Large Molecules in the Envelope Surrounding IRC+10216

Mon. Not. R. Astron. Soc. 316, 195±203 (2000) Large molecules in the envelope surrounding IRC110216 T. J. Millar,1 E. Herbst2w and R. P. A. Bettens3 1Department of Physics, UMIST, PO Box 88, Manchester M60 1QD 2Departments of Physics and Astronomy, The Ohio State University, Columbus, OH 43210, USA 3Research School of Chemistry, Australian National University, ACT 0200, Australia Accepted 2000 February 22. Received 2000 February 21; in original form 2000 January 21 ABSTRACT A new chemical model of the circumstellar envelope surrounding the carbon-rich star IRC110216 is developed that includes carbon-containing molecules with up to 23 carbon atoms. The model consists of 3851 reactions involving 407 gas-phase species. Sizeable abundances of a variety of large molecules ± including carbon clusters, unsaturated hydrocarbons and cyanopolyynes ± have been calculated. Negative molecular ions of chemical 2 2 formulae Cn and CnH 7 # n # 23 exist in considerable abundance, with peak concen- trations at distances from the central star somewhat greater than their neutral counterparts. The negative ions might be detected in radio emission, or even in the optical absorption of background field stars. The calculated radial distributions of the carbon-chain CnH radicals are looked at carefully and compared with interferometric observations. Key words: molecular data ± molecular processes ± circumstellar matter ± stars: individual: IRC110216 ± ISM: molecules. synthesis of fullerenes in the laboratory through chains and rings 1 INTRODUCTION is well-known (von Helden, Notts & Bowers 1993; Hunter et al. The possible production of large molecules in assorted astronomi- 1994), the individual reactions have not been elucidated. Bettens cal environments is a problem of considerable interest. The & Herbst (1995) were thus forced to hypothesize which reactions synthesis of PAH-type species is thought to occur in the inner would be most favourable in an interstellar setting, and to deter- envelopes of carbon-rich stars by high-temperature processes mine the rates and products of many such reactions theoretically. (Frenklach & Feigelson 1989), although the efficiency is low They utilized a simple version of a well-known statistical theory (Cherchneff, Barker & Tielens 1992). It is even more difficult to (the so-called RRKM theory) to deduce product branching produce significant abundances of these species by the standard fractions. Results of this theory include the diminishing of low-density chemical processes assumed to occur in interstellar photodissociation rates and the changeover from dissociative to clouds. Because of the low reactivity of molecular hydrogen with radiative recombination as molecular size increases. many molecular ions, ion±molecule reactions tend to produce Bettens & Herbst (1996, 1997) applied their extended gas-phase rather unsaturated (hydrogen-poor) organic molecules such as chemical models to both diffuse and dense interstellar clouds. Two carbon chains, cyanopolyynes, and radicals of the sort CnH types of models were used ± one an extended version of the so- (Millar, Leung & Herbst 1987). The synthesis of even the simplest called `new standard' model, and the other an extended version of PAH ± benzene ± under dense interstellar cloud conditions is `Model 4.' The former model includes fewer neutral±neutral rather inefficient (McEwan et al. 1999). reactions overall, but does include reactions between O and N atoms Several years ago, Bettens & Herbst (1995) extended standard and linear bare carbon chains (Cn). The latter model includes more models of gas-phase interstellar chemistry to produce unsaturated neutral±neutral reactions, but does not allow reactions between O molecules as large as fullerenes. (These and other chemical and N atoms and linear Cn. Both models contain negative ions of 2 2 networks referred to in the text are listed and described in Table 1.) the type Cn and CnH , since the neutral species have very large The synthesis proceeds through linear carbon chains until, at an electron affinities, and attachment of thermal electrons is thought to estimated 24 carbon atoms in size, the chains spontaneously be efficient for species with more than <5 carbon atoms (see convert into monocyclic rings. The monocyclic ring species Terzieva & Herbst 2000 for a detailed calculation of some continue to grow, but eventually change into tricyclic rings via attachment rates.) In general, the growth of large molecules is condensation-type reactions. Finally, the tricyclic rings are con- more efficient with the use of extended Model 4; use of this model verted into fullerenes by reactions capable of overcoming in its normal (non-extended) form for dense clouds results, however, considerable activation energy barriers. As molecules grow, they in worse agreement with observation for the well-studied dark cloud are also destroyed both chemically and by photons. Although the TMC-1 (Terzieva & Herbst 1998). An extension of the analysis of Bettens & Herbst to full-sized dust particles in supernova w E-mail: [email protected] remnants has been made by Clayton, Liu & Dalgarno (1999). q 2000 RAS 196 T. J. Millar, E. Herbst and R. P. A. Bettens Table 1. Assorted chemical networks. Network Description Reference new standard (nsm) basic gas-phase network Herbst et al. (2000) UMIST basic gas-phase network Millar et al. (1997) new neutral±neutral enhanced neutral reactions Terzieva & Herbst (1998) Model 4 moderately enhanced neutral reactions Terzieva & Herbst (1998) extended nsm through fullerenes Bettens & Herbst (1995, 1996, 1997) extended Model 4 through fullerenes Bettens & Herbst (1995, 1996, 1997) modified extended nsm through 23 carbon atoms only Ruffle et al. (1999) In order to produce significant abundances of large molecules in molecules produced under LTE or near-LTE conditions in the diffuse clouds, Bettens & Herbst (1996) found that it was inner envelope close to the stellar photosphere and blown necessary to consider time-dependent physical conditions. In outwards in a spherically symmetric outflow. particular, they adopted `dispersive' models, in which dark clouds In a previous paper (Millar & Herbst 1994) we showed that the of spherical shape expand isothermally at constant radial velocity. inclusion of newly measured (and analogous but unstudied) rapid Such an expansion allows larger molecules to form under dense neutral±neutral reactions does not hurt the agreement between the cloud conditions, so that when external radiation is finally able to outflow photochemical model and observation, unlike the situ- penetrate the now diffuse cloud, the molecules produced are ation in dense interstellar clouds, if a large number of unmeasured relatively immune to photodissociation. With the extended Model reactions are added (Herbst et al. 1994; Bettens, Lee & Herbst 4 network, the production of fullerenes, especially those with 60 1995). We also showed that the observed angular sizes of some of carbon atoms, can be sufficiently efficient that the assignment of the molecules could be well explained. Since then, a model by 1 two diffuse interstellar bands to C60 (Foing & Ehrenfreund 1994) Doty & Leung (1998) has appeared with a more realistic treatment cannot be ruled out. of the radiative transfer. With this treatment, the calculated radial Despite the ability of Model 4 to produce significant distribution of neutral atomic carbon is in closer agreement with abundances of fullerenes in dispersive clouds, neither model is observation than in the treatment by Millar & Herbst. However, able to produce large abundances of linear carbon clusters and while the column densities of some of the smaller observed hydrocarbons without the use of `seeds,' or carbon-containing molecules are in equal or slightly better agreement with molecules of intermediate size that desorb from dust particles. In a observation than in our earlier work, the calculated column recent study of diffuse clouds, the extended new standard model densities of some of the larger molecules are too low, indicating was used with the assumption of seeds to assess the possibility that perhaps that the chemical network of Doty & Leung is not as 2 the species C7 can be synthesized in sufficient abundance to be a complete as ours. Another recent model, by Mackay & Charnley likely carrier of the 4±5 diffuse interstellar bands which have been (1999), considers the silicon chemistry. assigned to it (Ruffle et al. 1999; Tulej et al. 1998). A large reason Although it might appear that the latest chemical models of for the relative inefficiency of the models in diffuse clouds (and to IRC110216 represent the outer envelope reasonably well, a a lesser extent in dense clouds) is that these sources are oxygen- critique of the entire approach to the chemical modelling of this rich; i.e., there is more elemental oxygen than carbon. Under source has been reiterated by GueÂlin, Neininger & Cernicharo oxygen-rich conditions, gas-phase models produce atomic O, (1998a). These authors wrote that `the models predict the longer which tends to deplete reactive carbon-containing neutrals, C-chains form from the shorter chains and peak at a larger although the different networks contain differing assumptions radius, while the observations show that all C-chains are about exactly which species react efficiently with O. During this concentrated in a single very thin shell (<103 AU) and must and earlier studies, it seemed evident that a more efficient form quasi-simultaneously,' such

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