Outbursting Quasi-Hilda Asteroid P/2010 H2 (Vales)

Abstract

Abstract Quasi-Hilda asteroid P/2010 H2 (Vales) underwent a spectacular photometric outburst by ≥7.5 mag (factor of ≥103) in 2010. Here, we present our optical observations of this event in the four month period from April 20 to August 10. The outburst, starting UT 2010 April 15.70, released dust particles of total cross-section 17,600 km2 (albedo 0.1 assumed) and mass ∼1.2 × 109 kg, this being about 10−4 of the mass of the nucleus, taken as a sphere of radius 1.5 km and density 500 kg m−3. While the rising phase of the outburst was very steep (brightness doubling time of hours), subsequent fading occurred slowly (fading timescales increasing from weeks to months), as large, low velocity particles drifted away from the nucleus. A simple model of the fading lightcurve indicates that the ejected particles occupied a broad range of sizes, from ∼1 μm to 6 cm, and followed a differential power-law distribution with index 3.6 ± 0.1 (similar to that in other comets). The fastest particles had speeds ≥210 m s−1, indicating gas-drag acceleration of small grains well coupled to the flow. Low-energy processes known to drive mass loss in active asteroids, including rotational disruption; thermal and desiccation stress cracking; and electrostatic repulsion, cannot generate the high particles speeds measured in P/Vales, and are discounted. Impact origin is unlikely given the short dynamical lifetimes of the quasi-Hildas and the low collision probabilities of these objects. The specific energy of the ejecta is estimated at 220 J kg−1. The outburst follows a series of encounters with Jupiter in the previous century, consistent with the delayed activation of buried supervolatiles (and/or the crystallization of subsurface amorphous ice) by conducted heat following an inward displacement of the perihelion. A potential origin in the debris cloud produced by avalanche is also considered.