster.
The atmospheric parameter determination of these stars is complex, because their spectra are dominated by molecules. To contribute to bridge the gap between observations and models of AGB stars we initiated a detailed investigation of S stars. This study started with devising a methodology to constrain the atmospheric parameters of S stars. Our iterative method makes use of the recently released Gaia parallaxes and the high-resolution HERMES spectra. The derivation of the atmospheric parameters made it possible to expand our study to the abundance analysis of S stars, to get insights into AGB nucleosynthesis.
We located the intrinsic and extrinsic S stars in the Hertzsprung-Russell (HR) diagram to constrain their evolutionary status. An important result of our HR diagram of S stars was the discovery of low-mass AGB (initial mass ~ 1 Msun) stars. Such an evidence for third dredge-up occurrence at low-mass and solar metallicity was not accounted for by most AGB models.
The abundances of S stars reveal their rich nucleosynthetic history. The zirconium-niobium pair helps distinguishing extrinsic from intrinsic stars and is as such, a diagnostic as reliable as technetium detection. We also discovered a special class of 'bitrinsic' S stars, which are new TP-AGB stars that also show signatures of previous binary interaction with a former AGB companion.
The evolutionary and chemical study of the S stars shed light on several important aspects of AGB stars like the third dredge-up and s-process nucleosynthesis.
We tackled several challenges while determining the parameters and abundances of S stars. Our methodology and atomic list of 'gold' lines can be used in further investigations. We build-up a systematic set of observational constraints on the onset of the third dredge-up and the associated nucleosynthesis. These observational constraints are essential clues to better understand AGB nucleosynthesis