The comet 17P/Holmes 2007 outburst: the early motion of the outburst material

Context. On October 24, 2007 the periodic comet 17P/Holmes underwent an astonishing outburst that increased its apparent total brightness from magnitude V ~17 up to V ~2.5 in roughly two days. In this contribution we report on Wendelstein 0.8 m telescope (WST) photometric observations of the early evolution stages of the outburst. Aims. We studied the evolution of the structure morphology and its kinematic and provide an estimate of the ejected dust mass. Methods. We analyzed 126 images of the comet in the BVRI photometric bands spread between October 26, 2007 and November 20, 2007. The bright comet core appeared to be separated from a quickly expanding dust cloud in all the data, and the bulk of the cloud was contained in the field of view of our instrument during the days soon after the outburst, allowing precise estimates both of the separation velocities of the two luminous baricenters and of the expansion velocity of the dust cloud. The ejected dust mass was derived on the basis of differential photometry on background stars occulted by the moving cloud. Results. The two cores were moving apart from each other at a relative, projected constant velocity of (9.87 ± 0.07) arcsec/day (0.135 ± 0.001 km s -1 ). In the inner regions of the dust cloud we observed a linear increase in size at a mean constant velocity of (14.6 ± 0.3) arcsec/day (0.200 ± 0.004 km s -1 ). Evidence of a radial velocity gradient in the expanding cloud was also found. Our estimate for the expanding coma's mass was approximately 10$^{-2}{-}1$ comet's mass, implying a significant disintegration event. Conclusions. We interpret our observations in the context of an explosive scenario that was more probably triggered by some internal instability processes rather than by an impact with an asteroidal body. Due to the peculiar characteristics of this event, further observations and investigations are necessary to bring the nature of the physical processes that determined it to light.