Abstract The circumstellar envelopes of carbon-rich AGB stars show a chemical complexity that is exemplified by the prototypical object IRC +10216, in which about 60 different molecules have been detected to date. Most of these species are carbon chains of the type C n H, C n H 2 , C n N, HC n N. We present the detection of new species (CH 2 CHCN, CH 2 CN, H 2 CS, CH 3 CCH and C 3 O) achieved thanks to the systematic observation of the full 3 mm window with the IRAM 30m telescope plus some ARO 12m observations. All these species, known to exist in the interstellar medium, are detected for the first time in a circumstellar envelope around an AGB star. These five molecules are most likely formed in the outer expanding envelope rather than in the stellar photosphere. A pure gas phase chemical model of the circumstellar envelope is reasonably successful in explaining the derived abundances, and additionally allows to elucidate the chemical formation routes and to predict the spatial distribution of the detected species

A J Apponi

E D Tenenbaum

Expósito J P Fonfría

J Cernicharo

J R Pardo

L M Ziurys

M Agúndez

M Guélin

· E D Tenenbaum

· M Guélin

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Noname manuscript No.
(will be inserted by the editor)
Understanding the chemical complexity in Circumstellar
Envelopes of C-Rich AGB stars: The case of IRC +10216
M. Agúndez, J. Cernicharo, J. R. Pardo, J. P. Fonfŕıa Expósito · M.
Guélin · E. D. Tenenbaum, L. M. Ziurys, A. J. Apponi
Received: date / Accepted: date
Abstract The circumstellar envelopes of carbon-rich
AGB stars show a chemical complexity that is exempli-
fied by the prototypical object IRC +10216, in which
about 60 different molecules have been detected to date.
Most of these species are carbon chains of the type
CnH, CnH2, CnN, HCnN. We present the detection of
new species (CH2CHCN, CH2CN, H2CS, CH3CCH and
C3O) achieved thanks to the systematic observation of
the full 3 mm window with the IRAM 30m telescope
plus some ARO 12m observations. All these species,
known to exist in the interstellar medium, are detected
for the first time in a circumstellar envelope around an
AGB star. These five molecules are most likely formed
in the outer expanding envelope rather than in the stel-
lar photosphere. A pure gas phase chemical model of
the circumstellar envelope is reasonably successful in
explaining the derived abundances, and additionally al-
lows to elucidate the chemical formation routes and to
predict the spatial distribution of the detected species.
Keywords astrochemistry · circumstellar matter ·
molecular processes · AGB stars · IRC +10216
M. Agúndez, J. Cernicharo, J. R. Pardo, J. P. Fonfŕıa Expósito
Departamento de Astrof́ısica Molecular e Infrarroja, Instituto de
Estructura de la Materia, CSIC, Serrano 121, E–28006, Madrid,
Spain
Tel.: +34 915616800
Fax: +34 915645557
E-mail: marce@damir.iem.csic.es
M. Guélin
Institut de Radioastronomie Millimétrique, 300 rue de la Piscine,
F–38406 St. Martin d’Héres, France
E. D. Tenenbaum, L. M. Ziurys, A. J. Apponi
Departments of Chemistry and Astronomy, University of Ari-
zona, 933 North Cherry Avenue, Tucson, AZ 85721
1 Introduction
IRC +10216 was discovered in the late sixties as an ex-
tremely bright object in the mid infrared (Becklin et al.
1969). Since then and with the development of radioas-
tronomy during the seventies it was recognized as one
of the richest molecular sources in the sky together with
some others such as the Orion nebula, Sagittarius-B2
and the Taurus molecular cloud complex.
IRC +10216 consists of a central carbon-rich AGB
star, i.e. C/O>1 in the photosphere, losing mass at
a rate of 2-4×10−5 M⊙ yr
−1 in the form of a quasi-
spherical wind that produces an extended circumstel-
lar envelope (CSE) from which most of the molecular
emission arises. To date, some 60 different molecules
have been detected in this source, most of which are or-
ganic molecules consisting of a linear and highly unsat-
urated backbone of carbon atoms. Among these species
there are cyanopolyynes (HC2n+1N) and their radicals
(C2n+1N), polyyne radicals (CnH), carbenes (H2Cn),
radicals (HC2nN) as well as S-bearing (CnS) and Si-
bearing (SiCn) species (Glassgold 1996; Cernicharo et
al. 2000).
It is nowadays accepted that the formation of these
molecules occurs either under chemical equilibrium in
the dense (n>1010 cm−3) and hot (Tk∼2000 K) vicin-
ity of the stellar photosphere or in the colder and less
dense outer envelope when the interstellar UV field
photodissociate/ionize the molecules flowing out from
the star producing radicals/ions which undergo rapid
neutral-neutral and ion-molecule reactions (Lafont et
al. 1982; Millar et al. 2000). This picture of circum-
stellar photochemistry resembles that occurring in cold
dense clouds (Tk=10 K, n∼10
4 cm−3) such as TMC-1
(Kaifu et al. 2004). In both places organic molecules
are mostly unsaturated, which is typical of low temper-
2
Fig. 1 Rotational lines of CH2CHCN detected toward IRC +10216 with the IRAM 30m telescope.
ature non-equilibrium chemistry and simply reflects the
trend of gas phase bimolecular reactions in ejecting an
hydrogen atom and the low reactivity of H2 with most
hydrocarbons.
In this contribution we report on the detection to-
ward IRC +10216 of the partially saturated C-bearing
species CH2CHCN, CH2CN and CH3CCH; the S-bearing
molecule H2CS and the oxygen-carbon chain C3O. All
these species are known to exist in cold dense clouds
(Matthews & Sears 1983; Irvine et al. 1988; Irvine et
al. 1981; Irvine et al. 1989; Brown et al. 1985). Thus,
their detection in IRC +10216 stress the similarity be-
tween the chemistry taking place in cold dense clouds
and in the CSEs of C-rich AGB stars.
2 Observations
The observations of CH2CHCN, CH2CN, H2CS and
CH3CCH were achieved with the IRAM 30m telescope
(see e.g. Fig. 1) while C3O was observed with both the
IRAM 30m and ARO 12m telescopes.
The IRAM 30m observations were carried out dur-
ing several sessions from 1990 to 2005, most of them
after 2002 in the context of a λ 3 mm line survey of
IRC+10216 from 80 to 115.75 GHz (Cernicharo et al.,
in preparation). Two 3 mm SIS receivers with orthog-
onal polarizations were used in single-sideband mode
with image sideband rejections >20 dB. The standard
wobbler switching mode was used with an offset of 4’.
The back end was a 512 two-pole filter with half-power
widths and spacings equal to 1.0 MHz.
The ARO 12m observations were done in several
runs between 2003 and 2005. The receivers were dual-
channel cooled SIS mixers at 2 and 3 mm, operated in
single-sideband mode with ∼18 dB rejection of the im-
age sideband. The back ends were two 256 channel filter
banks with 1 and 2 MHz resolutions, configured in par-
allel mode (2×128 channels) for the two receiver chan-
nels. A millimeter auto correlator spectrometer with
2048 channels of 768 kHz resolution was operated simul-
taneously to confirm features seen in the filter banks.
Data were taken in beam switching mode with ±2’ sub-
reflector throw.
3 Molecular column densities
The number of CH2CHCN, CH2CN and C3O lines de-
tected was large enough to allow us to construct rota-
tional temperature diagrams (see e.g. Fig. 2). The ro-
tational temperatures derived (see table 1) are within
the range of other shell distributed molecules in IRC
+10216: 20-50 K (Cernicharo et al. 2000). For CH3CCH
and H2CS we observed only a few transitions with sim-
ilar upper level energies and it was not possible to con-
strain the rotational temperature which was assumed
to be 30 K.
In Table 1 we give the column densities1 (averaged
over the IRAM 30m beam, θMB=21-31” at λ 3 mm) of
the species detected for the first time (in bold) as well as
values or upper limits derived for other related species.
The column densities of the new species are in the range
1012-1013 cm−2. Note for example that both CH3CN
and CH3CCH have very similar column densities al-
though the lines of CH3CCH are some 30 times weaker
1 For CH2CN and H2CS, the two molecules which have two
interchangeable nuclei with non zero spin, we have assumed an
ortho-to-para ratio 3:1 when deriving their column densities.
3
Fig. 2 Rotational temperature diagram of CH2CHCN.
than those of CH3CN mostly because the different elec-
tric dipole moments (0.780 D vs. 3.925 D). The two re-
lated species CH3CN and CH2CN have similar column
densities which may indicate a common chemical ori-
gin (see Sec 4). The carbon-rich nature of IRC +10216
makes oxygen-bearing species to have a low abundance.
For example thioformaldehyde is more abundant than
formaldehyde despite the cosmic abundance of oxygen
being 50 times larger than that of sulphur.
4 Molecular synthesis in the outer envelope
In order to explain how the detected species are formed
we have performed a detailed chemical model of the
outer envelope. The chemical network consists of 385
gas phase species linked by 6547 reactions, whose rate
constants have been taken from the astrochemical databases
UMIST99 (Le Teuff et al. 2000) and osu.2003 (Smith
et al. 2004), recently revised2. The temperature and
density radial profiles as well as other physical param-
eters are taken from Agúndez & Cernicharo (2006).
The resulting abundance radial profiles for CH2CHCN,
CH2CN, CH3CCH, H2CS and related species are plot-
ted in Fig. 3. The model predicts that these four molecules
together with C3O (not plotted in Fig. 3, see Tenen-
baum et al. (2006) for a detailed discussion) are formed
with an extended shell-type distribution (r∼20”) via
several gas phase reactions.
Vinyl cyanide (CH2CHCN) is formed by the reac-
tion between CN and ethylene (C2H4), which is most
likely the main formation route in dark clouds (Herbst
& Leung 1990). Fortunately, the reaction has been stud-
ied in the laboratory and has been found to be very
rapid at low temperature (Sims et al. 1993) and to pro-
duce vinyl cyanide (Choi et al. 2004). The predicted
2 See http://www.udfa.net and
http://www.physics.ohio-state.edu/∼eric/research.html
Table 1 Column densities of some molecules in IRC +10216
Molecule Trot Ntot
(K) (cm−2)
observed calculated
This Work MI00
CH2CHCN 44 5.1(12) 1.4(13) 2.2(11)
CH3CN 40 3.0(13) 6.3(12) 6.8(12)
CH2CN 49 8.4(12) 4.6(12) 1.4(13)
CH3C3N 30a <1.3(12) 1.2(11) 1.4(12)
CH3CCH 30a 1.6(13) 1.1(12) 8.0(12)
CH3C4H 30a <9.7(12) 8.2(12) 9.0(12)
H2CO 28 5.4(12) [6] 2.8(12) –
H2CS 30a 1.0(13) 1.3(12) 4.4(11)
C2O 30a <7.0(12) [19] 7.0(11) –
C3O 27 2.6(12) [19] 2.8(12) –
C4O 30a <6.0(12) [19] – –
C5O 30a <3.0(12) [19] – –
Notes: a(b) refers to a×10b. The total column density across
the source Ntot is twice the radial column density Nrad. A
superscript ”a” indicates an assumed value.
References: (MI00) Chemical model of Millar et al. (2000).
column density agrees reasonably well with the obser-
vational value.
In our model both CH2CN and CH3CN are mostly
formed (>90 %) through the dissociative recombination
(DR) of CH3CNH
+
CH+
H2
−→CH
+
2
H2
−→CH
+
3
HCN
−→CH3CNH
+ e
−
−→CH2CN
e
−
−→CH3CN
whereas the major destruction process (>90 %) for both
species is photodissociation. The branching ratios in the
DR of CH3CNH
+ are not known and are assumed to be
equal. Assuming, as we do, that the photodissociation
rate of CH3CN and CH2CN are equal, the observed
CH3CN/CH2CN ratio suggests branching ratios in the
DR of CH3CNH
+ of 0.8 for the (CH3CN + H) channel
and 0.2 for (CH2CN + H2 or 2H). This estimate will
be strongly affected if the photodissociation rates of
CH3CN and CH2CN are very different but not if, as has
been suggested by Herbst & Leung (1990) and Turner
et al. (1990), CH2CN does indeed react with atomic
oxygen, the abundance of which is too low at the radius
where CH2CN is present.
The synthesis of CH3CCH involves ion-molecule re-
actions with the dissociative recombination of the C3H5
+
and C4H
+
5 ions as the last step. The model underpro-
duces it by an order of magnitude, probably due to un-
certainties and/or incompleteness in the chemical net-
work, which affect the formation rate of the last step
species C3H5
+ and C4H
+
5 . The heavier chain CH3C4H
is also predicted with a column density even higher than
4
Fig. 3 Abundances of CH2CHCN, CH2CN, CH3CCH and H2CS
(solid lines) and related species (dotted and dashed lines) given
by the chemical model, as a function of radius (bottom axis) and
angular distance (top axis) for an assumed stellar distance of 150
pc.
that of CH3CCH. The larger rotational partition func-
tion works against lines being detectable, but the higher
dipole moment (1.21 D vs. 0.78 D) could result in line
intensities similar to those of CH3CCH.
Thioformaldehyde is formed by the reaction S +
CH3 and in less extent through the DR of H3CS
+ (see
Agúndez & Cernicharo 2006 for a detailed discussion).
The order of magnitude of discrepancy between model
and observations reduces to a factor 4 with further non-
local non-LTE radiative transfer calculations. We note,
however, that a significant fraction of both H2CO and
H2CS could be formed in grain surfaces by hydrogena-
tion of CO and CS respectively.
The detection of C3O in IRC +10216 (Tenenbaum
et al. 2006), only previously detected in the dark clouds
TMC-1 (Brown et al. 1985) and Elias 18 (Trigilio et al.
2007), stress both the similarity between dark clouds
and C-rich CSEs chemistries and also the non-negligible
oxygen chemistry taking place in C-rich CSEs. Actually
astrochemical networks consider that C3O is formed
through DR of the molecular ions HnCmO
+ (see Fig. 4).
However, the C3O column density derived in IRC +10216
is an order of magnitude higher than calculated which
could imply additional chemical routes for its forma-
tion, e.g. neutral-neutral reactions of atomic oxygen
with carbon chain radicals such as those suggested in
Fig. 4.
Acknowledgements We would like to thank the IRAM 30m
telescope staff for their assistance during the observations. MA
acknowledges a grant from Spanish MEC: AP2003-4619.
C O3
HC O4
+
e
_
C H4 2
+
O COO CO
photon
C +CO2
l-C H3 3
+
C H2 2
+
C H2 3
+
HC O3
+
H C O2 3
+
H C O3 3
+
C4l-C3H
O O
????
Fig. 4 Scheme showing the main chemical formation routes to
C3O. From Tenenbaum et al. (2006).
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Understanding the chemical complexity in Circumstellar Envelopes of C-Rich AGB stars: The case of IRC +10216

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Understanding the chemical complexity in Circumstellar Envelopes of C-Rich AGB stars: The case of IRC +10216

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