1 Introduction

The discovery of now more than 100 extra-solar giant planets opened a wide range of questions regarding the understanding of the mechanisms of planetary formation. To find a solution for the many problems risen, we need observational constraints. These can come, for example, from the analysis of orbital parameters of the known planets, like the distribution of planetary masses (Jorissen et al. 2001; Zucker & Mazeh 2001; Udry et al. 2001), eccentricities, or orbital periods (for a review see e.g. Udry et al. 2001; Mayor & Santos 2002). In fact, as new and longer period planets are found, more interesting correlations are popping up. Examples of these are the discovery that there is a paucity of high mass companions orbiting in short period orbits (Zucker & Mazeh 2002; Udry et al. 2002a), or the interesting lack of long-period low-mass planets (Udry et al. 2002b).

But further evidences are coming from the study of the planet host stars themselves. Precise spectroscopic studies have revealed that stars with planets seem to be particularly metal-rich when compared with "single'' field dwarfs (Gonzalez 1997; Fuhrmann et al. 1997; Gonzalez 1998; Santos et al. 2000; Gonzalez et al. 2001; Santos et al. 2001a,b). Furthermore, the frequency of planets seems to be a strong function of [Fe/H] (Santos et al. 2001a, hereafter Paper II). These facts, that were shown not to result from any sampling bias (Paper II), are most probably telling us that the metallicity plays a key role in the formation of a giant planet, or at least of a giant planet like the ones we are finding now.

The source of this metallicity "excess'' has, however, been a matter of debate. Some authors have suggested that the high metal content of the planet host stars may have an external origin: it results from the addition of metal-rich (hydrogen poor) material into the convective envelope of the star, a process that could result from the planetary formation process itself (Gonzalez 1998; Laughlin 2000; Gonzalez et al. 2001; Smith et al. 2001; Murray & Chaboyer 2002). Evidences for the infall of planetary material have in fact been found for a few planet host stars (e.g. Israelian et al. 2001; Laws & Gonzalez 2001; Israelian et al. 2002), although not necessarily able to change considerably the overall metal content (see e.g. Sandquist et al. 2002). In fact, most evidences today suggest that the metallicity "excess'' as a whole has a "primordial'' origin (Pinsonneault et al. 2001; Santos et al. 2001a,b, 2002b; Sadakane et al. 2002), and thus that the metal content of the cloud giving birth to the star and planetary system is indeed a key parameter to form a giant planet.

Besides the simple correlation between the presence of a planet and the high metal-content of its host star, there are some hints that the metallicity might be correlated with the planetary orbital properties. For example, Gonzalez (1998) and Queloz et al. (2000) have shown some evidences that stars with very short-period planets (i.e. small semi-major axes) may be particularly metal-rich, even amongst the planetary hosts. More recent studies have, however, failed to confirm this relation (Paper II).

With the number of new planets growing every day, it is extremely important to survey the samples for new emerging correlations between the stellar properties and the characteristics (minimum masses and orbital parameters) of the planetary companions. In this paper we focus exactly on this point. We have obtained high-resolution and high-S/N spectra of more than 50 extra-solar planet host stars, most of them without any previous detailed spectroscopic analysis. These new determinations bring to about 80 our sample of planet host stars with homogeneous derived spectroscopic parameters. The structure of this article goes as follows. In Sect. 2 we describe the observations and the chemical analysis. In Sect. 3 we review the current status of the metallicity distribution of stars with planets, further discussing its origin, and in Sect. 4 we analyze the relation between the orbital parameters and the metallicity. In Sect. 5 we finally analyze the space velocities of planet and non-planet host stars, comparing them with the stellar metallicity. We conclude in Sect. 6.