THE HIPPARCOS HERTZSPRUNG-RUSSELL DIAGRAM OF S-STARS - PROBING NUCLEOSYNTHESIS AND DREDGE-UP

HIPPARCOS trigonometrical parallaxes make it possible to compare the l ocation 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 bri ghter than Tc-poor S stars. The comparison with the Geneva evolutionar y tracks reveals that the line marking the onset of thermal pulses on the asymptotic giant branch (AGE) matches well the observed limit betw een Tc-poor and Tc-rich S stars. Tc-rich S stars are, as expected, ide ntified with thermally-pulsing AGE stars of low and intermediate masse s, whereas Tc-poor S stars comprise mostly low-mass stars (with the ex ception of 57 Peg) located either on the red giant branch or on the ea rly AGE. Like barium stars, Tc-poor S stars are known to belong exclus ively to binary systems, and their location in the HR diagram is consi stent with the average mass of 1.6 +/- 0.2 M-circle dot derived from t heir orbital mass-function distribution (Jorissen et al. 1997, A&A, su bmitted). A comparison with the S stars identified in the Magellanic C louds and in the Fornax dwarf elliptical galaxy reveals that they have luminosities similar to the galactic Tc-rich S stars. However, most o f the surveys of S stars in the external systems did not reach the low er luminosities at which galactic Tc-poor S stars are found. The deep Westerlund survey of carbon stars in the SMC uncovered a family of fai nt carbon stars that may be the analogues of the low-luminosity, galac tic Tc-poor S stars.