Broad- and narrowband visible imaging of comet 9P/Tempel 1 at ESO around the time of the Deep Impact event

Abstract

Context.On 4 July 2005 at 05:52UT the impactor of NASA's DeepImpact (DI) mission exploded at comet 9P/Tempel 1. The ejecta material of the impact expanded into the coma that is produced by normal cometary activity. The La Silla and Paranal sites of the European Southern Observatory ESO in Chile participated in the world-wide campaign to observe this event. Aims.The gas and dust content of the cometary coma is observed around the time of the DI event to identify signatures of the impact, like changes in the gas production (CN, C₃, C₂, NH₂ and Na) and in the dust properties. The study also describes the normal activity pattern in the coma and the expanding ejecta cloud. Methods.The gas production rates and the dust reddening slope are measured in images obtained through narrowband cometary filters. The ejecta cloud and the features of normal cometary activity, as imaged in narrow- and broadband filter images, are studied with respect to geometry, intensity, and persistence. Results.The production of CN, C₃, and C₂ gas by the nucleus is at similar level on 3 July and 9+10 July 2005. The mixing ratios of these gases are in the range of those for "typical" comets. NH₂ and Na gas are not detected above the dust continuum flux in the coma. The reddening slope of the dust continuum changes from a value of 9%/100 nm, constant throughout 20 000 km distance around the nucleus, on 3 July to about 19-20%/100 nm on 9+10 July 2005. An increase of the slope with radial distance is found. Both changes might be due to the presence of dust from the ejecta cloud. The dust coma of the comet showed a porcupine pattern of 9 coma jets before impact which remained intact after impact. It can be interpreted as being due to embedded fan structures produced by at least 4 active regions on the rotating nucleus. The ejecta cloud contains dust grains of likely absorbing material and possibly dielectric admixtures with solar radiation pressure factors beta of 0.2-1.9 and ejection velocities of 160-370 m s^-1. The ejecta cloud is optically thin 21 h after impact and shows a higher surface brightness and redder color in the sunward sector compared to the tailward side.