Forum Springer Astron. Astrophys.
Forum Whats New Search Orders

Astron. Astrophys. 322, 147-154 (1997)

Previous Section Next Section Title Page Table of Contents

1. Introduction

The kinematics of Main Sequence A type stars in the solar neighborhood shows certain particular features which cannot be explained by the classical theory of ellipsoidal velocity distribution. To analyze the mechanisms responsible for features such as the presence of moving groups, the strong vertex deviation, and the observed increase in velocity dispersion with age, large stellar samples are required with both accurate velocities and positions and precise physical data, especially ages and masses.

The imminent publication of the most accurate astrometric data ever measured -the Hipparcos mission observations- will constitute a clear advance in the first requirement; the new proper motions and trigonometric parallaxes together with the new radial velocities obtained from the ground will provide us with very accurate velocities and positions of stars in the solar neighborhood.

In most studies on the kinematic evolution of neighboring main sequence stars individual ages are not considered. Instead, mean ages assigned from spectral type are used (e.g. Jahreiss & Wielen, 1983; Gómez et al, 1990). Although this is a reasonable first approximation for very early type stars, the increase in age dispersion as more advanced spectral types are considered can strongly affect the conclusions derived. Here we study the individual assignation of ages and masses to Main Sequence and moderately evolved B, A and F type stars (Sect.  2) from their atmospheric parameters and a given set of stellar evolutionary models. Special care is taken with stars in the Overlap Region, where the correspondence between the pair (age, mass) and the atmospheric parameters is not univocal.

When dealing with large samples of stars both [FORMULA] and Geneva photometric systems have proved to be efficient tools in the atmospheric parameter determination. Photometric indices, once dereddened, are taken as input parameters in [FORMULA] and [FORMULA] calibrations (e.g. Moon and Dworetsky, 1985, Napiwotzki et al., 1993, Kobi and North, 1990); a detailed description of the full procedure can be found in Figueras et al. (1991) and Jordi et al. (1994, 1996). On the other hand, Z can also be provided by [FORMULA] and Geneva photometry, in which the [FORMULA] m0 and [FORMULA] m2 indices, respectively, are very well correlated with [Fe/H] for spectral types A3 to G2 (Smalley, 1993; Berthet, 1990; North & Nicolet, 1990).

In Sect.  3a detailed computation of the age and mass error assignation is developed, and the variation of these quantities as a function of the position of the star in the HR diagram is presented. Finally, in Sect.  4fundamental values of radii and masses of detached, double-lined eclipsing binary systems allow us to select the most realistic stellar models from recent publications.

Previous Section Next Section Title Page Table of Contents

© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998