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Astron. Astrophys. 317, 90-98 (1997)

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1. Introduction

The improved efficiency of the Hubble Space Telescope is opening to the investigation the still unexplored field of very faint stars in galactic globular clusters at last. According to long debated expectations (see, e.g., Castellani 1979), now deep CM diagrams are disclosing the evidence for the sequence of cooling white dwarfs, extending in the same time our knowledge of main sequence stars down to about their natural limit, i.e., towards the lower mass limit for the ignition of nuclear H burning in the structures.

In a series of previous papers we presented theoretical computations covering the evolution of globular cluster stars from the phases of central H burning up to the final cooling as White Dwarfs (see, e.g., Castellani et al. 1989, Castellani et al. 1994 and references therein). According to new observational evidence, in this paper evolutionary computations devoted to extend previous evaluations to less massive stars will be presented, in order to implement - as far as possible- the theoretical scenario with suitable evaluations for stars well below the Turn-Off, along the cluster main sequence.

The physical inputs of present computations will be discussed in the next section. Section 3 will deal with the theoretical evolutionary behavior of very low mass (VLM) models, discussing some features which have received scarce attention in the previous literature and which could be of interest for people concerned with the theory of stellar structures. In the next section (Sect. 4) the evolutionary results regarding the lower main sequence of metal poor structures, will be presented comparing theoretical data with HST observations of the galactic globular NGC 6397. The evolutionary scenario for the sequence of dwarfs with known parallaxes, will be finally revisited, shortly discussing metal rich VLM structures. tables of selected evolutionary data will be provided both in the theoretical and in the observational ( [FORMULA], V-I) plane, to be used for comparison with previous work on the argument and with observational data.

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