The puzzling case of the Nysa-Polana family

The Nysa-Polana dynamical family is one of the most complex groupings ident ified in the asteroid main belt. It has a generally twofold structure and i s located in a region of the belt showing an unusual abundance of the fairl y rare F-type taxonomic class. We present here new spectra of 22 family mem bers, observed at ESO (European Southern Observatory) and at CASLEO (Comple jo Astronomico El Leoncito, Argentina). We find evidence that the family is actually formed by two distinct groupings mutually overlapping in the spac e of orbital proper elements. The first group of objects, consisting of dar k asteroids including several F-type members, can be named after its least- numbered member, 142 Polana. The second group consists of S-type asteroids and should be named after its most plausible least-numbered member, 878 Mil dred. Some detailed physical analysis of the two groupings can be already a ttempted at this stage, including a reasonable reconstruction of the origin al ejection velocity field for the Mildred family. However, we face here a number of intriguing problems that deserve further analysis. For instance, it is not clear how to interpret the role of the two largest nominal member s of the clan, 44 Nysa and 135 Hertha, as well as the presence of several l arge F-type asteroids apparently not related, but very close to the clan. M oreover, recent discoveries of common spectral features indicating hydrated minerals in the spectra of Nysa and Hertha add complexity to an already pu zzling scenario. (C) 2001 Academic Press.