Astron. Astrophys. 324, 556-565 (1997)

1. Introduction

Since the founder publication of Iben (1965), many works have been devoted to the study of the pre-main sequence evolutionary phase. From that time, the constitutive physics to be included in stellar models has been significantly improved (see Sect. 2.1.1). As mentioned by several authors (Mazzitelli 1989; Forestini 1994 and hereafter F94; D'Antona & Mazzitelli 1994, hereafter DM94) many uncertainties still remain in this field. Effects engendered by various treatments of surface conditions or different convection models influence the location of the tracks and isochrones in the HRD.

During its PMS phase, a star undergoes a quasi static contraction that will end with the onset of central hydrogen burning. Its structure remains fully convective along the almost vertical Hayashi track and rapidly turns radiative at the center where the opacity decreases with increasing temperature. The important nucleosynthetic events during this early phase sum up in deuterium, lithium and beryllium depletions. Apart during the short deuterium burning phase, the energetics is supplied by the gravitational contraction until the arrival on the Zero-Age-Main Sequence (ZAMS) where the nuclear energy production becomes dominant.

Synthetic HRD consist in generating, from the computed HRD tracks, a theoretical star distribution for a given age, age dispersion and mass distribution. This tool is well suited to study the HRD morphology of observed open clusters, i.e. their main-sequence (MS) extension and width and stellar density in the different regions of the HRD. The synthetic diagrams modify this morphology in function of various factors, namely the initial mass function determining the expected stellar mass distribution, the age dispersion that establishes the duration of the cluster star formation process and the binarity rate among the formed stars.

In Sect. 2 we briefly present the new physical ingredients as well as the computed evolutionary sequences. We take advantage of this section to present our isochrones and compare ZAMS location and main sequence width with observations. Sect. 3 is devoted to the synthetic HRD. We first present the method used to generate such distributions, describe the effect engendered by the relevant parameters and test this method to reproduce the morphology of the Pleiades clusters. We finally conclude in Sect. 4.

Let us finally stress that many recent observations clearly outline that PMS stars are accreting matter from a circumstellar disk that could influence the structural evolution during that phase. We are currently investigating that question (see also Bernasconi & Maeder 1996, Siess et al. 1996).

© European Southern Observatory (ESO) 1997

Online publication: May 26, 1998

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