large molecules in the envelope surrounding...

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 1QD2Departments of Physics and Astronomy, The Ohio State University, Columbus, OH 43210, USA3Research School of Chemistry, Australian National University, ACT 0200, Australia

Accepted 2000 February 22. Received 2000 February 21; in original form 2000 January 21

A B S T R A C T

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

formulae C2n and CnH2 �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.

1 I N T R O D U C T I O N

The possible production of large molecules in assorted astronomi-

cal environments is a problem of considerable interest. The

synthesis of PAH-type species is thought to occur in the inner

envelopes of carbon-rich stars by high-temperature processes

(Frenklach & Feigelson 1989), although the efficiency is low

(Cherchneff, Barker & Tielens 1992). It is even more difficult to

produce significant abundances of these species by the standard

low-density chemical processes assumed to occur in interstellar

clouds. Because of the low reactivity of molecular hydrogen with

many molecular ions, ion±molecule reactions tend to produce

rather unsaturated (hydrogen-poor) organic molecules such as

carbon chains, cyanopolyynes, and radicals of the sort CnH

(Millar, Leung & Herbst 1987). The synthesis of even the simplest

PAH ± benzene ± under dense interstellar cloud conditions is

rather inefficient (McEwan et al. 1999).

Several years ago, Bettens & Herbst (1995) extended standard

models of gas-phase interstellar chemistry to produce unsaturated

molecules as large as fullerenes. (These and other chemical

networks referred to in the text are listed and described in Table 1.)

The synthesis proceeds through linear carbon chains until, at an

estimated 24 carbon atoms in size, the chains spontaneously

convert into monocyclic rings. The monocyclic ring species

continue to grow, but eventually change into tricyclic rings via

condensation-type reactions. Finally, the tricyclic rings are con-

verted into fullerenes by reactions capable of overcoming

considerable activation energy barriers. As molecules grow, they

are also destroyed both chemically and by photons. Although the

synthesis of fullerenes in the laboratory through chains and rings

is well-known (von Helden, Notts & Bowers 1993; Hunter et al.

1994), the individual reactions have not been elucidated. Bettens

& Herbst (1995) were thus forced to hypothesize which reactions

would be most favourable in an interstellar setting, and to deter-

mine the rates and products of many such reactions theoretically.

They utilized a simple version of a well-known statistical theory

(the so-called RRKM theory) to deduce product branching

fractions. Results of this theory include the diminishing of

photodissociation rates and the changeover from dissociative to

radiative recombination as molecular size increases.

Bettens & Herbst (1996, 1997) applied their extended gas-phase

chemical models to both diffuse and dense interstellar clouds. Two

types of models were used ± one an extended version of the so-

called `new standard' model, and the other an extended version of

`Model 4.' The former model includes fewer neutral±neutral

reactions overall, but does include reactions between O and N atoms

and linear bare carbon chains (Cn). The latter model includes more

neutral±neutral reactions, but does not allow reactions between O

and N atoms and linear Cn. Both models contain negative ions of

the type C2n and CnH2, since the neutral species have very large

electron affinities, and attachment of thermal electrons is thought to

be efficient for species with more than <5 carbon atoms (see

Terzieva & Herbst 2000 for a detailed calculation of some

attachment rates.) In general, the growth of large molecules is

more efficient with the use of extended Model 4; use of this model

in its normal (non-extended) form for dense clouds results, however,

in worse agreement with observation for the well-studied dark cloud

TMC-1 (Terzieva & Herbst 1998). An extension of the analysis of

Bettens & Herbst to full-sized dust particles in supernova

remnants has been made by Clayton, Liu & Dalgarno (1999).

q 2000 RAS

w E-mail: [email protected]

In order to produce significant abundances of large molecules in

diffuse clouds, Bettens & Herbst (1996) found that it was

necessary to consider time-dependent physical conditions. In

particular, they adopted `dispersive' models, in which dark clouds

of spherical shape expand isothermally at constant radial velocity.

Such an expansion allows larger molecules to form under dense

cloud conditions, so that when external radiation is finally able to

penetrate the now diffuse cloud, the molecules produced are

relatively immune to photodissociation. With the extended Model

4 network, the production of fullerenes, especially those with 60

carbon atoms, can be sufficiently efficient that the assignment of

two diffuse interstellar bands to C160 (Foing & Ehrenfreund 1994)

cannot be ruled out.

Despite the ability of Model 4 to produce significant

abundances of fullerenes in dispersive clouds, neither model is

able to produce large abundances of linear carbon clusters and

hydrocarbons without the use of `seeds,' or carbon-containing

molecules of intermediate size that desorb from dust particles. In a

recent study of diffuse clouds, the extended new standard model

was used with the assumption of seeds to assess the possibility that

the species C27 can be synthesized in sufficient abundance to be a

likely carrier of the 4±5 diffuse interstellar bands which have been

assigned to it (Ruffle et al. 1999; Tulej et al. 1998). A large reason

for the relative inefficiency of the models in diffuse clouds (and to

a lesser extent in dense clouds) is that these sources are oxygen-

rich; i.e., there is more elemental oxygen than carbon. Under

oxygen-rich conditions, gas-phase models produce atomic O,

which tends to deplete reactive carbon-containing neutrals,

although the different networks contain differing assumptions

about exactly which species react efficiently with O. During this

and earlier studies, it seemed evident that a more efficient

synthesis of many large molecules, including negative ions, could

be achieved without seeds in a carbon-rich rather than an oxygen-

rich environment. In such a source, the role of O atoms would be

reduced, and the differences between the two extended models

lessened if not obliterated.

In this paper we report the use of an extension of the new

standard model to study large molecules in a carbon-rich

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

have been detected in IRC110216 (Olofsson 1994, 1997), and

interferometry has been utilized to study the detailed radial and

angular structure of some of the species (Dayal & Bieging 1993;

GueÂlin, Lucas & Cernicharo 1993). Ground-breaking model

studies by Glassgold and co-workers (Huggins & Glassgold

1982; Glassgold, Lucas & Omont 1986; Glassgold et al. 1987;

Mamon, Glassgold & Huggins 1988; Cherchneff, Glassgold &

Mamon 1993; Cherchneff & Glassgold 1993) and by Morris &

Jura (1983) and Nejad & Millar (1987) showed that the gas-phase

chemistry is instigated by the photoprocessing of `parent'

molecules produced under LTE or near-LTE conditions in the

inner envelope close to the stellar photosphere and blown

outwards in a spherically symmetric outflow.

In a previous paper (Millar & Herbst 1994) we showed that the

inclusion of newly measured (and analogous but unstudied) rapid

neutral±neutral reactions does not hurt the agreement between the

outflow photochemical model and observation, unlike the situ-

ation in dense interstellar clouds, if a large number of unmeasured

reactions are added (Herbst et al. 1994; Bettens, Lee & Herbst

1995). We also showed that the observed angular sizes of some of

the molecules could be well explained. Since then, a model by

Doty & Leung (1998) has appeared with a more realistic treatment

of the radiative transfer. With this treatment, the calculated radial

distribution of neutral atomic carbon is in closer agreement with

observation than in the treatment by Millar & Herbst. However,

while the column densities of some of the smaller observed

molecules are in equal or slightly better agreement with

observation than in our earlier work, the calculated column

densities of some of the larger molecules are too low, indicating

perhaps that the chemical network of Doty & Leung is not as

complete as ours. Another recent model, by Mackay & Charnley

(1999), considers the silicon chemistry.

Although it might appear that the latest chemical models of

IRC110216 represent the outer envelope reasonably well, a

critique of the entire approach to the chemical modelling of this

source has been reiterated by GueÂlin, Neininger & Cernicharo

(1998a). These authors wrote that `the models predict the longer

C-chains form from the shorter chains and peak at a larger

radius, while the observations show that all C-chains are

concentrated in a single very thin shell (<103 AU) and must

form quasi-simultaneously,' such as via desorption from grains.

Although this statement ignores the possibility that complex

chemical networks with widely varying formation and destruction

rate coefficients can lead to non-intuitive results, it raises the issue

that the models must reproduce the detailed radial distributions of

molecules. So a secondary purpose of this paper is to compare

theoretical radial distributions with observed values (Dayal &

Bieging 1993; GueÂlin et al. 1993) carefully.

2 M O D E L

The gas is assumed to expand in a spherically symmetric outflow

with a velocity of 14 km s21 and a mass-loss rate of 3 �1025 M( yr21: We note that this smooth outflow is an over-

simplification, since Mauron & Huggins (1999) have reported

B- and V-band images which reveal the envelope to consist of

discrete, concentric shells. We follow the chemistry from an inner

radius ri of 1 � 1016 cm; at which distance the total hydrogen

density n, where

n � n�H�1 2n�H2�; �1�

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)

196 T. J. Millar, E. Herbst and R. P. A. Bettens

q 2000 RAS, MNRAS 316, 195±203

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

T�r� � max�100�r=ri�20:79; 10�; �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

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

negatively charged carbon clusters and neutral carbon clusters,

already included in the interstellar version of the new standard

model, are an important route to the synthesis of hydrocarbons in

IRC110216.

The ion±molecule and neutral±neutral chemistry leading to the

production of cyanopolyyne species has been extended from that

of the interstellar model so that cyanopolyynes as complex as

HC23N are included. Reactions involving the radical CN and

hydrocarbons are involved in the formation of cyanopolyynes, as

they are in dense clouds, but reactions between the radical C2H

and smaller cyanopolyynes are far more important in IRC110216.

As in the interstellar model, the production of benzene (McEwan

et al. 1999) is included as well as that of the polar species

C6H5CN. Note that atomic O is not produced in abundance until

the outflow reaches regions far outside the place where most

organic molecules have their peak abundance, because the atomic

oxygen must be formed by photodissociation of CO, which occurs

slowly. Consequently, destruction of large species by reaction with

O is minimal in the regions where these molecules have sizeable

abundances. Destruction by photons is important at all distances,

as is destruction via a wide assortment of neutral±neutral and ion±

molecule reactions.

Table 3 contains some of the more important classes of

synthetic reactions for the larger species in IRC110216. Figs 1

and 2 show the dominant routes to the neutral C16Hn hydrocarbons

and the cyanopolyynes with 15 carbon atoms respectively, at a

radius of 3:2 � 1016 cm: At this time and at this molecular size, the

hydrocarbons are produced mainly through negative ion routes,

while the cyanopolyynes are produced through neutral±neutral

and positive ion±molecule reactions. The laboratory evidence for

Table 2. Adopted initial fractional abundances ofparent species with respect to n(H2).

Species Initial Abundance Doty & Leung Value

He 1:5 � 1021 ±CO 6:0 � 1024 8:0 � 1024

C2H2 5:0 � 1025 4:0 � 1025

CH4 2:0 � 1026 4:0 � 1026

HCN 8:0 � 1026 1:2 � 1025

NH3 2:0 � 1026 4:0 � 1026

N2 2:0 � 1024 3:7 � 1025

H2S 1:0 � 1026 ±

Table 3. Some major classes of syn-thetic reactions for larger species.

1. C2H� CnH2 ! Cn�2H2 � H2. C2 � CnH! Cn�2 �H3. C2

n � Cm ! C2n�m � hn

4. C2H�2 � CnHm ! Cn�2H�m�1 � H5. C2H� HC2n�1N! HC2n�3N�H

Figure 1. Major synthetic routes to neutral hydrocarbons with 16 carbon

atoms at a radius of 3:2 � 1016 cm:

Figure 2. Major synthetic routes to neutral cyanopolyynes with 15 carbon

atoms at a radius of 3:2 � 1016 cm:

Large molecules in IRC110216 197

q 2000 RAS, MNRAS 316, 195±203

Table 4. Calculated radial column densities (cm22).


q 2000 RAS, MNRAS 316, 195±203

many of the synthetic neutral±neutral reactions included in the

network is minimal, and most of the assumed rate coefficients are

based by analogy on a small number of studied reactions. The

reaction network is available upon request.

3 R E S U LT S

Calculated radial column densities for all species in the model are

listed in Table 4. These values represent integrations from the

inner to the outer radius, and are not multiplied by a factor of 2.

Table 5 contains a comparison between these results and recently

observed values for selected organic molecules, along with

previously calculated values by us (Millar & Herbst 1994) and

by Doty & Leung (1998) with analogous but smaller chemical

models. In general, the current model and our previous one yield

similar values for observed species, while the model of Doty &

Leung yields somewhat smaller abundances for the more complex

cyanopolyynes. Comparison between observation and theory is

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

the entire envelope, whereas the observed column densities are

sensitive to the properties of the gas which gives rise to the

emission detected. For example, the fact that the abundance of H2

is less than 1:6 � 103 cm23 beyond a radius of 1 � 1017 cm means

that collisional excitation of several complex molecules may be

inefficient. Species with peak abundances that are predicted to lie

beyond 1 � 1017 cm may be difficult to observe because of this,

although radiative excitation is expected to be important in

IRC110216 (Morris 1975).

Table 4 ± continued


q 2000 RAS, MNRAS 316, 195±203

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

column density of the cyanopolyyne HC9N is 5:8 � 1013 cm22; in

excellent agreement with observation, while the next two larger

members of the HC2n11N family considered �5 # n # 6� are

calculated to have lower column densities by factors of 4.5 and 21,

respectively. In the CnH radical series, the newly calculated

column density of the largest observed radical ± C8H ± is 1:1 �1014 cm22; which is 20 times larger than the observed value,

while the column densities of C9H and C10H are 0.24 and 0.16

times that of C8H. Even if our prediction of relatively shallow

declines in abundance with increasing size is correct, radio

searches for more complex molecules in IRC110216 will still

take large amounts of integration time, although this may be

offset to some degree by the fact that dipole moments tend to

increase with chain length for certain molecules, particularly the

cyanopolyynes.

The calculated column densities of negative ions are substantial

because of the efficiency of electron sticking to larger carbon

clusters. For example, the column density of the ion C27 ; a

probable carrier of 4±5 diffuse interstellar bands (Tulej et al.

1998), is 1:4 � 1013 cm22; which is 0.1 times the column density

of the neutral C7. If we focus on the species CnH2, which are

Figure 3. Calculated and observed radial column densities for cyanopo-

lyyne molecules HC2n+1N versus n. The MH results refer to Millar &

Herbst (1994), while the DL results refer to Model C of Doty & Leung

(1998).

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 &Leung (1998); (1) Keene et al. (1993); (2) Hinkle et al. (1988); (3)Bernath et al. (1989); (4) GueÂlin et al. (1997); (5) Groesbeck et al.(1994); (6) Avery et al. (1992); (7) Kawaguchi et al. (1995); (8) Dayal& Bieging (1993); (9) Cernicharo et al. (1991), the observations referonly to the cumulene form; (10) GueÂlin et al. (1998b); (11) Olofsson(1997); (12) GueÂlin & Cernicharo (1991).

Figure 4. Calculated and observed radial column densities for CnH

radicals versus n. The MH results refer to Millar & Herbst (1994), while

the DL results refer to Model C of Doty & Leung (1998). When

observational results differ strongly, a vertical line is drawn between the

points.


q 2000 RAS, MNRAS 316, 195±203

polar and therefore detectable in the radio once their laboratory

spectra are measured, we see that the calculated column density of

the ion C8H2 is 2:7 � 1013 cm22; which is 1=4 of the calculated

abundance of the neutral!

Although radial column densities provide one method of

comparison between theory and observation, perhaps a more

detailed form of comparison is given by the radial distribution of

individual species. This form of comparison has been discussed in

recent models (Millar & Herbst 1994; Doty & Leung 1998), but

has been given more emphasis by the recent statement of GueÂlin

et al. (1998a) quoted in the introduction. In Figs 5±7, we show,

respectively, calculated fractional abundances of molecules (with

respect to H2) in the CnH, HC2n11N, and CnH2 families versus

radius from 1016±18 cm. One can see from these figures that as size

increases in any family, the peak fractional abundance does drift to

larger radius, but the effect can be rather gradual. Thus, for the

CnH family, the radius of peak abundance goes from 1016.6 cm for

C2H to 1016.85 cm for C7H, a difference of 3:1 � 1016 cm; or

2000 au. The negatively charged species tend to peak at larger

radii than neutral species, possibly because the fractional electron

abundance tends to get larger as the density decreases.

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

(with respect to H2) of 1:8 � 1026 is in excellent agreement with

our value of 2:3 � 1026; although the observed peak occurs at a

radius of 2:5 � 1016 cm; whereas ours occurs at a radius of 4:2 �1016 cm: Dayal & Bieging assume a distance to IRC110216 of

100 pc, a mass-loss rate of 3 � 105 M( yr21; and an outflow

velocity of 13:8 km s21: Assuming their distance to be correct, we

can eliminate the radial discrepancy by choosing a larger

photodissociation rate, which in turn can be accomplished with

a somewhat larger radiation field. If the distance to IRC110216 is

instead 200 pc (GueÂlin et al. 1993), then the observed peak

abundance lies at a radius much closer to our calculated result.

In the absence of derived plots of abundance versus radius from

observers, it is unclear how best to compare theory and

interferometric observations of radial dependence. In the limit of

low rotational temperature, and in the situation where rotation is

collisionally excited, it is perhaps best to plot the fractional

abundance multiplied by the square of the gas density, which we

label the `intensity,' and which is proportional to the collisional

excitation rate assuming that the temperature is constant. Such a

plot is shown for the CnH family of molecules in Fig. 8. As

compared with Fig. 5, the radii of peak intensity tend to move

inward. There is still a considerable difference between the peak

-12

-11

-10

-9

-8

-7

-6

-5lo

gfr

actio

nala

bund

ance

16.0 16.2 16.4 16.6 16.8 17.0 17.2 17.4 17.6 17.8 18.0

log Radius (cm)

log C8H

log C7H

log C6H

log C5H

log C4H

log C3H

log C2H

C H Familyn

Figure 5. A plot of fractional abundance versus the log of radius (cm) for

assorted CnH radicals.

-15

-14

-13

-12

-11

-10

-9

-8

-7

-6

-5

log

frac

tion

alab

unda

nce

16.0 16.2 16.4 16.6 16.8 17.0 17.2 17.4 17.6 17.8 18.0

log Radius (cm)

log HC15N

log HC13N

log HC11N

log HC9N

log HC7N

log HC5N

log HC3N

HC N Family2n+1


assorted cyanopolyynes.

-17

-16

-15

-14

-13

-12

-11

-10

-9

-8

log

frac

tiona

labu

ndan

ce

16.0 16.2 16.4 16.6 16.8 17.0 17.2 17.4 17.6 17.8 18.0

log Radius (cm)

log C22H-

log C16H-

log C14H-

log C12H-

log C10H-

log C8H-

Negative Ions


assorted negative ions of the C2nH2 family.

-2

-1

0

1

2

log

inte

nsit

y

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 C8H

log C7H

log C6H

log C5H

log C4H

log C3H

log C2H

C H Familyn

Figure 8. A plot of `intensity' versus the log of radius (cm) for assorted

CnH radicals detected in IRC� 10216:


q 2000 RAS, MNRAS 316, 195±203

of the C2H distribution, which occurs at 1016.4 cm, and that of the

C7H distribution, which occurs at 1016.75 cm, with other values

falling in between these limits. Since infrared radiative excitation

is perhaps more important than collisional excitation in

IRC110216 (Morris 1975; see also Yamamoto et al. 1987),

plots of our `intensity' do not necessarily coincide with radio

intensity plots.

4 D I S C U S S I O N

Our new model of the outer envelope of IRC110216 contains

predicted abundances for a sizeable number of carbon-containing

species. We have not treated, or have only partially treated, other

classes of interesting molecules such as sulphur-bearing, silicon-

bearing, and metal-bearing species. Many of the large organic

molecules in our model have significant fractional abundances

with respect to molecular hydrogen. Since IRC110216 does not

contain much material, however, total column densities are not

overly substantial. It should be possible to observe larger

molecules than heretofore detected, given known laboratory

frequencies (of special importance for negative ions, which are

generally poorly understood) and considerable amounts of

integration time. It is also likely that we have underestimated

the abundances of certain large molecules considerably, because

it was necessary to truncate the size of the reaction network. For

example, reactions involving the radical C2H are important in the

synthesis of complex molecules. Analogous synthetic reactions

involving more complex radicals such as C4H, C6H, etc. are

generally not included, despite the fact that the abundances of

the more complex radicals decline only slowly with increasing

size.

In order to study the extent of this problem, we reran the model

with additional (unstudied) reactions between C4H and neutrals of

the type

C4H 1 CnH2 ! Cn14H2 1 H �3�

C4H 1 HC2n11N! HC2n15N 1 H: �4�

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 � 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

always attempted. GueÂlin and co-workers (e.g. GueÂlin et al. 1993,

1998a) have criticized gas-phase models because they believe the

radial distributions of a variety of molecules to be nearly identical,

a fact most easily understood in terms of a common origin for

molecules (e.g., desorption from dust) as well as equal rates of

destruction.

In their 1993 paper, GueÂlin et al. wrote that the positions of C4H

and C3H coincide within #1016 cm. In our calculations the two

distributions appear very similar in the intensity plot (C3H does

not have a well-defined maximum), while in the fractional

abundance plot the radii of peak abundance are separated by

�1±2� � 1016 cm: The radius of peak abundance for HC5N, another

species mentioned by GueÂlin et al. (1993), is nearly identical with

that of C4H. So, given the evidence for these three species, it does

not appear to us that gas-phase theories of molecule formation

must be disgarded. If, on the other hand, more detailed analyses of

the observational data show that the radial distributions of species

as diverse as C4H and C7H are `identical,' then gas-phase theories

will be more severely challenged, although it will be necessary for

proponents of other mechanisms, such as desorption from dust, to

estimate how such a process can possess the necessary efficiency.

Bettens & Herbst (1996) have looked at the question of

photodesorption rates, and estimated that with an extraordinarily

high efficiency per photon of 1 per cent, the rate coefficient (s21)

for photodesorption is <1029 in the absence of shielding,

assuming a typical interstellar radiation field incident on the

source. Given a time-scale of 200 yr for the possible desorption to

take place (GueÂlin et al. 1993), the photodesorption suggestion

does not seem reasonable unless far more photons penetrate into

the envelope than is customarily thought. Assuming that

desorption does indeed occur with extraordinary efficiency, it

should be easy enough to model the subsequent gas-phase

photochemistry.

Interestingly, the calculated radial distributions of the largest

molecules in our model still show significant decreases in

abundance at large radial distances despite their generally slower

photodestruction rates. As shown by Bettens & Herbst (1995),

however, photodestruction is still operative until the species

become somewhat larger than the ones considered here. Had we

considered molecules with <30 carbon atoms and more, we would

have seen much flatter radial distributions.

Our models contain the assumption of a smooth outflow

Figure 9. Calculated radial column densities for cyanopolyyne molecules

HC2n+1N versus n with (open circles) and without (filled squares)

additional reactions involving C4H.


q 2000 RAS, MNRAS 316, 195±203

velocity/mass-loss. Yet, Mauron & Huggins (1999) have recently

shown that the envelope of IRC110216 consists of discrete,

incomplete, concentric shells, at least in B- and V-band images

that reveal dust-scattered light. Chemical models with a

modulated mass-loss are certainly feasible and may help to shed

light on the radial distributions of molecules in the outer envelope.

The images by Mauron & Huggins (1999) show that there are

field stars seen through the envelope of IRC110216, which

suggests that optical absorption studies through a path offset from

the centre can be performed. If so, the large column densities of

negative ions predicted in our model could lead to the observation

of analogous features to the diffuse interstellar bands (DIBs) if

negative ions are important carriers of these bands (Tulej et al.

1998). Our predicted radial column density of C27 ; for example, is

an order of magnitude greater than its apparent value in diffuse

interstellar clouds (Ruffle et al. 1999).

5 S U M M A RY

We have constructed a new chemical model of the outer

circumstellar envelope of IRC110216 in which carbon-bearing

molecules through 23 carbon atoms in size are included. On a

relative scale, significant abundances of different types of large

molecules, including carbon clusters, unsaturated hydrocarbons

and cyanopolyynes, are obtained. Negative ions of formulae C2n

and CnH2 �7 # n # 23� are found to have possibly detectable

abundances, and to influence the synthesis of the neutral

hydrocarbons. Optical absorption studies through the outer

envelope might show the presence of DIBs if negative ions in

the diffuse interstellar medium are indeed carriers. The calculated

radial column densities for the smaller carbon-bearing species are

in reasonable agreement with observations and with values from

earlier, smaller models. The calculated radial distributions,

whether plotted as fractional abundance versus radius or as

`intensity' versus radius, show that as molecular size in a given

family increases, the peak abundance shifts slowly to larger

radius. Whether this small effect is in conflict with interferometric

observations is currently unclear.

AC K N OW L E D G M E N T S

Astrophysics at UMIST is supported by a grant from PPARC. EH

acknowledges the support of the National Science Foundation

(US) for his research in astrochemistry.

R E F E R E N C E S

Avery L. et al., 1992, ApJS, 83, 363

Bernath P. F., Hinkle K. H., Keady J. J., 1989, Sci, 244, 562

Bettens R. P. A., Herbst E., 1995, Int. J. Mass Spectrom. Ion Proc., 149/

150, 321

Bettens R. P. A., Herbst E., 1996, ApJ, 468, 686

Bettens R. P. A., Herbst E., 1997, ApJ, 478, 585

Bettens R. P. A., Lee H.-H., Herbst E., 1995, ApJ, 443, 664

Cernicharo J., Gottlieb C. A., GueÂlin M., Killian T. C., Thaddeus P.,

Vrtilek J. M., 1991, ApJ, 368, L43

Cernicharo J., Yamamura I., GonzaÂlez-Alfonso E., De Jong T., Heras A.,

Escribano R., Ortigoso J., 1999, ApJ, 526, L41

Cherchneff I., Glassgold A. E., 1993, ApJ, 419, L41

Cherchneff I., Barker J. R., Tielens A.G.G.M., 1992, ApJ, 401, 269

Cherchneff I., Glassgold A. E., Mamon G. A., 1993, ApJ, 410, 188

Clayton D. D., Liu W., Dalgarno A., 1999, Sci, 283, 1290

Dayal A., Bieging J. H., 1993, ApJ, 407, L37

Doty S. D., Leung C. M., 1998, ApJ, 502, 898

Foing B. H., Ehrenfreund P., 1994, Nat, 369, 296

Frenklach M., Feigelson E. D., 1989, ApJ, 341, 372

Glassgold A. E., Lucas R., Omont A., 1986, A&A, 157, 35

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

GueÂlin M., Cernicharo J., 1991, A&A, 244, L21

GueÂlin M., Lucas R., Cernicharo J., 1993, A&A, 280, L19

GueÂlin M. et al., 1997, A&A, 317, L1

GueÂlin M., Neininger N., Cernicharo J., 1998a, in Ossenkopf V., ed., The

Physics and Chemistry of the Interstellar Medium, Abstract Book.

Shaker Verlag, Aachen, p. 170

GueÂlin M., Neininger N., Cernicharo J., 1998b, A&A, 335, L1

Herbst E., Lee H.-H., Howe D. A., Millar T. J., 1994, MNRAS, 268, 335

Herbst E., Terzieva R., Talbi D., 2000, MNRAS, 311, 869

Hinkle K. H., Keady J. J., Bernath P. F., 1988, Sci, 241, 1319

Huggins P. J., Glassgold A. E., 1982, ApJ, 252, 201

Hunter J. M., Fye J. L., Roskamp E. J., Jarrold M. F., 1994, J. Phys. Chem.,

98, 1810

Kawaguchi K., Kasai Y., Ishikawa S., Kaifu N., 1995, PASJ, 47, 853

Keene J., Young K., Phillips T. G., Buttgenbach T. H., 1993, ApJ, 415,

L131

Mackay D. D. S., Charnley S. B., 1999, MNRAS, 302, 793

McEwan M. J., Scott G. B. I., Adams N. G., Babcock L. M., Terzieva R.,

Herbst E., 1999, ApJ, 513, 287

Mamon G. A., Glassgold A. E., Huggins P. J., 1988, ApJ, 328, 797

Mauron N., Huggins P. J., 1999, A&A, 349, 203

Millar T. J., Herbst E., 1994, A&A, 288, 561

Millar T. J., Leung C. M., Herbst E., 1987, A&A, 183, 109

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

Nejad L. A. M., Millar T. J., 1987, A&A, 183, 279

Olofsson H., 1994, in Jorgensen U. G., ed., Molecules in the Stellar

Environment. Springer, Berlin, p. 113

Olofsson H., 1997, in van Dishoeck E. F., ed., Molecules in Astrophysics:

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

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

von Helden G., Gotts N. G., Bowers M. T., 1993, Nat, 363, 60

Yamamoto S., Saito S., GueÂlin M., Cernicharo J., Suzuki H., Ohishi M.,

1987, ApJ, 323, L149

This paper has been typeset from a TEX/LATEX file prepared by the author.


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