High resolution observation of 17P/ Holmes during the outburst event in 2007

¹ INAF - IASF, Area Ricerca di Tor Vergata, Rome Italy
e-mail: mariateresa.capria@iasf-roma.inaf.it
² INAF - Padua Observatory, vicolo dell’Osservatorio 5, Padua, Italy

Received: 16 December 2009
Accepted: 12 April 2010

Abstract

Aims. On October 24, 2007 the comet 17P/ Holmes underwent an outburst that was extraordinary in both duration and size. We attempted to determine the cause of the outburst by acquiring and analyzing high resolution spectra of the comet.

Methods. On two nights, October 25 and December 2, we acquired four high resolution spectra in the visible range at the Telescopio Nazionale Galileo in La Palma. The oxygen lines intensity ratio was estimated for the observation performed on October 25, and computed for the observation performed on December 2.

Results. Many emission lines were detected in the spectra, most of them attributable to C₂, NH₂, CN and CH. The atomic O green line and one of the red doublet lines were detected in a spectrum acquired on October 25. All the three atomic O lines were detected in a spectrum acquired on December 2. The value obtained for the first night, 0.1 ± 0.11, implies that the H₂O molecule was the main parent of the emission lines, while the value obtained for the second night, 0.3 ± 0.1, implies that the main parent was CO or CO₂ instead than H₂O.

Conclusions. Our results imply that in the days around the outburst the water production was exceptionally high for that heliocentric distance. By December, however, the behavior of 17P/ Holmes had returned to that of a comet orbiting at 2.6 AU from the Sun, in which state the activity is probably sustained mostly by gases more volatile than water, such as CO and CO₂.