Observations of CN and dust activity of comet 9P/Tempel 1 around Deep Impact

Abstract

Aims. We present observations of CN emission and the scattered solar light on cometary dust particles around the impact time of the Deep Space spacecraft (NASA) into the nucleus of comet 9P/Tempel 1. The purpose of the observations was to compare post-impact activity to the conditions pre-impact to search for new spectral emission lines after impact, to quantify the increase in gas activity due to the impact and to study the long-term activity changes. Methods. We performed long-slit spectroscopy observations of comet 9P/Tempel 1 at the VLT, ESO, using the FORS instruments from July 2 to July 12, 2005. A wavelengths range of 370–920 nm was covered using two grisms. Four different position angle settings of the slit were applied each night with the projected Sun-comet line as standard setting, for which we report results here. Results. The optical spectra of comet 9P/Tempel 1 showed the usual emission bands in the optical wavelengths range of the radicals: CN, C3 ,C 2 and NH2. No new emission bands were detected after impact. The ejecta cloud of gas and dust caused by the impacting spacecraft into the cometary nucleus could be followed over the observing period. The projected expansion velocities have been determined. The night after impact we observed about (3.9 ± 1.2) × 10 29 molecules of the CN parent in the ejected cloud. However, after five days the appearance of the gas and dust coma was back to pre-impact conditions.