Technetium and the third dredge up in AGB stars

Free access article

Issue		A&A Volume 411, Number 3, December I 2003


Page(s)		533 - 542
Section		Stellar atmospheres
DOI		http://dx.doi.org/10.1051/0004-6361:20031458

A&A 411, 533-542 (2003)
DOI: 10.1051/0004-6361:20031458

I. Field stars

T. Lebzelter and J. Hron

Institute for Astronomy (IfA), University of Vienna, Türkenschanzstrasse 17, 1180 Vienna, Austria
(Received 28 January 2003 / Accepted 12 September 2003)

Abstract
We searched for Tc in a sample of long period variables selected by stellar luminosity derived from Hipparcos parallaxes. Tc, as an unstable s-process element, is a good indicator for the evolutionary status of stars on the asymptotic giant branch (AGB). In this paper we study the occurrence of Tc as a function of luminosity to provide constraints on the minimum luminosity for the third dredge up as estimated from recent stellar evolution models.

A large number of AGB stars above the estimated theoretical limit for the third dredge up are found not to show Tc. We confirm previous findings that only a small fraction of the semiregular variables show Tc lines in their spectra. Contrary to earlier results by Little et al. (1987) we find also a significant number of Miras without Tc.

The presence and absence of Tc is discussed in relation to the mass distribution of AGB stars. We find that a large fraction of the stars of our sample must have current masses of less than 1.5 $M_{\sun}$ . Combining our findings with stellar evolution scenarios we conclude that the fraction of time a star is observed as a SRV or a Mira is dependent on its mass.

Key words: stars: late-type -- stars: AGB and post-AGB -- stars: evolution

Offprint request: T. Lebzelter, lebzelter@astro.univie.ac.at

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