The chemistry of C₃and C₂in cometary comae

Abstract

Context. It is widely accepted that C 3 and in particular C 2 play an important role in the compositional classification of comets, and the most well-established classification scheme to date is indeed based on the Haser production rates of these two radicals. A link between both C 3 and C 2 and their actual parent molecules would therefore be desirable to allow both a physical and chemical interpretation of the compositional classification of comets. A first detailed study was performed by Helbert and collaborators for comet C/1995 O1 (Hale-Bopp), which suggested a link between these two radicals and the parent species C 2 H 2 , C 2 H 6 , and C 3 H 4 . Aims. We extend previous studies of the formation of C 3 and C 2 to other comets at smaller heliocentric distances. The proposed model for the formation of these two radicals is tested for these comets. Methods. We compare the observed radial column densities of C 3 and C 2 in the comae of the comets C/2001 Q4 (NEAT), C/2002 T7 (LINEAR), and 9P/Tempel 1 with the results of a one-dimensional multi-fluid coma chemistry model. The shape of the modelled radial column density profiles are compared with the observed profiles, and the production rates of the parent species are computed by fitting the observational data with the model. Results. We do not find that C 2 H 6 is a significant parent species of the observed cometary C 2 . Furthermore, electron impact reactions do not play an important role in the formation of C 3 . The model for the formation of C 3 and C 2 derived from comet Hale-Bopp is inconsistent with observations of these radicals in other comets.

Chemical Composition of Comet C/2007 N3 (Lulin): Another ‘‘Atypical’’ Comet

“…C 2 H 5 and C 2 H 4 photo-dissociate to C 2 H 2 and then to C 2 , with branching ratios that, while uncertain, have been estimated to provide up to a 50% yield of C 2 from C 2 H 6 (Kobayashi et al 2010;Helbert et al 2005). However, we also note that Weiler (2012) argued that C 2 H 6 could not be a significant contributor to the observed C 2 . If we include a possible C 2 H 6 contribution, we find that C 2 H 2 and C 2 H 6 could potentially account for most of the C 2 production for comet Lulin.…”

Inventory of the volatiles on comet 67P/Churyumov-Gerasimenko from Rosetta/ROSINA

“…Therefore, a classification in the same sense cannot be made from our data. Assuming that the contribution to C 2 is mainly from C 2 H 2 (see discussion in Weiler 2012) Tables 2 and 4). The summer hemisphere has the highest outgassing and is most appropriate for comparison with remote sensing.…”

TRAPPIST photometry and imaging monitoring of comet C/2013 R1 (Lovejoy): Implications for the origin of daughter species

“…The C 3 morphology is similar to the C 2 morphology. Since HCN, C 2 H 2 , and C 2 H 6 usually have the same spatial distribution (Dello Russo et al 2009;Villanueva et al 2011), this could indicate that, while most of the CN is the product of HCN photodissociation, only part of the C 2 is produced by the photodissociation of C 2 H 2 (and C 2 H 6 but C 2 H 6 is not very efficient to release C 2 , see Weiler 2012). The rest of the C 2 is probably coming from organic-rich grains.…”