The location in the () plane of a sample of over one hundred nearby stars, covering a metallicity range of , with Hipparcos parallaxes of relative accuracy better than 5 per cent, has been obtained. Most stars have bolometric magnitudes directly measured, and effective temperatures derived from the Infra-Red Flux Method, by Alonso et al. (1996a). The metallicities (LTE) have been derived from the 1996 edition of the catalogue of determinations by Cayrel de Strobel et al. (1997).
The HR diagram shows an unexpected clumping of stars of low metallicity, only slightly separated from the solar metallicity sequence, as found much earlier, from ground-based parallaxes by Perrin et al. (1977). However the explanation of this fact by a variation of the helium content with metallicity is no more tenable, with the narrow range of variation of Y between and . A comparison of the data with the standard theoretical isochrones, computed with improved opacities, show that unevolved stars with solar metallicities agree more or less with the theoretical expectations, whereas stars with metallicities below -0.5 dex deviate from the isochrones having the same metallicity. The case of µ Cas, a binary of metallicity -0.7, is particularly illustrative.
Two effects, recently studied in the literature have been investigated to explain this observation. The first one concerns only the atmosphere of the star. Until now, non-LTE abundances for iron were not currently available in the literature. Thévenin & Idiart (1999) have studied more than one hundred stars, including µ Cas and many other metal-poor stars, and found a non-LTE correction of +0.15 dex for in µ Cas. Morel & Baglin (1999) have studied the microscopic diffusion of helium and heavier elements in metal-poor stars of various mass. They found that the atmosphere is depleted in iron after 10 Gyr, and that the effective temperature of the star is shifted by about -30 to -200 K for its luminosity. The combination of the two effects brings µ Cas back on its theoretical sequence. The bulk of the combined correction is due to the fact that both the non-LTE correction, and the diffusion of heavy elements, make the atmospheric LTE iron abundance not representative of the mean inside abundance of iron.
Among stars with metallicities closer to the solar metallicity there is a global agreement between the observations and the isochrones with a scaled helium abundance, although some scatter may be present. New computations of sedimentation at these metallicities would be necessary to further resolve this issue.
Also an enlarged sample of stars, with distances known with an accuracy of 1 or 2 per cent, appears necessary for a good determination of the helium content of individual stars. The GAIA mission is very promising in this respect (Perryman et al. 1997). With GAIA the mean precision on parallax measurements will be at the 10 micro-arc-second (mas) level up to V=15, to be compared with 1 mas with Hipparcos up to V=9. In parallel, progresses in the analyses of stellar spectra are relevant, as exemplified by the introduction of departures from LTE of the iron atom.
More stellar masses are also deeply needed, not a single mass being known for stars more metal-poor than µ Cas.
© European Southern Observatory (ESO) 1999
Online publication: October 4, 1999