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

Published online by Cambridge University Press:  30 November 2022

Shreeya Shetye Open the ORCID record for Shreeya Shetye [Opens in a new window] ,
Sophie Van Eck ,
Alain Jorissen ,
Lionel Siess  and
Stephane Goriely
Shreeya Shetye
Affiliation:
Institute of Physics, Laboratory of Astrophysics, École Polytechnique Fédérale de Lausanne (EPFL), Observatoire de Sauverny, 1290 Versoix, Switzerland email: [email protected] Institute of Astronomy and Astrophysics (IAA), Université Libre de Bruxelles (ULB), CP 226, Boulevard du Triomphe, B-1050 Bruxelles, Belgium
Sophie Van Eck
Affiliation:
Institute of Astronomy and Astrophysics (IAA), Université Libre de Bruxelles (ULB), CP 226, Boulevard du Triomphe, B-1050 Bruxelles, Belgium
Alain Jorissen
Affiliation:
Institute of Astronomy and Astrophysics (IAA), Université Libre de Bruxelles (ULB), CP 226, Boulevard du Triomphe, B-1050 Bruxelles, Belgium
Lionel Siess
Affiliation:
Institute of Astronomy and Astrophysics (IAA), Université Libre de Bruxelles (ULB), CP 226, Boulevard du Triomphe, B-1050 Bruxelles, Belgium
Stephane Goriely
Affiliation:
Institute of Astronomy and Astrophysics (IAA), Université Libre de Bruxelles (ULB), CP 226, Boulevard du Triomphe, B-1050 Bruxelles, Belgium
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Abstract

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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 9999Tc, 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.

Keywords

Stars: abundancesStars: AGB and post-AGBHertzsprung-Russell and C-M diagramsNuclear reactions nucleosynthesis abundancesStars: interiors
Type
Contributed Paper
Information
Proceedings of the International Astronomical Union , Volume 16 , Symposium S366: The Origin of Outflows in Evolved Stars , November 2020 , pp. 259 - 264
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Copyright
© The Author(s), 2022. Published by Cambridge University Press on behalf of International Astronomical Union

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