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

TL;DR

This study uses HIPPARCOS parallaxes to analyze the positions of S stars in the HR diagram, revealing their evolutionary status, nucleosynthesis processes, and binary nature, and compares galactic and extragalactic S star populations.

Contribution

It provides the first precise HR diagram placement of S stars using HIPPARCOS data, clarifying their evolutionary stages and nucleosynthesis, and compares galactic and extragalactic S star populations.

Findings

Tc-rich S stars are cooler and brighter than Tc-poor S stars.

The HR diagram positions match the onset of thermal pulses on the AGB.

Most Tc-poor S stars are low-mass, binary systems on the red giant branch or early AGB.

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 1.6 +- 0.2 Msun derived from their orbital mass-function distribution. 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.