Mature and Fresh Surfaces on New-Born Asteroid Karin

We report a near-infrared (J, H, and K bands) spectroscopy of the brightest asteroid 832 Karin among the Karin cluster group, which was formed by collisional breakup only 5.8 million years ago. The spectroscopic observation was performed by the Subaru telescope with Cooled Infrared Spectrograph and Camera for OHS (CISCO) on 2003 September 14. To obtain a wide range spectrum, grisms named zJ (0.88-1.40 micron), JH (1.06-1.82 micron), and wK (1.85-2.51 micron) were used. We obtained 3 sets of spectra corresponding to the rotational phase 0.30-0.34, 0.35-0.38, and 0.45-0.50 in comparison with lightcurve observations. Near infrared (0.9-1.4micron) reflectance slope of the 1st set was twice as steep as that of later spectra. The range, where the most significant spectral change was detected, was observed both by zJ and JH bands. Gradual change of the spectral slope is detected through zJ(1st) - JH(1st) - zJ(2nd) - JH(2nd) data . We verified that spectra of a reference star SAO165395 (zJ) were not changed before the 1st set and after the 2nd set of Karin observation, which should remove the possibility that the spectral change was caused by instrumental or atmospheric (and hour angle) effect through the observation of the 1st set and the 2nd set of Karin.

For different rotational phases of Karin, we derived different spectra such as a reddened spectrum like that of S-type asteroid and an un-reddened spectrum like that of ordinary chondrite. Karin would be an impact fragment which not only has new surface but also preserves old surface. Probably it would be one of cone-shaped fragments at low-velocity impact forming Karin cluster group. Our result supports the idea that S-type asteroids are parent bodies of ordinary chondrites.