Spectrophotometry of asteroids 32 Pomona, 145 Adeona, 704 Interamnia, 779 Nina, 330825, and 2012 QG42 and laboratory study of possible analog samples

Six asteroids including two NEAs, one of which is PHA, accessible for observation in September 2012 were investigated using a low-resolution spectrophotometry in the range 0.35-0.90 um with the aim to study features of their reflectance spectra. For the first time we discovered likely spectral signs (as a maximum at 0.4-0.6 um in reflectance spectra) of simultaneous sublimation activity and presence of a temporal coma on three primitive-type main-belt asteroids, Adeona, Interamnia, and Nina, being at perihelion distances or approaching to it. We suggest that such a cometary-like activity may be a common phenomenon at the highest subsolar surface temperatures for C and close type asteroids including considerable amounts of H2O and CO2 ices beneath the surface. However, excavation of fresh ice at recent impact event(s) could be an alternative explanation of the phenomenon. Similar absorption bands centered at 0.38, 0.44 and 0.67-0.71 um registered in the reflectance spectra of Adeona, Interamnia, and Nina clearly point to predominantly silicate surface matter. To specify its content, we performed laboratory investigations of ground samples of known carbonaceous chondrites (Orguel, Mighei, Murchison, and Boriskino) and seven samples of low-iron Mg serpentines as possible analogs of the asteroids. In particular, we found that the equivalent width of the band centered at 0.44 um in reflectance spectra of the low-Fe serpentine samples has a high correlation with content of Fe3+ (octahedral and tetrahedral). It means that the absorption feature can be used as an indicator of ferric iron in oxidized and hydrated low-Fe silicate compounds on asteroids (abridged).