Introduction

The library presented in this paper was compiled in order to provide a grid of reference spectra for the determination of reliable atmospheric parameters from high-resolution spectra taken with the echelle spectrograph ELODIE at the Observatoire de Haute-Provence. It was the first step of a chemical and kinematical probe of the Galaxy that we have been conducting for five years with different spectrographs, and for which we have recorded 600 spectra of field stars at various resolutions and signal to noise ratios (S/N). Some 132 of our brighter target stars ($11\leq V \leq 14$) have been observed with ELODIE, and we have also observed in the same conditions, but higher S/N, 211 among the most studied spectroscopic standards, including the Sun, in the temperature interval [4000 K, 6300 K] to build up a library of reference spectra. This temperature interval has been chosen because it covers the full span of stars born from the beginning of the Galaxy. The metallicity of these stars is supposed to reflect the metallicity of the interstellar material from which they were formed, tracing galactic evolution. In hotter stars, the metallicity might be altered by physical processes, and spectra of cooler stars are crowded by molecular bands and might lead to erroneous results. Classical methods estimating [Fe/H], where equivalent widths of weak lines are measured, are not applicable on low S/N spectra. Furthermore the metallicity cannot be extracted independently of the two other atmospheric parameters on which the spectrum depends : effective temperature $T_\mathrm{eff}$, and gravity $\log g$. Therefore, in order to estimate at the same time the three parameters, we have developed a method and a software, TGMET, relying on the least-square comparison of a target spectrum to the library of reference spectra for which the atmospheric parameters are well-known. A version of the software is now available for observers working on ELODIE at the Observatoire de Haute-Provence, and allows on-line estimation of ($T_\mathrm{eff}$, $\log g$, [Fe/H]) for stars in the spectral range F5 to K7, observed at a S/N ratio greater than 10. The method is fully described in a companion paper (Katz et al. 1998, paper I).

In order to use the library as a reference grid, one has to know precisely the atmospheric parameters of all the standards. This is not a simple task, since even for stars which have been studied at high resolution and high S/N by several authors, the results of detailed spectral analyses can differ significantly due to the use of different models of atmospheres and comparison stars. Section 3 is devoted to the problem of finding the best atmospheric parameters for the standards, which must be consistent with the literature and also with spectral analogies in the library. Reliable determinations of ($T_\mathrm{eff}$, $\log g$, [Fe/H]) were compiled from the literature, but a simple average of these values was not satisfactory, and for several stars, disagreements were found between the mean atmospheric parameters and their spectrum. The avalaibility of an homegeneous set of high resolution, high S/N, wide wavelength range spectra led to the obvious idea that it could help finding the most consistent parameters, in the sense that stars having very similar spectra should have very similar atmospheric parameters. As TGMET is a tool which was conceived especially to measure similarities and discrepancies between spectra, it was used iteratively taking in turn each reference star as an unknown object to adjust its parameters with respect to the rest of the library. Finally an homogeneous set of parameters and a self consistent library were obtained, as described in Sect. 3.

This library can be used for several astrophysical purposes like calibrations, differential studies of lines, or spectral classification. As TGMET includes very careful preparation of the spectra ( continuum flattening, spurious radiation events removal, etc...), the spectra of the library are ready to be used. The observational material and preparation of the spectra are briefly described in Sect. 2, as well as the basic data of these stars : coordinates, magnitudes (including the bolometric absolute magnitudes), spectral types, distances, radial velocities, spatial velocities.