Near Infrared Observations of Comet-Like Asteroid (596) Scheila

Abstract

steroid (596) Scheila was reported to exhibit a cometary appearance and an increase in brightness on UT 2010 December 10.4. We used the IRCS spectrograph on the 8-m Subaru telescope to obtain medium-resolution spectra of Scheila in the HK-band (1.4 - 2.5$\mu$m) and low-resolution spectra in the KL-band (2.0 - 4.0$\mu$m) on UT 2010 December 13 and 14. In addition, we obtained low-resolution spectroscopy using the SpeX spectrograph on the 3-m NASA Infrared Telescope Facility (IRTF) telescope on UT 2011 January 04 and 05. The spectrum of Scheila shows a consistent red slope from 0.8 to 4.0$\mu$m with no apparent absorption features, resembling spectra of D-type asteroids. An intimate mixing model suggests that the amount of water ice that might be present on the surface of Scheila is no more than a few percent. The spectrum of the Tagish Lake chondrite matches the asteroid's spectrum at shorter wavelengths ($\lambda < 2.5 \mu$m), but no hydration features are observed at longer wavelengths on Scheila. Our analysis corroborates other studies suggesting that the comet-like activity of Scheila is likely not caused by the sublimation of water ice. The dust coma and tail may be results of a recent impact event.