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Context. On October 24, 2007 the periodic comet 17P/Holmes underwent anastonishing outburst that increased its apparent total brightness frommagnitude V\sim17 up to V\sim2.5 in roughly two days. We report on Wendelstein0.8 m telescope (WST) photometric observations of the early evolution stages ofthe outburst. Aims. We studied the evolution of the structure morphology, itskinematic, and estimated the ejected dust mass. Methods. We analized 126 imagesin the BVRI photometric bands spread between 26/10/2007 and 20/11/2007. Thebright comet core appeared well separated from that one of a quickly expandingdust cloud in all the data, and the bulk of the latter was contained in thefield of view of our instrument. The ejected dust mass was derived on the baseof differential photometry on background stars occulted by the moving cloud.Results. The two cores were moving apart from each other at a relativeprojected constant velocity of (9.87 +/- 0.07) arcsec/day (0.135 +/-0.001km/sec). In the inner regions of the dust cloud we observed a linear increasein size at a mean constant velocity of (14.6+/-0.3) arcsec/day (0.200+/-0.004km/sec). Evidence of a radial velocity gradient in the expanding cloud was alsofound. Our estimate for the expanding coma's mass was of the order of 10^{-2}-1comet's mass implying a significant disintegration event. Conclusions. Weinterpreted our observations in the context of an explosive scenario which wasmore probably originated by some internal instability processes, rather than animpact with an asteroidal body. Due to the peculiar characteristics of thisevent, further observations and investigations are necessary in order toenlight the nature of the physical processes that determined it.

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