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 hydro- carbons 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 as via desorption from grains. synthesis of many large molecules, including negative ions, could Although this statement ignores the possibility that complex be achieved without seeds in a carbon-rich rather than an oxygen- chemical networks with widely varying formation and destruction rich environment. In such a source, the role of O atoms would be rate coefficients can lead to non-intuitive results, it raises the issue reduced, and the differences between the two extended models that the models must reproduce the detailed radial distributions of lessened if not obliterated. molecules. So a secondary purpose of this paper is to compare In this paper we report the use of an extension of the new theoretical radial distributions with observed values (Dayal & standard model to study large molecules in a carbon-rich Bieging 1993; GueÂlin et al. 1993) carefully. environment, the envelope of the carbon-rich star IRC110216. This relatively nearby source has been well studied, both observationally and theoretically. Over 50 different molecules 2 MODEL have been detected in IRC110216 (Olofsson 1994, 1997), and The gas is assumed to expand in a spherically symmetric outflow interferometry has been utilized to study the detailed radial and with a velocity of 14 km s21 and a mass-loss rate of 3  25 21 angular structure of some of the species (Dayal & Bieging 1993; 10 M( yr : We note that this smooth outflow is an over- GueÂlin, Lucas & Cernicharo 1993). Ground-breaking model simplification, since Mauron & Huggins (1999) have reported studies by Glassgold and co-workers (Huggins & Glassgold B- and V-band images which reveal the envelope to consist of 1982; Glassgold, Lucas & Omont 1986; Glassgold et al. 1987; discrete, concentric shells. We follow the chemistry from an inner 16 Mamon, Glassgold & Huggins 1988; Cherchneff, Glassgold & radius ri of 1  10 cm; at which distance the total hydrogen Mamon 1993; Cherchneff & Glassgold 1993) and by Morris & density n, where Jura (1983) and Nejad & Millar (1987) showed that the gas-phase chemistry is instigated by the photoprocessing of `parent' n ˆ n H† 1 2n H2†; 1†

q 2000 RAS, MNRAS 316, 195±203 Large molecules in IRC110216 197

Table 2. Adopted initial fractional abundances of Table 3. Some major classes of syn- parent species with respect to n(H2). thetic reactions for larger species.

Species Initial Abundance Doty & Leung Value 1. C2H ‡ CnH2 ! Cn‡2H2 ‡ H 2. C ‡ C H ! C ‡ H 21 2 n n‡2 He 1:5  10 ± 3. C2 ‡ C ! C2 ‡ hn 24 24 n m n‡m CO 6:0  10 8:0  10 4. C H‡ ‡ C H ! C H‡ ‡ H 25 25 2 2 n m n‡2 m‡1 C2H2 5:0  10 4:0  10 5. C H ‡ HC N ! HC N ‡ H 26 26 2 2n‡1 2n‡3 CH4 2:0  10 4:0  10 HCN 8:0  1026 1:2  1025 26 26 NH3 2:0  10 4:0  10 24 25 N2 2:0  10 3:7  10 26 H2S1:0  10 ± is 3:209  105 cm23: The inner radius is chosen because here the photochemistry becomes non-negligible. The chemistry is followed to an outer radius of 3  1018 cm; and the journey from inner to outer radius takes approximately 7  104 yr: The number density of the gas follows an r22 distribution, while the temperature profile T(r) in K is given by the relation

20:79 T r†ˆmax‰100 r=ri† ; 10Š; 2† so that the temperature does not fall to a value under 10 K. The rates of photodissociation and photoionization as functions of the visual extinction AV are determined via the numerical approach of Nejad & Millar (1987). The strength of the unextinguished radiation field is set at the standard interstellar value. The model parameters are similar to, but not identical with, those chosen by Doty & Leung (1998). Parent species are injected into the outward flow at the inner Figure 1. Major synthetic routes to neutral hydrocarbons with 16 carbon atoms at a radius of 3:2  1016 cm: radius with specific fractional abundances with respect to n(H2). These are listed in Table 2, along with similar values chosen by

Doty & Leung (1998). Our value for C2H2 is close to the value inferred from ISO/SWS observations by Cernicharo et al. (1999), while our value for HCN is somewhat lower. The extended new standard model (Ruffle et al. 1999) has been modified to be appropriate for IRC110216. Hydrocarbons with more than 23 atoms are not included in the network, so that no monocyclic, tricyclic, or fullerene species are in the model. The network contains 407 species connected by 3851 reactions. Some important classes of synthetic neutral±neutral reactions, as discussed in our earlier paper (Millar & Herbst 1994), have been added to the network to produce the larger hydrocarbons.

These involve the radicals C2 and C2H, which are quite important in hydrocarbon growth in IRC110216, whereas in dense clouds growth occurs more by reactions with neutral and ionized atomic carbon. We also find that radiative association reactions between Figure 2. Major synthetic routes to neutral cyanopolyynes with 15 carbon negatively charged carbon clusters and neutral carbon clusters, atoms at a radius of 3:2 Â 1016 cm: already included in the interstellar version of the new standard model, are an important route to the synthesis of hydrocarbons in oxygen must be formed by photodissociation of CO, which occurs IRC110216. slowly. Consequently, destruction of large species by reaction with The ion±molecule and neutral±neutral chemistry leading to the O is minimal in the regions where these molecules have sizeable production of cyanopolyyne species has been extended from that abundances. Destruction by photons is important at all distances, of the interstellar model so that cyanopolyynes as complex as as is destruction via a wide assortment of neutral±neutral and ion±

HC23N are included. Reactions involving the radical CN and molecule reactions. hydrocarbons are involved in the formation of cyanopolyynes, as Table 3 contains some of the more important classes of they are in dense clouds, but reactions between the radical C2H synthetic reactions for the larger species in IRC110216. Figs 1 and smaller cyanopolyynes are far more important in IRC110216. and 2 show the dominant routes to the neutral C16Hn hydrocarbons As in the interstellar model, the production of benzene (McEwan and the cyanopolyynes with 15 carbon atoms respectively, at a et al. 1999) is included as well as that of the polar species radius of 3:2 Â 1016 cm: At this time and at this molecular size, the

C6H5CN. Note that atomic O is not produced in abundance until hydrocarbons are produced mainly through negative ion routes, the outflow reaches regions far outside the place where most while the cyanopolyynes are produced through neutral±neutral organic molecules have their peak abundance, because the atomic and positive ion±molecule reactions. The laboratory evidence for q 2000 RAS, MNRAS 316, 195±203 198 T. J. Millar, E. Herbst and R. P. A. Bettens

Table 4. Calculated radial column densities (cm22).

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Table 4 ± continued

many of the synthetic neutral±neutral reactions included in the similar values for observed species, while the model of Doty & network is minimal, and most of the assumed rate coefficients are Leung yields somewhat smaller abundances for the more complex based by analogy on a small number of studied reactions. The cyanopolyynes. Comparison between observation and theory is reaction network is available upon request. not straightforward, despite the fact that the geometry of the envelope is relatively simple. Our theoretical column densities are found by summing the molecular distributions radially throughout 3 RESULTS the entire envelope, whereas the observed column densities are Calculated radial column densities for all species in the model are sensitive to the properties of the gas which gives rise to the listed in Table 4. These values represent integrations from the emission detected. For example, the fact that the abundance of H2 inner to the outer radius, and are not multiplied by a factor of 2. is less than 1:6 Â 103 cm23 beyond a radius of 1 Â 1017 cm means Table 5 contains a comparison between these results and recently that collisional excitation of several complex molecules may be observed values for selected organic molecules, along with inefficient. Species with peak abundances that are predicted to lie previously calculated values by us (Millar & Herbst 1994) and beyond 1 Â 1017 cm may be difficult to observe because of this, by Doty & Leung (1998) with analogous but smaller chemical although radiative excitation is expected to be important in models. In general, the current model and our previous one yield IRC110216 (Morris 1975). q 2000 RAS, MNRAS 316, 195±203 200 T. J. Millar, E. Herbst and R. P. A. Bettens

Table 5. Comparison of observed and calculated column densities (cm22).

Species Present MH DL (C) Observed C 1.0E116 2.7E116 1.3E116 1.1E116 (1) C3 6.5E114 4.7E114 3.1E114 1E115 (2) C5 7.5E114 1.1E115 9.6E113 1E114 (3) C2H 5.7E115 1.8E116 7.7E115 3±5E115 (4,5,6) C3H 1.4E114 1.4E114 2.1E114 3E113 (4,7) C4H 1.0E115 5.5E115 4.7E114 2±9E115 (4,6,7,8) C5H 8.7E113 5.5E113 3.5E113 2±50E113 (4,7) C6H 5.8E114 4.5E114 8.2E113 3±30E113 (4,7) C7H 4.5E113 5.4E112 3.8E113 1E112 (4) C8H 1.1E114 3.6E113 4.0E112 5E112 (4) C3H2 2.1E113 3.0E113 7.0E113 2E113 (7) C4H2 2.9E115 4.7E115 8.6E114 3±20E112 (7,9) C3N 3.2E114 2.2E114 1.9E114 2±4E114 (7,10) C5N 1.4E114 2.3E114 6.4E113 3E112 (10) HC3N 1.8E115 2.8E115 1.4E115 1±2E115 (7,10) HC5N 7.1E114 1.2E115 1.1E114 2±3E114 (7,10) HC7N 2.2E114 2.6E114 7.8E112 1E114 (7) HC9N 5.8E113 5.1E113 3.8E112 3E113 (7) HCO1 2.4E112 ± ± 3E112 (11) CH3CN 3.4E112 ± ± 6E112 (12) References. MH Millar & Herbst (1994); DL (C) model C of Doty & Figure 3. Calculated and observed radial column densities for cyanopo- Leung (1998); (1) Keene et al. (1993); (2) Hinkle et al. (1988); (3) lyyne molecules HC2n+1N versus n. The MH results refer to Millar & Bernath et al. (1989); (4) GueÂlin et al. (1997); (5) Groesbeck et al. Herbst (1994), while the DL results refer to Model C of Doty & Leung (1994); (6) Avery et al. (1992); (7) Kawaguchi et al. (1995); (8) Dayal (1998). & Bieging (1993); (9) Cernicharo et al. (1991), the observations refer only to the cumulene form; (10) GueÂlin et al. (1998b); (11) Olofsson (1997); (12) GueÂlin & Cernicharo (1991).

Despite this caveat, all three models are in reasonable (better than order-of-magnitude) agreement with observation for 80± 90 per cent of the molecules using the criterion of column density. The level of agreement is shown for the cyanopolyyne family of molecules in Fig. 3, and the CnH radical family in Fig. 4. The cyanopolyynes seem to be well represented by our present and former models, while the column densities of the largest observed

CnH radicals are significantly lower than our current values. The range of observed values for smaller radicals often makes com- parison to within a factor of a few difficult. The molecule least well reproduced by the models is the C5N radical, which is over- produced compared with observation by more than an order of magnitude. The major formation mechanism of this radical is photodissociation of HC5N, while its only major depletion is also photodissociation. The photodissociation rates are highly uncertain. Some salient features of the calculated column densities are the generally slow but inexorable fall-off in abundance of increasingly larger members of molecular families and the significant abundances of large negative ions of carbon clusters and unsaturated hydrocarbons. For example, our newly calculated Figure 4. Calculated and observed radial column densities for C H 13 22 n column density of the cyanopolyyne HC9Nis5:8  10 cm ; in radicals versus n. The MH results refer to Millar & Herbst (1994), while excellent agreement with observation, while the next two larger the DL results refer to Model C of Doty & Leung (1998). When members of the HC2n11N family considered 5 # n # 6† are observational results differ strongly, a vertical line is drawn between the calculated to have lower column densities by factors of 4.5 and 21, points. respectively. In the CnH radical series, the newly calculated column density of the largest observed radical ± C8H ± is 1:1  increase with chain length for certain molecules, particularly the 1014 cm22; which is 20 times larger than the observed value, cyanopolyynes. while the column densities of C9H and C10H are 0.24 and 0.16 The calculated column densities of negative ions are substantial times that of C8H. Even if our prediction of relatively shallow because of the efficiency of electron sticking to larger carbon 2 declines in abundance with increasing size is correct, radio clusters. For example, the column density of the ion C7 ; a searches for more complex molecules in IRC110216 will still probable carrier of 4±5 diffuse interstellar bands (Tulej et al. take large amounts of integration time, although this may be 1998), is 1:4  1013 cm22; which is 0.1 times the column density 2 offset to some degree by the fact that dipole moments tend to of the neutral C7. If we focus on the species CnH , which are

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-5

CnH Family Cn H Family -6

-7 2

-8

log C2H 1 -9 log C3H log C2H log C4H log intensity log C3H 0

log fractional abundance log C5H -10 log C4H log C6H log C5H log C7H -11 -1 log C6H log C8H log C7H log C8H -12 16.0 16.2 16.4 16.6 16.8 17.0 17.2 17.4 17.6 17.8 18.0 -2 16.0 16.1 16.2 16.3 16.4 16.5 16.6 16.7 16.8 16.9 17.0 log Radius (cm) log Radius (cm) Figure 5. A plot of fractional abundance versus the log of radius (cm) for Figure 8. A plot of `intensity' versus the log of radius (cm) for assorted assorted CnH radicals. CnH radicals detected in IRC ‡ 10216:

-5 Although radial column densities provide one method of -6 HC N Family 2n+1 comparison between theory and observation, perhaps a more -7 detailed form of comparison is given by the radial distribution of

-8 individual species. This form of comparison has been discussed in log HC3N recent models (Millar & Herbst 1994; Doty & Leung 1998), but log HC5N -9 log HC7N has been given more emphasis by the recent statement of GueÂlin -10 log HC9N et al. (1998a) quoted in the introduction. In Figs 5±7, we show, log HC11N -11 respectively, calculated fractional abundances of molecules (with log HC13N 2 respect to H2) in the CnH, HC2n11N, and CnH families versus log fractional abundance -12 log HC15N radius from 1016±18 cm. One can see from these figures that as size -13 increases in any family, the peak fractional abundance does drift to larger radius, but the effect can be rather gradual. Thus, for the -14 16.6 CnH family, the radius of peak abundance goes from 10 cm for -15 16.85 16 16.0 16.2 16.4 16.6 16.8 17.0 17.2 17.4 17.6 17.8 18.0 C2Hto10 cm for C7H, a difference of 3:1 Â 10 cm; or

log Radius (cm) 2000 au. The negatively charged species tend to peak at larger radii than neutral species, possibly because the fractional electron Figure 6. A plot of fractional abundance versus the log of radius (cm) for abundance tends to get larger as the density decreases. assorted cyanopolyynes. One species for which a radial distribution of the fractional

abundance has been derived from interferometric studies is C4H (Dayal & Bieging 1993). The observed peak fractional abundance Negative Ions 26 -8 (with respect to H2)of1:8 Â 10 is in excellent agreement with our value of 2:3 Â 1026; although the observed peak occurs at a -9 radius of 2:5 Â 1016 cm; whereas ours occurs at a radius of 4:2 Â -10 1016 cm: Dayal & Bieging assume a distance to IRC110216 of 5 21 -11 100 pc, a mass-loss rate of 3 Â 10 M( yr ; and an outflow velocity of 13:8kms21: Assuming their distance to be correct, we -12 can eliminate the radial discrepancy by choosing a larger -13 log C8H- photodissociation rate, which in turn can be accomplished with

-14 log C10H- a somewhat larger radiation field. If the distance to IRC110216 is log fractional abundance log C12H- instead 200 pc (GueÂlin et al. 1993), then the observed peak -15 log C14H- abundance lies at a radius much closer to our calculated result. log C16H- -16 In the absence of derived plots of abundance versus radius from log C22H- -17 observers, it is unclear how best to compare theory and 16.0 16.2 16.4 16.6 16.8 17.0 17.2 17.4 17.6 17.8 18.0 interferometric observations of radial dependence. In the limit of log Radius (cm) low rotational temperature, and in the situation where rotation is Figure 7. A plot of fractional abundance versus the log of radius (cm) for collisionally excited, it is perhaps best to plot the fractional 2 assorted negative ions of the C2nH family. abundance multiplied by the square of the gas density, which we label the `intensity,' and which is proportional to the collisional polar and therefore detectable in the radio once their laboratory excitation rate assuming that the temperature is constant. Such a spectra are measured, we see that the calculated column density of plot is shown for the CnH family of molecules in Fig. 8. As 2 13 22 the ion C8H is 2:7 Â 10 cm ; which is 1=4 of the calculated compared with Fig. 5, the radii of peak intensity tend to move abundance of the neutral! inward. There is still a considerable difference between the peak q 2000 RAS, MNRAS 316, 195±203 202 T. J. Millar, E. Herbst and R. P. A. Bettens

In general, the effects are minimal, but the abundances of the larger cyanopolyynes increase considerably, as is shown in Fig. 9, where greater than order-of-magnitude increases can be seen for the largest species. The larger hydrocarbons do not show such a sensitivity, because their synthesis is dominated by ion±molecule processes. Thus we cannot claim with confidence that our model is converged with respect to the abundances of the larger species, especially the cyanopolyynes. Indeed, the fall-off in abundance with increasing size is likely to be even smaller than that of the

upper curve shown in Fig. 9, assuming that reactions with CnH radicals are as synthetic as we have chosen them to be. On the other hand, since our calculated column densities for the larger

observed CnH radicals n ˆ 7; 8† appear to be too large, our calculated values for still larger radicals n . 8† may also be too large. Chemical models of the outer envelope of IRC110216 should be able to reproduce the detailed radial distribution of each observed species as well as its total column density. The first criterion requires extensive data analysis, such as a deconvolution of smoothing (Bieging, private communication), which is not Figure 9. Calculated radial column densities for cyanopolyyne molecules always attempted. GueÂlin and co-workers (e.g. GueÂlin et al. 1993,

HC2n+1N versus n with (open circles) and without (filled squares) 1998a) have criticized gas-phase models because they believe the additional reactions involving C4H. radial distributions of a variety of molecules to be nearly identical, a fact most easily understood in terms of a common origin for 16.4 of the C2H distribution, which occurs at 10 cm, and that of the molecules (e.g., desorption from dust) as well as equal rates of 16.75 C7H distribution, which occurs at 10 cm, with other values destruction. falling in between these limits. Since infrared radiative excitation In their 1993 paper, GueÂlin et al. wrote that the positions of C4H is perhaps more important than collisional excitation in 16 and C3H coincide within #10 cm. In our calculations the two IRC110216 (Morris 1975; see also Yamamoto et al. 1987), distributions appear very similar in the intensity plot (C3H does plots of our `intensity' do not necessarily coincide with radio not have a well-defined maximum), while in the fractional intensity plots. abundance plot the radii of peak abundance are separated by 16 1±2†Â10 cm: The radius of peak abundance for HC5N, another  4 DISCUSSION species mentioned by Guelin et al. (1993), is nearly identical with that of C4H. So, given the evidence for these three species, it does Our new model of the outer envelope of IRC110216 contains not appear to us that gas-phase theories of molecule formation predicted abundances for a sizeable number of carbon-containing must be disgarded. If, on the other hand, more detailed analyses of species. We have not treated, or have only partially treated, other the observational data show that the radial distributions of species classes of interesting molecules such as sulphur-bearing, silicon- as diverse as C4H and C7H are `identical,' then gas-phase theories bearing, and metal-bearing species. Many of the large organic will be more severely challenged, although it will be necessary for molecules in our model have significant fractional abundances proponents of other mechanisms, such as desorption from dust, to with respect to molecular hydrogen. Since IRC110216 does not estimate how such a process can possess the necessary efficiency. contain much material, however, total column densities are not Bettens & Herbst (1996) have looked at the question of overly substantial. It should be possible to observe larger photodesorption rates, and estimated that with an extraordinarily molecules than heretofore detected, given known laboratory high efficiency per photon of 1 per cent, the rate coefficient (s21) frequencies (of special importance for negative ions, which are for photodesorption is <1029 in the absence of shielding, generally poorly understood) and considerable amounts of assuming a typical interstellar radiation field incident on the integration time. It is also likely that we have underestimated source. Given a time-scale of 200 yr for the possible desorption to the abundances of certain large molecules considerably, because take place (GueÂlin et al. 1993), the photodesorption suggestion it was necessary to truncate the size of the reaction network. For does not seem reasonable unless far more photons penetrate into example, reactions involving the radical C2H are important in the the envelope than is customarily thought. Assuming that synthesis of complex molecules. Analogous synthetic reactions desorption does indeed occur with extraordinary efficiency, it involving more complex radicals such as C4H, C6H, etc. are should be easy enough to model the subsequent gas-phase generally not included, despite the fact that the abundances of photochemistry. the more complex radicals decline only slowly with increasing Interestingly, the calculated radial distributions of the largest size. molecules in our model still show significant decreases in In order to study the extent of this problem, we reran the model abundance at large radial distances despite their generally slower with additional (unstudied) reactions between C4H and neutrals of photodestruction rates. As shown by Bettens & Herbst (1995), the type however, photodestruction is still operative until the species become somewhat larger than the ones considered here. Had we C4H 1 CnH2 ! Cn14H2 1 H 3† considered molecules with <30 carbon atoms and more, we would have seen much flatter radial distributions. C4H 1 HC2n11N ! HC2n15N 1 H: 4† Our models contain the assumption of a smooth outflow

q 2000 RAS, MNRAS 316, 195±203 Large molecules in IRC110216 203 velocity/mass-loss. Yet, Mauron & Huggins (1999) have recently Bettens R. P. A., Herbst E., 1997, ApJ, 478, 585 shown that the envelope of IRC110216 consists of discrete, Bettens R. P. A., Lee H.-H., Herbst E., 1995, ApJ, 443, 664 incomplete, concentric shells, at least in B- and V-band images Cernicharo J., Gottlieb C. A., GueÂlin M., Killian T. C., Thaddeus P., that reveal dust-scattered light. Chemical models with a Vrtilek J. M., 1991, ApJ, 368, L43 modulated mass-loss are certainly feasible and may help to shed Cernicharo J., Yamamura I., GonzaÂlez-Alfonso E., De Jong T., Heras A., Escribano R., Ortigoso J., 1999, ApJ, 526, L41 light on the radial distributions of molecules in the outer envelope. Cherchneff I., Glassgold A. E., 1993, ApJ, 419, L41 The images by Mauron & Huggins (1999) show that there are Cherchneff I., Barker J. R., Tielens A.G.G.M., 1992, ApJ, 401, 269 field stars seen through the envelope of IRC110216, which Cherchneff I., Glassgold A. E., Mamon G. A., 1993, ApJ, 410, 188 suggests that optical absorption studies through a path offset from Clayton D. D., Liu W., Dalgarno A., 1999, Sci, 283, 1290 the centre can be performed. If so, the large column densities of Dayal A., Bieging J. H., 1993, ApJ, 407, L37 negative ions predicted in our model could lead to the observation Doty S. D., Leung C. M., 1998, ApJ, 502, 898 of analogous features to the diffuse interstellar bands (DIBs) if Foing B. H., Ehrenfreund P., 1994, Nat, 369, 296 negative ions are important carriers of these bands (Tulej et al. Frenklach M., Feigelson E. D., 1989, ApJ, 341, 372 2 Glassgold A. E., Lucas R., Omont A., 1986, A&A, 157, 35 1998). Our predicted radial column density of C7 ; for example, is an order of magnitude greater than its apparent value in diffuse Glassgold A. E., Mamon G. A., Omont A., Lucas R., 1987, A&A, 180, 183 Groesbeck T. D., Phillips T. G., Blake G. A., 1994, ApJS, 94, 147 interstellar clouds (Ruffle et al. 1999). GueÂlin M., Cernicharo J., 1991, A&A, 244, L21 GueÂlin M., Lucas R., Cernicharo J., 1993, A&A, 280, L19 5SUMMARY GueÂlin M. et al., 1997, A&A, 317, L1 We have constructed a new chemical model of the outer GueÂlin M., Neininger N., Cernicharo J., 1998a, in Ossenkopf V., ed., The Physics and Chemistry of the Interstellar Medium, Abstract Book. circumstellar envelope of IRC110216 in which carbon-bearing Shaker Verlag, Aachen, p. 170 molecules through 23 carbon atoms in size are included. On a GueÂlin M., Neininger N., Cernicharo J., 1998b, A&A, 335, L1 relative scale, significant abundances of different types of large Herbst E., Lee H.-H., Howe D. A., Millar T. J., 1994, MNRAS, 268, 335 molecules, including carbon clusters, unsaturated hydrocarbons Herbst E., Terzieva R., Talbi D., 2000, MNRAS, 311, 869 2 and cyanopolyynes, are obtained. Negative ions of formulae Cn Hinkle K. H., Keady J. J., Bernath P. F., 1988, Sci, 241, 1319 2 and CnH 7 # n # 23† are found to have possibly detectable Huggins P. J., Glassgold A. E., 1982, ApJ, 252, 201 abundances, and to influence the synthesis of the neutral Hunter J. M., Fye J. L., Roskamp E. J., Jarrold M. F., 1994, J. Phys. Chem., hydrocarbons. Optical absorption studies through the outer 98, 1810 envelope might show the presence of DIBs if negative ions in Kawaguchi K., Kasai Y., Ishikawa S., Kaifu N., 1995, PASJ, 47, 853 the diffuse interstellar medium are indeed carriers. The calculated Keene J., Young K., Phillips T. G., Buttgenbach T. H., 1993, ApJ, 415, L131 radial column densities for the smaller carbon-bearing species are Mackay D. D. S., Charnley S. B., 1999, MNRAS, 302, 793 in reasonable agreement with observations and with values from McEwan M. J., Scott G. B. I., Adams N. G., Babcock L. M., Terzieva R., earlier, smaller models. The calculated radial distributions, Herbst E., 1999, ApJ, 513, 287 whether plotted as fractional abundance versus radius or as Mamon G. A., Glassgold A. E., Huggins P. J., 1988, ApJ, 328, 797 `intensity' versus radius, show that as molecular size in a given Mauron N., Huggins P. J., 1999, A&A, 349, 203 family increases, the peak abundance shifts slowly to larger Millar T. J., Herbst E., 1994, A&A, 288, 561 radius. Whether this small effect is in conflict with interferometric Millar T. J., Leung C. M., Herbst E., 1987, A&A, 183, 109 observations is currently unclear. Millar T. J., Farquhar P. R. A., Willacy K., 1997, A&AS, 121, 139 Morris M., 1975, ApJ, 197, 603 Morris M., Jura M., 1983, ApJ, 264, 546 ACKNOWLEDGMENTS Nejad L. A. M., Millar T. J., 1987, A&A, 183, 279 Olofsson H., 1994, in Jorgensen U. G., ed., Molecules in the Stellar Astrophysics at UMIST is supported by a grant from PPARC. EH Environment. Springer, Berlin, p. 113 acknowledges the support of the National Science Foundation Olofsson H., 1997, in van Dishoeck E. F., ed., Molecules in Astrophysics: (US) for his research in astrochemistry. Probes and Processes. Kluwer, Dordrecht, p. 457 Ruffle D. P., Bettens R. P. A., Terzieva R., Herbst E., 1999, ApJ, 523, 678 Terzieva R., Herbst E., 1998, ApJ, 501, 207 REFERENCES Terzieva R., Herbst E., 2000, Int. J. Mass Spectrom., in press Tulej M., Kirkwood D. A., Pachkov M., Maier J. P., 1998, ApJ, 506, L69 Avery L. et al., 1992, ApJS, 83, 363 von Helden G., Gotts N. G., Bowers M. T., 1993, Nat, 363, 60 Bernath P. F., Hinkle K. H., Keady J. J., 1989, Sci, 244, 562 Yamamoto S., Saito S., GueÂlin M., Cernicharo J., Suzuki H., Ohishi M., Bettens R. P. A., Herbst E., 1995, Int. J. Mass Spectrom. Ion Proc., 149/ 1987, ApJ, 323, L149 150, 321 Bettens R. P. A., Herbst E., 1996, ApJ, 468, 686 This paper has been typeset from a TEX/LATEX file prepared by the author.

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