Chemical Properties of Comets

Figure 1: Image and isophotes of the tails of comet McNaught 2006P1 on 2007 January 12.5 UT, taken by the STEREO satellite. --The archlike tail corresponds to the solid white line that follows the crest of the isophotes on the left of the main dust tail. The dashed lines are the model ion tails moving at the given solar wind velocity. The solid line is the theoretical Fe tail.

 We have started a program of high resolution observations of comets with the echelle spectrograph SARG on the TNG telescope on La Palma in order to catalogue all the emission lines measured in the visible spectral range. This work will be very important to catalogue molecular bands, to identify unknown emissions, to calculate isotopic ratio, to determine spin temperature providing information on the formation of the Solar System, and to study some physical and chemical processes occurring in the coma. The catalogued of emission lines is a real mine of ideas and new discoveries, as it is shown in the works we have already done. We have applied a recent model to some NH2 calculating the ratio between some emissions and obtaining the spin temperature of the parent molecule NH3 (ammonia) of 32 K for the long period comet Ikeya-Zhang providing information on the comet formation in the solar nebula (Kawakita et al. 2004). Then we have analysed the spectral regions including the three emission lines of Oxygen in order to get information on the processes that gave them origin, as they are related to water, CO, and CO2 that are the molecules responsible of the cometary activity (Capria et al., 2005). We have observed several comets, but it takes a lot of time the reduction and the cataloguing of all the emissions and at the same time we work on specific and short spectral ranges following new ideas. Up to now we have published only one catalogue of emission lines for the comet Ikey-Zhang, reporting 8468 emissions with 1862 lines unidentified, on the spectral range of 462-792 nm (Cremonese et al., 2007). Even if each comet may have a composition and an activity very different from each other, most of the emission lines are common being due to very few common molecules, as C2, CN and NH2. The catalogue of comet Ikeya-Zhang has been helpful for the discovery of a new feature on another comet, as the iron tail on comet McNaught nearby the Sun (Fulle et al., 2007). After this new discovery we started to search for iron emissions in the unidentified lines of our catalogue.

People: G. Cremonese

Collaboration: M.T.Capria, M.C.De Sanctis (INAF-IASF, Roma); M.Fulle (INAF OA Trieste); H.Kawakita (Gunma Astronomical Observatory, Japan), J.Watanabe (NAOJ, Japan); A.Bhardwaj (Space Physics Laboratory, Trivarundrum, India)

Recent Publications: Cremonese et al. (2007), A&A 461,789; Fulle et al. (2007), ApJ, 661,93; Leblanc et al. (2008), A&apm;A, 482,293; Capria et al. (2008), A&apm;A, 479,257