5 Summary

In this section we summarize the important points presented in this paper:

• Most of the observed 36 members of Themis family show featureless spectra similar to C-, B-, G- and F-type. The distribution of these spectra is completely contained in the range of the above classes;
• A limited spread of the distribution of superficial composition is apparent, confirming the dynamical indication of a probable common origin from the fragmentation of a parent-body;
• Two objects, 461 Saskia and 1171 Rusthawelia, display a spectra similar to P-type suggesting that they might not be members of the Themis family but just background objects;
• Indication of aqueous alteration is clear in 15 objects and is possibly present in 11 more. This result tends to indicate that the parent-body of Themis family was probably thermally altered. This hypothesis is also supported by the larger percentage of objects presenting aqueous alteration among the big asteroids than among the small ones;
• A slight correlation between albedos and diameters is found: as the asteroid diameter decreases a larger dispersion of albedos is present and higher albedos are displayed by smaller asteroids. The same slight correlation is apparent in our spectra as discussed above.
• Comparison between Themis and Veritas families, which lie nearly at the same heliocentric distance, shows a greater inhomogeneity among the latter one. On the other hand, the presence of aqueous alteration is more prominent in the Themis family. One of the possible explanations is that since the original parent body of Themis was larger than that of Veritas a thermal event leading to metamorphism (probably due to the ²⁶Al decay) was more efficient in the former.

From the analysis of 36 members of Themis family we can conclude that a compositional homogeneity is present as would be expected from the collisional breakup of an original parent body. On the other hand, a slight heterogeneity is also apparent from the presence or not of aqueous alteration and from slightly distinct albedo populations among the family.

Acknowledgements

We acknowledge the technical staff of the ESO for their prompt help whenever needed. We are also grateful to V. Zappalà for valuable comments and remarks which much improved this paper. M.F. was supported by CAPES while D.L., T.M-D., C.A.A., and A.S.B. by CNPq, through diverse fellowships and grants. Support by FAPERJ is also acknowledged.