Massive outburst of a previously unknown comet or a disrupted Hilda asteroid?

On 2010 April 16.0, Jan Vales of the Crni Vrh observatory, Slovenia, discovered a 13th magnitude 'star' moving slowly across the sky. Observations using larger telescopes some hours later showed the object was slightly more diffuse than the adjacent stars and it was subsequently identified as a new comet, one that seems to have undergone an outburst of 7 magnitudes or more within a time interval of less than 14 hr.  This unusual comet, P/2010 H2 was observed on April 19.4 with the 2.0-m Faulkes Telescope North by myself using six different filters in all.  At this time, the comet has a very bright slightly elliptical central region some 3.3"x3.9" in size (fwhm) and the outer coma extends some 35"x40" tilted at a position angle of 70/250 deg.  A set of 13 images were tracked and stacked using Astrometrica software and then subjected to rotational gradient processing using IRIS software to bring out the faint details within the inner coma as shown in the accompanying image.

The orbit of the object shows that the parent body is one of the family of asteroids known as the Hildas, which are held in their orbit through resonance with the planet Jupiter.  We may have witnessed an event whereby the asteroid in question has collided with a smaller body causing it to disintegrate into fragments, releasing dust and gas in the process.  On the other hand, some internal process may have triggered what is essentially a cometary outburst - we just do not know at this stage!

We already know of almost 20 Hilda-type comets and P/2010 H2 appears to be the newest member to join this group.  In the latest image shown here, outflowing jets of dust can be seen emanating in at least 5 different directions. However, the majority of material reflecting sunlight still remains within less than 5 arcseconds of the nucleus almost 4 days after the initial outburst, which means that most of the mass of the parent body is moving away in all directions at less than 30 m/s, i.e. something of a snail's pace for such a relatively massive outburst.  Further observations to follow the evolution of this new comet, which reaches perihelion around May 21, will help to understand its true nature.

Richard Miles

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