The Nucleus of Active Asteroid 311P/(2013 P5) PANSTARRS

Abstract

The unique inner-belt asteroid 311P/PANSTARRS (formerly P/2013 P5) is notable for its sporadic, comet-like ejection of dust in nine distinct epochs spread over similar to 250 days in 2013. This curious behavior has been interpreted as the product of localized, equatorward landsliding from the surface of an asteroid rotating at the brink of instability. We obtained new Hubble Space Telescope observations to directly measure the nucleus and to search for evidence of its rapid rotation. We find a nucleus with mid-light absolute magnitude H-V = 19.14 +/- 0.02, corresponding to an equal-area circle with radius 190. +/-. 30 m (assuming geometric albedo p(V) = 0.29). However, instead of providing photometric evidence for rapid nucleus rotation, our data set a lower limit to the light-curve period, P >= 5.4 hr. The dominant feature of the light curve is a V-shaped minimum, similar to 0.3 mag deep, which is suggestive of an eclipsing binary. Under this interpretation, the time-series data are consistent with a secondary/ primary mass ratio, m(s)/m(p) similar to 1:6, a ratio of separation/primary radius, r/r(p) similar to 4 and an orbit period similar to 0.8 days. These properties lie within the range of other asteroid binaries that are thought to be formed by rotational breakup. While the light-curve period is long, centripetal dust ejection is still possible if one or both components rotate rapidly (less than or similar to 2 hr) and have small light-curve variation because of azimuthal symmetry. Indeed, radar observations of asteroids in critical rotation reveal " muffin-shaped" morphologies, which are closely azimuthally symmetric and which show minimal light curves. Our data are consistent with 311P being a close binary in which one or both components rotates near the centripetal limit. The mass loss in 2013 suggests that breakup occurred recently and could even be on-going. A search for fragments that might have been recently ejected beyond the Hill sphere reveals none larger than effective radius r(e) similar to 10 m.