1 Introduction

The access to precise Hipparcos parallaxes opens the possibility to go further in the knowledge of astrophysical stellar parameters when these parallaxes are coupled with other data.

The luminosity function of early-type stars allows to study the recent history of our Galaxy up to about 1 Gyr. We have selected the A0-type stars as representative of young objects which are long-lived enough on the main sequence to be present, in significant percentage, in the solar neighbourhood.

On one hand the use of field stars has the advantage, compared to cluster stars, to provide a larger sample of bright stars for which more accurate photometric and spectroscopic data can be obtained. On the other hand the use of field stars requires the knowledge of precise individual distances.

The present study is based on a sample of 71 A0 stars. This sample is extracted from a large set of data collected in the framework of an ESO Key Program (Gerbaldi et al. 1989), for studying B8 to F2 stars belonging to the Hipparcos Input Catalogue (Turon et al. 1992). These stars are classified non supergiant A0 in the Bright Star Catalogue (Hoffleit & Jaschek 1982) [hereafter named BSC]; for the few stars not belonging to the BSC, the spectral type is taken from the CDS data base. Stars classified as Ap and Am stars were discarded from the observational program. Among these 71 stars (limited to the Southern Hemisphere) 50 are classified A0 V and represent 21% of the BSC A0 V stars; these stars are expected to be at a distance $d \leq 150$ pc.

The purpose of this study is to select a reference sample of non supergiant A0 stars to be used for the construction of an accurate HR diagram where M_V is derived from the Hipparcos parallaxes.

The spectroscopic observations are used to verify, through the comparison between observed and computed spectra, the accuracy of the photometrically derived atmospheric parameters; several criteria are used to detect possible peculiarities.

The determination of $T_{\mathrm{eff}}$ and logg is discussed in Sect. 3; the computations of the grid of synthetic spectra and the fit on the observed ones are described in Sect. 4; the results obtained from the spectral analysis are presented in Sects. 5 and 6. A subsample of non binary, non peculiar stars is defined (Sect. 7) for which the resulting HR diagram is discussed in Sect. 8.1; in Sect. 8.2 the gravity derived from atmospheric properties (photometric and spectroscopic data) are compared with that obtained combining stellar evolution models and luminosity computed from the Hipparcos parallaxes. In Sect. 8.3 the Mass-Luminosity relationship is analyzed and compared to the empirical one, derived from the eclipsing binaries.