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1. Introduction

A comprehensive spectroscopic stellar library at high resolution provides a valuable tool for facing fundamental problems in different branches of astronomy. For instance, it can be used for determining the atmospheric parameters of stars by comparison with observed spectra (Cayrel et al. 1991a, b). On the other hand, synthetic spectra have proved to be an important (and sometimes the only) brick for the investigation of the behavior of prominent spectral features in terms of the stellar atmospheric parameters (e.g., Jones et al. 1996). Once this behavior has been properly modelled in stars, we can, in principle, predict the spectroscopic properties of stellar aggregates (e.g., Proc. of the conference "From Stars To Galaxies: The Impact Of Stellar Physics On Galaxy Evolution'', Leitherer et al. 1996).

Out of the several aspects connected with the understanding of the evolutionary histories of stellar aggregates, we are particularly involved in the analysis and synthesis of spectral features typical of old stellar populations.

Along this line, we have computed a set of synthetic spectra in the wavelength interval 4850-5400 Å. This spectral region has been chosen because it contains conspicuous features, such as the Mgb triplet, the MgH molecular bands and FeI absorptions, which are easily observable in the integrated spectra of distant stellar systems. Moreover, the sensitivity of these lines to variations in surface gravity, effective temperature and metal content allows the determination of these parameters for individual stars in a self-consistent way.

We have not extended this region to include the blue wing of Htex2html_wrap_inline1017 because of the poor fit between the observed and theoretical solar fluxes (Gulati 1989): the theoretical continuum appears too high with respect to the observed one.

Several reasons have been suggested to explain the discrepancy, among them problems in the convection treatment in the models or some underestimation of the continuum opacity (see, for instance, Castelli et al. 1997).

The spectra, computed with the upgraded versions of Kurucz's models and codes (Kurucz 1993a-c), and based on revised atomic and molecular data, cover a parameter space suitable for the study of F, G and K stars, which are the dominant components in old stellar systems.

So far, these spectra have been already used for synthesizing a set of spectral features for population studies of clusters and galaxies observed at low and moderate resolution (Chavez et al. 1995, 1996). Since this application does not exhaust the informational content of the synthetic spectra, we are making available to the community the original high resolution spectra.

This paper describes the main characteristics and the format of the Atlas which can be obtained from the authors.

In Sect. 2 we present a short description of the line data used in the computations. Section 3 is devoted to present the characteristics of the spectra included in the Atlas. In Sect. 4 a subsample of the Atlas is presented in pictorial form suitable to highlight low resolution features.

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