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arXiv · 2112.04318

Luminosities and masses of single Galactic Post-Asymptotic Giant Branch (Post-AGB) stars with distances from $Gaia$ EDR3: The revelation of an $s$-process diversity

Devika Kamath·Hans Van Winckel·Paolo Ventura·Maksym Mohorian·Bruce Hrivnak·Flavia Dell'Agli·Amanda Karakas
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Abstract

Post-AGB stars are exquisite probes of AGB nucleosynthesis. However, the previous lack of accurate distances jeopardised comparison with theoretical AGB models. The $Gaia$ Early Data Release 3 ($Gaia$ EDR3) has now allowed for a breakthrough in this research landscape. In this study, we focus on a sample of single Galactic post-AGBs for which chemical abundance studies were completed. We combined photometry with geometric distances to carry out a spectral energy distribution (SED) analysis and derive accurate luminosities. We subsequently determined their positions on the HR-diagram and compared this with theoretical post-AGB evolutionary tracks. While most objects are in the post-AGB phase of evolution, we found a subset of low-luminosity objects that are likely to be in the post-horizontal branch phase of evolution, similar to AGB-manqu\'e objects found in globular clusters. Additionally, we also investigated the observed bi-modality in the $s$-process enrichment of Galactic post-AGB single stars of similar Teff, and metallicities. This bi-modality was expected to be a direct consequence of luminosity with the $s$-process rich objects having evolved further on the AGB. However, we find that the two populations: the $s$-process enriched and non-enriched, have similar luminosities (and hence initial masses), revealing an intriguing chemical diversity. For a given initial mass and metallicity, AGB nucleosynthesis appears inhomogeneous and sensitive to other factors which could be mass-loss, along with convective and non-convective mixing mechanisms. Modelling individual objects in detail will be needed to investigate which parameters and processes dominate the photospheric chemical enrichment in these stars.

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Devika Kamath, Hans Van Winckel, Paolo Ventura, Maksym Mohorian, Bruce Hrivnak, Flavia Dell'Agli, Amanda Karakas. 2021-12-08. Luminosities and masses of single Galactic Post-Asymptotic Giant Branch (Post-AGB) stars with distances from $Gaia$ EDR3: The revelation of an $s$-process diversity. https://doi.org/10.3847/2041-8213%2Fac5686

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