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Astron. Astrophys. 329, 971-985 (1998)

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The HIPPARCOS Hertzsprung-Russell diagram of S stars: probing nucleosynthesis and dredge-up *

S. Van Eck 1, A. Jorissen 1, S. Udry 2, M. Mayor 2 and B. Pernier 2

1 Institut d'Astronomie et d'Astrophysique, Université Libre de Bruxelles, C.P. 226, Boulevard du Triomphe, B-1050 Bruxelles, Belgium
2 Observatoire de Genève, 51 Chemin des Maillettes, CH-1290 Sauverny, Switzerland

Received 23 May 1997 / Accepted 29 July 1997

Abstract

HIPPARCOS trigonometrical parallaxes make it possible to compare the location of Tc-rich and Tc-poor S stars in the Hertzsprung-Russell (HR) diagram: Tc-rich S stars are found to be cooler and intrinsically brighter than Tc-poor S stars.

The comparison with the Geneva evolutionary tracks reveals that the line marking the onset of thermal pulses on the asymptotic giant branch (AGB) matches well the observed limit between Tc-poor and Tc-rich S stars. Tc-rich S stars are, as expected, identified with thermally-pulsing AGB stars of low and intermediate masses, whereas Tc-poor S stars comprise mostly low-mass stars (with the exception of 57 Peg) located either on the red giant branch or on the early AGB. Like barium stars, Tc-poor S stars are known to belong exclusively to binary systems, and their location in the HR diagram is consistent with the average mass of [FORMULA] M [FORMULA] derived from their orbital mass-function distribution (Jorissen et al. 1997, A&A, submitted).

A comparison with the S stars identified in the Magellanic Clouds and in the Fornax dwarf elliptical galaxy reveals that they have luminosities similar to the galactic Tc-rich S stars. However, most of the surveys of S stars in the external systems did not reach the lower luminosities at which galactic Tc-poor S stars are found. The deep Westerlund survey of carbon stars in the SMC uncovered a family of faint carbon stars that may be the analogues of the low-luminosity, galactic Tc-poor S stars.

Key words: stars: AGB – H-R diagram – stars: evolution – stars: fundamental parameters – stars: carbon

* Based on data from the HIPPARCOS astrometry satellite

Send offprint requests to: S. Van Eck
(e-mail: svaneck@astro.ulb.ac.be)

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© European Southern Observatory (ESO) 1998

Online publication: December 16, 1997
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