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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
Received 23 May 1997 / Accepted 29 July 1997
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 M 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
Send offprint requests to: S. Van Eck
© European Southern Observatory (ESO) 1998
Online publication: December 16, 1997