1 Introduction

Themis is one of the most statistically reliable family in the asteroid belt. First discovered by Hirayama (1918), along with Eos and Koronis, it has been identified as a family in all subsequent works (see Zappalà et al. 1995, and references therein). According to a modern view (Farinella et al. 1992) the term "family'' should be attributed only to clusters of asteroids whose physical properties support a common origin. Thus, the study of surface composition of Themis members is fundamental to define or, at least, constrain its collisional origin. Furthermore, if there is no doubt, from a statistical and physical point of view, that these are indeed fragments resulting from the breakup of a parent-body, thus important constraints on the latter can be obtained. The information about whether the parent-body was differentiated or primitive will lead, consequently, to constraints on the primordial temperature of the solar nebula which are fundamental for a correct modelling of the Solar System formation.

A very striking feature of Themis family is its closeness to the border of the 2/1 mean motion resonance, as shown by Morbidelli et al. (1995). This resonance corresponds to the Hecuba gap, the widest of all the gaps in the asteroid belt, on which much work has been done in the last years trying to understand its depletion mechanism (see Henrard et al. 1995; Morbidelli 1996; Moons 1997; Ferraz-Mello et al. 1998). According to Morbidelli et al. (1995), when the Themis family is plotted in an appropriate resonant phase space, its rightmost limit is coincident with the border of the 2/1 resonance. As a consequence they suggest that the catastrophic event which gave rise to Themis family might once have populated the resonance zone. Due to some depletion mechanism all the objects were then swept off this region, creating the gap. If this assumption is true, then it implies that fragments, similar to those present today in the Themis family, might have been "transported'' to other regions of the asteroid belt. It has also been suggested (Dermott et al. 1992, and references therein) that collisions in the Themis family could contribute to the formation of the Zodiacal Cloud. A precise surface characterization of asteroids members of Themis family can, therefore, help to investigate the fate of fragments injected in the Hecuba gap.

In the present paper we report spectroscopic observations of 36 asteroids, members of the Themis family, and discuss the implications of these results on the formation and evolution of this family. In the next section we present a short characterization of the Themis family and discuss some hypothesis on the fragmentation which gave origin to it. The observations and data reduction are presented in Sect. 3 along with a discussion of their implications on the origin and evolution of the family. A summary of the main results of the present work is given in the last section.