Evidence for an Impact Event on (493) Griseldis

An extended feature associated with the main-belt asteroid (493) Griseldis has been detected in three R-band exposures of 330 to 350 sec duration spanning 5 hours taken on 2015 Mar 17 UT with the HyperSuprimeCam instrument on the 8-m Subaru telescope. Additional observations of Griseldis were taken with the 6.5-m Magellan telescope on 2015 Mar 21 UT, and the extended feature was still detected, though weaker. No extended feature was detected in one unfiltered 600 sec exposure taken with the 2.2-m University of Hawaii telescope on 2015 Mar 24 UT, or in Magellan images taken on 2015 Apr 18 and May 21 UT. Griseldis is a 46 km diameter P-type asteroid with semimajor axis of 3.12 AU, eccentricity of 0.17, inclination of 15 deg, and Tisserand parameter of 3.187 relative to Jupiter. The heliocentric distance on 2015 Mar 17 was 3.33 AU, thus the asteroid was closer to aphelion than it was to perihelion. The most recent perihelion passage was on 2013 Aug 17, and the orbital period is 5.5 years. Additional images in the Subaru and CTIO DECam archives from 2010 and 2012, respectively, show no signs of activity. The position angle of the extended feature does not match either the antisolar direction or the negative velocity vector of the asteroid. The rotational lightcurve of Griseldis is known to show a peak-to-peak amplitude of up to 0.45 mag over a 52-hour period. No anomalous brightening of the asteroid is evident in the 2015 photometry reported along with the astrometry of this object. The observations are consistent with the occurrence of an impact event on this asteroid. Additional analysis of the images is in progress.