Observational Evidence for an Impact on the Main-belt Asteroid (596) Scheila

An unexpected outburst was observed around (596) Scheila in 2010 December. We observed (596) Scheila soon after the impact using ground-based telescopes. We succeeded in the detection of a faint linear tail after 2011 February, which provides a clue to determine the dust ejection date. It is found that the dust particles ranging from 0.1-1 μm to 100 μm were ejected into the interplanetary space impulsively on December 3.5 ±1.0 day. The ejecta mass was estimated to be (1.5-4.9)×10⁸ kg, suggesting that an equivalent mass of a 500-800 m diameter crater was excavated by the event. We also found that the shape of the light curve changed after the impact event probably because fresh material was excavated around the impact site. We conclude that a decameter-sized asteroid collided with (596) Scheila only eight days before the discovery.