Tc-rich M stars: platypuses of low-mass star evolution

TL;DR

This paper investigates Tc-rich M stars, particularly S Her, revealing their unique chemical compositions and evolutionary status, which challenge existing models of stellar evolution and dredge-up processes in low-mass stars.

Contribution

It provides the first detailed chemical analysis of a Tc-rich M star, offering new insights into early AGB evolution and s-process nucleosynthesis in low-mass stars.

Findings

Confirmed detection of technetium lines in S Her

Analyzed its carbon and s-process element abundances

Constrained the evolutionary status of Tc-rich M stars

Abstract

The technetium-rich (Tc-rich) M stars reported in the literature (Little-Marenin & Little (1979); Uttenthaler et al. (2013)) are puzzling objects since no isotope of technetium has a half-life longer than a few million years, and 99Tc, the longest-lived isotope along the s-process path, is expected to be detected only in thermally-pulsing stars enriched with other s-process elements (like zirconium). Carbon should also be enriched, since it is dredged up at the same time, after each thermal pulse on the asymptotic giant branch (AGB). However, these Tc-enriched objects are classified as M stars, meaning that they neither have any significant zirconium enhancement (otherwise they would be tagged as S-type stars) nor any large carbon overabundance (in which case they would be carbon stars). Here we present the first detailed chemical analysis of a Tc-rich M-type star, namely S Her. We first confirm the detection of the Tc lines, and then analyze its carbon and s-process abundances, and draw conclusions on its evolutionary status. Understanding these Tc-rich M stars is an important step to constrain the threshold luminosity for the first occurrence of the third dredge-up and the composition of s-process ejecta during the very first thermal pulses on the AGB.