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

The Tc-rich M stars reported in the literature (Little-Marenin & Little 1979, Uttenthaler et al. 2013) are puzzling objects since 99Tc, a relatively short-lived isotope on the s-process path, is expected to be detected only in stars also enriched with other s-process elements (like zirconium) or carbon which is dredged-up at the same time, after the thermal pulses on the asymptotic giant branch. However these Tc-enriched objects are classified as M stars, meaning that they do not have any significant zirconium enhancement (otherwise they would be tagged as S-type stars) or any large carbon overabundance (in which case they would be carbon stars). Here we present the first detailed chemical analysis of a sample of Tc-rich M-type stars. We first confirm the detection of the Tc lines, and then analyse the carbon and s-process abundances of these objects, to draw conclusions on their evolutionary status, and on the luminosity of the third dredge-up.