Abstract

The HIPPARCOS mission will permit a decisive step forward in the comparison between observed and predicted global properties of stars, in producing distances and apparent magitudes with accuracies more than one order of magnitude higher than before. Nearby stars of intermediate and low mass will allow for statistical tests on the validity of the equation of state, like for instance the steepness of the main sequence.

La mission HIPPARCOS va permettre un pas en avant fondamental dans les tests des propriéés thermodynamiques des étoiles de masse intermédiaire en fournissant des distances et des magnitudes apparentes beaucoup plus précises que celles obtenues au sol.

Introduction.

Tests of the physical description of stellar interiors rely on a theory vs observation comparison. The stellar evolution theory predicts the variation with time of the state of the interior of a star and, also, of its fundamental, observable parameters, i.e. luminosity, surface temperature, for a given mass. The HIPPARCOS (High Precision PARallax COllecting Satellite) mission will permit a decisive step forward in this confrontation by producing distances and apparent magnitudes with accuracies more than one order of magnitude higher than before (Baglin, 1988).

For a description of the mission see for instance Perryman et al., 1992, and the ‘Hipparcos Input Catalogue’ (Turon et al., 1992).

Distances measurements.

HIPPARCOS measures parallaxes i.e. distances, and proper motions. Aproximately 120 000 stars brighter than mv ≈ 12.5 are observed; the survey is complete up to the apparent magnitude 7.5.