The Origin of Large Molecules in Primordial Autocatalytic ...
Large molecules in the envelope surrounding...
Transcript of 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
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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
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Table 4. Calculated radial column densities (cm22).
198 T. J. Millar, E. Herbst and R. P. A. Bettens
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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
Large molecules in IRC110216 199
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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.
200 T. J. Millar, E. Herbst and R. P. A. Bettens
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
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-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.
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-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
Figure 6. A plot of fractional abundance versus the log of radius (cm) for
assorted cyanopolyynes.
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log
frac
tiona
labu
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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
Figure 7. A plot of fractional abundance versus the log of radius (cm) for
assorted negative ions of the C2nH2 family.
-2
-1
0
1
2
log
inte
nsit
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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:
Large molecules in IRC110216 201
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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.
202 T. J. Millar, E. Herbst and R. P. A. Bettens
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.
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This paper has been typeset from a TEX/LATEX file prepared by the author.
Large molecules in IRC110216 203
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