P/2010 A2 Linear⋆

Comet P/2010 A2 LINEAR is an object on an asteroidal orbit within the inner main belt, therefore a good candidate for membership with the main belt comet family. It was observed with several telescopes (ESO New Technology Telescope, La Silla, Chile; Gemini North, Mauna Kea, Hawaii; University of Hawaii 2.2 m, Mauna Kea, Hawaii) from 14 Jan. until 19 Feb. 2010 in order to characterize and monitor it and its very unusual dust tail, which appears almost fully detached from the nucleus; the head of the tail includes two narrow arcs forming a cross. No evolution was observed during the span of the observations. Observations obtained during the Earth orbital plane crossing allowed an examination of the out-of-plane 3D structure of the tail. The immediate surroundings of the nucleus were found dust-free, which allowed an estimate of the nucleus radius of 80–90 m, assuming an albedo p = 0.11 and a phase correction with G = 0.15 (values typical for S-type asteroids). A model of the thermal evolution indicates that such a small nucleus could not maintain any ice content for more than a few million years on its current orbit, ruling out ice sublimation dust ejection mechanism. Rotational spin-up and electrostatic dust levitations were also rejected, leaving an impact with a smaller body as the favoured hypothesis. This is further supported by the analysis of the tail structure. Finston-Probstein dynamical dust modelling indicates the tail was produced by a single burst of dust emission. More advanced models (described in detail in a companion paper), independently indicate that this burst populated a hollow cone with a half-opening angle α ∼ 40◦ and with an ejection velocity vmax ∼ 0.2 m s−1, where the small dust grains fill the observed tail, while the arcs are foreshortened sections of the burst cone. The dust grains in the tail are measured to have radii between a = 1–20 mm, with a differential size distribution proportional to a−3.44±0.08. The dust contained in the tail is estimated to at least 8 × 108 kg, which would form a sphere of 40 m radius (with a density ρ = 3000 kg m−3 and an albedo p = 0.11 typical of S-type asteroids). Analysing these results in the framework of crater physics, we conclude that a gravity-controlled crater would have grown up to ∼100 m radius, i.e. comparable to the size of the body. The non-disruption of the body suggest this was an oblique impact.