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

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 re- veals that the line marking the onset of thermal pulses on the asymptoticgiantbranch(AGB)matcheswelltheobservedlimit between Tc-poor and Tc-rich S stars. Tc-rich S stars are, as ex- pected, identied 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 sys- tems,andtheirlocationintheHRdiagramisconsistentwiththe average mass of 1:6 0: 2M derived from their orbital mass- function distribution (Jorissen et al. 1997, A&A, submitted). A comparison with the S stars identied 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.