RSAA Colloquia / Seminars / Feast-of-Facts: Tuesday, 31 March 2015, 11:00-12:00; Duffield Lecture Theatre

Devika Kamath

"A new perspective on single/binary star evolution and nucleosynthesis based on post-AGB and post-RGB stars in the Magellanic Clouds"

Post-Asymptotic Giant Branch (post-AGB) stars bear signatures of the entire chemical and morphological changes that occur prior to and during the AGB phase of evolution. In our recent studies, we carried out an extensive low-resolution spectroscopic survey of post-AGB candidates in the Magellanic Clouds. In this talk, I will present two important outcomes of this survey. Firstly, this survey resulted in a well characterised sample of binary and single post-AGB stars. Detailed chemical abundance studies of some of these objects have shown that they are chemically much more diverse than anticipated. As expected, some of the single post-AGB objects are the most s-process enriched objects known to date while others, the likely binary objects, are not enriched at all. However, some objects show peculiar chemical signatures which do not correspond to the accepted single/binary star evolution and nucleosynthesis. In this talk, I will present this chemical diversity and its implications for widely accepted stellar evolution and nucleosynthesis theories. Secondly, one of the most important results of this survey is the serendipitous discovery of a group of ’post- Red Giant Branch (post-RGB)’ stars in the LMC and SMC. These are low-luminosity, evolved, dusty objects and they have mid-IR excesses and stellar parameters similar to those of post-AGB stars (late-G to late-A spectral types, low log g values, and low metallicities with [Fe/H]~1.0). However, their luminosities are much lower (200 - 2200Lsun) than that expected for post-AGB stars. It is likely that these objects are the result of a mass-loss that occurs via binary interaction on the RGB. In this talk, I will present our analysis on this unexplored phase in binary evolution via evolutionary connections between the possible precursors (Sequence-E variables) and successors (systems with white dwarf [WD] components like sub-dwarf B stars or He-WDs).