The Astrophysical Journal, 743:36 (8pp), 2011 December 10
C 2011.
doi:10.1088/0004-637X/743/1/36
The American Astronomical Society. All rights reserved. Printed in the U.S.A.
CIRCUMSTELLAR ION–MOLECULE CHEMISTRY: OBSERVATIONS OF HCO+
IN THE ENVELOPES OF O-RICH STARS AND IRC + 10216
1
R. L. Pulliam1,2,3 , J. L. Edwards1,2,3 , and L. M. Ziurys1,2,3,4
Department of Astronomy, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721, USA
2 Department of Chemistry, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721, USA
3 Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721, USA
4 Arizona Radio Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721, USA
Received 2011 June 11; accepted 2011 August 31; published 2011 November 21
ABSTRACT
Millimeter-wave observations of HCO+ have been conducted toward oxygen-rich circumstellar envelopes, as well
as IRC + 10216, using the facilities of the Arizona Radio Observatory (ARO). The J = 1 → 0 and 2 → 1 transitions
of this molecule were measured with the ARO 12 m antenna, while the J = 3 → 2 and 4 → 3 lines were
observed using the ARO Sub-Millimeter Telescope. HCO+ was detected toward the supergiant NML Cyg and the
asymptotic giant branch (AGB) stars IK Tau, TX Cam, and W Hya in at least two transitions. The J = 2 → 1 and
3 → 2 lines of this ion were also detected toward IRC + 10216, confirming the identification of HCO+ in this
object. The line profiles measured for HCO+ toward NML Cyg consist of red- and blueshifted components,
suggesting a non-spherical shell. Based on a radiative transfer analysis, the abundances in the O-rich envelopes were
f(HCO+ /H2 ) ∼ 0.15–1.3 × 10−7 , with the AGB stars typically exhibiting the higher values. In IRC + 10216,
f(HCO+ /H2 ) ∼ 4.1 × 10−9 , lower than the O-rich counterparts. The abundances of HCO+ were also found to peak
at considerable distances from the star, indicative of an outer envelope molecule. Comparison with H2 O and CO,
the main precursor species, suggests that HCO+ is more prevalent in envelopes that have substantial water, but
CO also plays a role in its formation. The abundance of HCO+ appears to increase inversely with mass-loss rate,
provided the rate is >10−6 M yr−1 . The common appearance of HCO+ in circumstellar gas indicates that, at some
level, ion–molecule reactions influence the chemistry of evolved stellar envelopes.
Key words: astrochemistry – circumstellar matter – ISM: molecules – radio lines: stars – stars: individual
(NML Cyg) – supergiants
Online-only material: color figures
should be present in circumstellar envelopes, with abundances
as high as 10−6 , relative to H2 (Mamon et al. 1987; Glassgold
1996; Cordiner & Millar 2009). However, as noted by Agúndez
& Cernicharo (2006), the presence of HCO+ in IRC + 10216,
the best-studied circumstellar shell, has been debated for years.
In 1990, Lucas & Guélin conducted a sensitive search for the
J = 1 → 0 transition of this molecule at 89 GHz, using the
IRAM 30 m telescope. These authors detected a weak feature
with an intensity of about 20 mK at the HCO+ frequency, but
additional transitions were never found. Avery et al. (1994)
attempted to confirm the identification by searching for the J =
4 → 3 and the J = 3 → 2 transitions with the James Clark
Maxwell Telescope. Although a weak line (TA∗ ∼ 25 mK) was
detected at the frequency of the J = 4 → 3 transition of this
ion, no emission was observed for the J = 3 → 2 line. It was
therefore concluded that the one feature arose from a molecule
other than HCO+ (Agúndez & Cernicharo 2006).
In 2007, HCO+ was at last conclusively detected in circumstellar gas, notably in the envelope of the oxygen-rich supergiant
star, VY Canis Majoris, or VY CMa (Ziurys et al. 2007, 2009).
The J = 1 → 0, 2 → 1, 3 → 2, and 4 → 3 transitions of the
molecule were observed toward the envelope of this star using
the telescopes of the Arizona Radio Observatory (ARO): the
12 m and the Submillimeter Telescope (SMT). This new identification suggests that HCO+ might be detectable in other O-rich
shells. Consequently, we have conducted a search for this ion toward other oxygen-rich circumstellar envelopes, using the ARO
12 m and the SMT. We also conducted more sensitive measurements of HCO+ in IRC + 10216. We have detected this ion in
the envelopes of NML Cyg, TX Cam, IK Tau, and W Hya, and
1. INTRODUCTION
One of the fundamental questions concerning the chemistry in circumstellar envelopes of evolved stars is the role
of ion–molecule reactions (Glassgold 1996; Agúndez &
Cernicharo 2006; Cordiner & Millar 2009). Such reactions are
thought to be significant contributors to molecule formation in
the outer part of the envelope, where they are initiated by either cosmic rays or UV photoionization (Mamon et al. 1987).
Ions such as C2 H+2 and C2 H+3 , for example, have been proposed
to lead to the high abundances of the long-chain hydrocarbon
species found in C-rich shells (Nejad & Millar 1987; Millar
et al. 2000). The surprising presence in IRC + 10216 of negative ions composed of acetylenic chains, including C4 H− and
C6 H− (McCarthy et al. 2006), also emphasizes the importance
of ion–molecule processes in circumstellar gas. Yet positive ions
have remained elusive in circumstellar shells, despite years of
astronomical hunting.
One molecular ion that has been sought for decades in
circumstellar material has been HCO+ . This species is thought
to be critical in the ion–molecule scheme, because it is formed
directly from H+3 and CO (Herbst & Klemperer 1976):
H+3 + CO → HCO+ + H2 .
(1)
HCO+ in fact is observed in virtually all phases of the dense
interstellar medium, including molecular clouds (Buhl & Snyder
1970), planetary nebulae (Bachiller et al. 1997), and diffuse
clouds (Lucas & Liszt 1994), as well as in comets (Veal et al.
1997; Milam et al. 2004). Chemical models predict that HCO+
1
The Astrophysical Journal, 743:36 (8pp), 2011 December 10
Pulliam, Edwards, & Ziurys
Table 1
Properties of Observed Starsa
Source
α (B1950.0)
δ (B1950.0)
R∗
(cm)
T∗
(K)
Type
Reference
IRC + 10216
NML Cyg
TX Cam
IK Tau
W Hya
9h 45m 14.s 8
20h 44m 33.s 8
04h 56m 40.s 6
03h 50m 43.s 6
13h 46m 12.s 2
13◦ 30 40
39◦ 55 57
56◦ 06 28
11◦ 15 32
−28◦ 07 07
3.5 × 1013
2.6 × 1014
1.95 × 1013
2.12 × 1013
2.73 × 1013
2300
2500
2600
2100
2500
Carbon-rich AGB
Oxygen-rich red supergiant
Oxygen-rich AGB
Oxygen-rich AGB
Oxygen-rich AGB
Men’shchikov et al. (2001)
Monnier et al. (1997)
Olofsson et al. (1991)
Duari et al. (1999)
Justtanont et al. (2004, 2005)
Note. a Also see Ziurys et al. (2009).
Table 2
Line Parameters of Observed Transitions of HCO + a
Source
IRC + 10216
NML Cyg
IK Tau
TX Cam
W Hya
Transition
Frequency
(MHz)
ηb or ηc
θ b ( )
VLSR
(km s−1 )
TA∗ or TR ∗ b
(K)
ΔV1/2
(km s−1 )
J=1→0c
J=2→1d
J=3→2e
J=1→0c
89188.5
178375.0
267557.6
89188.5
0.89
0.68
0.78
0.89
70
35
28
70
J=2→1
178375.0
0.68
35
J=3→2f
J=4→3g
J=1→0
J=2→1
J=3→2
J=1→0
J=2→1
J=3→2f
J=1→0c
J=2→1
J=3→2f
267557.6
356734.2
89188.5
178375.0
267557.6
89188.53
178375.0
267557.6
89188.5
178375.0
267557.6
0.78
0.70
0.89
0.68
0.78
0.89
0.68
0.78
0.89
0.68
0.78
28
21
70
35
28
70
35
28
70
35
28
−26.2 (3.4)
∼−26
−25.9 (2.2)
−20.9 (3.4)
10.9 (3.4)
−20.7 (3.4)
14.1 (3.4)
−22.3 (2.2)
−5.0
34.8 (6.7)
34.5
∼34
10.1 (6.7)
9.2
∼9
41.2 (3.4)
41.0
41.1 (2.4)
0.012 (0.004)
0.005 (0.003)
0.004 (0.001)
0.005 (0.002)
0.004 (0.002)
0.006 (0.002)
0.006 (0.002)
0.004 (0.001)
...
0.005 (0.002)
<0.015
0.003 (0.002)
0.003 (0.002)
<0.01
∼0.001
0.003 (0.001)
<0.01
0.003 (0.001)
30.2 (3.4)
∼30
26.9 (2.2)
14.0 (6.7)
16.8 (6.7)
15.0 (6.8)
20.0 (6.8)
∼15
...
33.6 (6.7)
...
∼35
26.9 (6.7)
...
∼30
13.5 (3.4)
...
17.9 (2.4)
Notes.
a Measured with 2 MHz resolution unless otherwise noted.
b T ∗ applies to the SMT data and T ∗ to 12 m data.
A
R
c Measured with 1 MHz resolution.
d Partially blended with U-line.
e Partially blended with CCC13 CH.
f On shoulder of SO : J
2 Ka,Kc = 133,11 → 132,12 transition; blueshifted component only for NML Cyg.
g Contaminated by the J
Ka,Kc = 104,6 → 103,7 transition of SO2 .
have confirmed its presence in IRC + 10216. From a radiative
transfer analysis of the data, abundances and source sizes for
HCO+ have been determined. Here we present our results and its
implications for ion–molecule chemistry in circumstellar gas.
ηc is the “corrected” beam efficiency. The backends consisted
of two 512 channel filter banks with 1 and 2 MHz resolution,
respectively, operated in parallel mode (2 × 256 channels each).
Observations at 1 mm and 0.8 mm were conducted at the SMT.
The dual polarization, 1 mm receiver utilized ALMA Band 6
SBS mixers with image rejection between 12 and 18 dB, intrinsic
in the device architecture. Measurements at 0.8 mm were carried
out using a double-sideband, dual-polarization SIS receiver. The
temperature scale at the SMT is TA∗ , the chopper wheel-corrected
antenna temperature. The radiation temperature is then defined
as TR = TA∗ /ηb , where ηb is the main-beam efficiency. The
backends consisted of a 2048 channel filter bank with 1 MHz
resolution, operated in parallel mode (2 × 1024 channels) for
the two polarizations.
All observations were conducted in beam-switching mode,
with a ±2 beam throw. Local oscillator shifts were conducted
at every observing frequency to protect against image contamination. The sources observed, their coordinates, and relevant
stellar parameters are listed in Table 1. Telescope beam sizes,
beam efficiencies, and observing frequencies are presented in
Table 2.
2. OBSERVATIONS
The measurements were conducted between 2006 June and
2009 May using the ARO SMT at Mt. Graham, AZ and
the ARO 12 m telescope on Kitt Peak. The receivers used
at the 12 m telescope consisted of dual-polarization, singlesideband SIS mixers, one at 2 mm and the other at 3 mm
(3 mm HI: 82–115 GHz). Image rejection, achieved by tuning
the mixer backshorts, was 18 dB at 3 mm and between 12
and 18 dB at 2 mm, respectively. In 2009 May, additional
observations for W Hya were conducted at 3 mm using a new
sideband separating (SBS) receiver employing ALMA Band 3
mixers from the Herzberg Institute for Astrophysics (HIA). The
intensity scale at the 12 m, given as TR∗ , is determined by the
chopper-wheel method, corrected for forward spillover losses.
The radiation temperature is defined as TR = TR∗ /ηc , where
2
The Astrophysical Journal, 743:36 (8pp), 2011 December 10
Pulliam, Edwards, & Ziurys
The Astrophysical Journal, 743:36 (8pp), 2011 December 10
Pulliam, Edwards, & Ziurys
The Astrophysical Journal, 743:36 (8pp), 2011 December 10
Pulliam, Edwards, & Ziurys
4. ANALYSIS
The molecular abundances, relative to H2 , and spatial distributions for HCO+ were determined by modeling the line profiles
with the non-LTE radiative transfer code of Bieging & Tafalla
(1993). This code assumes a spherically symmetric envelope
and utilizes the Sobolev approximation, solving the statistical
equilibrium equations for the rotational energy levels of a given
molecule. The sources of excitation include collisions and radiation from dust. The model convolves the molecular distribution
to the object distance and the telescope beam size. A density and
temperature profile must be specified. This model was used for
all sources except NML Cyg, where a non-spherical geometry
had to be assumed. More details are given in Ziurys et al. (2009).
The modeling parameters used for NML Cyg and W Hya were
taken from Ziurys et al. (2009). For IK Tau, new observations are
available from Decin et al (2010a). The gas kinetic temperature
profile was therefore inferred from these measurements:
γ
2 × 1015
TK (r) = 250
,
(2)
r
where gamma is equal to 0.6. Because the star is comparable in
size and mass-loss rate to IK Tau, the same temperature profile
was used for TX Cam. For IRC + 10216, the model parameters
were derived from Keady et al. (1988) and Crosas & Menten
(1997). Collisional excitation rates for HCO+ were taken from
Buffa et al. (2009) and Flower (1999). Vibrational excitation by
infrared dust emission of the first quantum of the low-energy
bending mode of HCO+ was considered.
HCO+ emission was modeled for all sources except NML Cyg
using a shell distribution, as described by
2
− r − rshell r
outer ,
f (r) = f0 e
(3)
Figure 4. Spectra of the J = 1 → 0, 2 → 1, and 3 → 2 transitions of HCO+
observed toward IRC + 10216 at 89 GHz, 178 GHz, and 267 GHz using the
ARO 12 m at 2 and 3 mm and ARO SMT at 1 mm. The radiative transfer model
fits are overlaid in red. Temperature scales are TR∗ for the 12 m data and TA∗
for the SMT spectra. Spectral resolution is 2 MHz (SMT data are smoothed),
except for the J = 1 → 0 line, where it is 1 MHz. The J = 2 → 1 transition is
partially contaminated by an unidentified feature toward higher LSR velocity,
while the J = 3 → 2 line is partly blended with the spin doublets of the N =
29 → 28 transition of CCC13 CH.
(A color version of this figure is available in the online journal.)
where router is the distance from rshell at which the abundance falls
by a factor of 1/e, and f0 is the peak abundance, occurring at rshell ,
relative to H2. To achieve the best fits to the observed spectra,
the values of rshell and router were cycled in 1 –3 increments
for a range of abundance values. The modeling was initiated
at a distance rinner , at roughly the radius of the dust formation
zone. The peak abundances f0 , derived from the modeling, are
presented in Table 3, along with the values for rHCO+ = rshell +
router , the radial distance at which this abundance falls by a
factor of 1/e. Attempts to fit the data with a simple spherical
distribution failed. Model fitting parameters are summarized in
Table 4. Predicted profiles from the modeling are superimposed
over the HCO+ spectra in Figures 1, 2 and 4, as mentioned.
The blue- and redshifted components in NML Cyg were
analyzed individually as separate outflows. As suggested by
the maser emission (e.g., Richards et al. 1996; Nagayama et al.
2008), these components have a bipolar geometry. In modeling
these flows, the same spherically symmetric envelope was
employed in the statistical equilibrium calculation. However,
instead of convolving the entire spherical envelope with the
telescope beam, only conical sections of the envelope are
integrated (see Ziurys et al. 2007, 2009). For the analysis, a
bipolar geometry was assumed, oriented 8◦ relative to the line
of sight (Nagayama et al. 2008). Both flows were best fit with a
cone angle of 60◦ and expansion velocities of 26 km s−1 for the
blueshifted component and 30 km s−1 for the redshifted flow.
The resulting peak abundances and radial distance from the star
where this abundance falls by a factor of 1/e (rHCO+ ) are listed
in Table 3, as well.
SO2 and HNC in VY CMa also closely reassemble the composite
spectrum for the OH masers in this object (see Ziurys et al.
2007, 2009). It is thought that the larger-scale structure of the
individual outflows is traced in thermal molecular emission,
while the masers arise from the ejecta close to the star (Ziurys
et al. 2007, 2009; Tenenbaum et al. 2010). It is therefore likely
that the double-peaked HCO+ profiles in NML Cyg trace similar
episodic outflows. The appearance of these velocity components
in HCO+ , but not obviously in CO or HCN, must be a chemical
effect.
The three transitions observed for HCO+ in IRC + 10216 are
shown in Figure 4. The J = 1 → 0 transition appears as a single
feature, as also observed by Lucas & Guelin (1990). The J = 2 →
1 line, in contrast, is blended with what appears to be a U-line
at higher velocity, while the J = 3 → 2 transition is partially
blended with CCC13 CH, as previously discussed. Model fits
to the data are superimposed on the spectra (see Section 4).
Detection of the J = 2 → 1 and J = 3 → 2 transitions of HCO+
confirms the presence of this molecule in IRC + 10216; the J =
3 → 2 line was previously reported by Tenenbaum et al. (2010)
in their spectral line survey.
5
The Astrophysical Journal, 743:36 (8pp), 2011 December 10
Pulliam, Edwards, & Ziurys
Table 3
HCO+ , CO, and H2 O Abundances and Distributions in Observed Starsa
Source
VY CMa
NML Cyg
TX Cam
IK Tau
W Hya
IRC + 10216
rHCO + ( )
f [HCO+ ]/[H2 ]
10−9 b
5×
2 × 10−8 e
1 × 10−8 f
2.8 × 10−8 e
1.5 × 10−8 f
1.3 × 10−7
4.4 × 10−8
2.4 × 10−8
4.1 × 10−9
rCO ( )
f [CO]/[H2 ]
10−5 b
4×
2 × 10−4 e
8 × 10−5 f
8 × 10−5 b
...
3.2 × 10−4 b
1–5 × 10−4 b,i,j
2–3 × 10−4 b,l
1.1 × 10−3 j
10
9
10
5
4
13
12
11
45
b
6
4.5 e
9f
14 b
...
50 b , 39 g
20 b , 25g , 30i,j
7–13 b,l
50 m
rH2 O ( )
f [H2 O]/[H2 ]
10−3 c,d
0.4–1 ×
...
...
0.4–1 × 10−4 c
...
3.0 × 10−4 h
0.7–3.5 × 10−4 h,k
1–2 × 10−3 h,l
2.4 × 10−6 n
>3 c
...
...
>2.5 c
...
4h
4h
>2 l,h
18 n
Notes.
a HCO+ abundance is f and r
0
HCO+ is rshell + router , from modeling of line profiles (see the text).
b From Ziurys et al. (2009); spherical flow for VY CMa.
c Zubko et al. (2004); assumes ortho:para = 1:1.
d Polehampton et al. (2010).
e Redshifted flow; VY CMa data from Ziurys et al. (2009).
f Blueshifted flow; VY CMa data from Ziurys et al. (2009).
g Castro-Carrizo et al. (2010).
h Maercker et al. (2008).
i Decin et al. (2010a).
j Teyssier et al. (2006).
k Decin et al. (2010b).
l Justtanont et al. (2005).
m Guelin et al. (1997).
n Hasegawa et al. (2006); based on ortho-H O only.
2
Table 4
Modeling Parameters
Source
NML Cyg
TX Cam
Distance
(pc)
Mass-loss Rate
(M yr−1 )
v exp
(km s−1 )
rinner a
(cm)
Tdust
(K)
1700
2.0 × 10−4
31
5 × 1015
400
390
4.4 ×
10−6
19
2×
1014
4.6 ×
10−6
18
2×
1014
1000
1000
IK Tau
300
W Hya
78
2.5 × 10−7
8
2 × 1014
1000
150
3.5 × 10−5
15
4 × 1014
400
IRC + 10216
Tgas
(K)
16 γ
270 1×10
r
15 γ
250 2×10
r
15 γ
250 2×10
r
15 γ
425 1×10
r
15 γ
200 1×10
r
γ
b
0.5
c
0.6
c
0.6
d
0.43
e
0.73
Notes.
a r
inner ∼ rdust.
b From Zubko et al. (2004).
c For IK Tau; Decin et al. (2010a).
d From Justtanont et al. (2005).
e From Keady et al. (1988) and Crosas & Menten (1997).
abundance of ∼2.4 × 10−8 . Therefore, the general abundance of
HCO+ in O-rich AGB envelopes, based on this limited sample,
is f0 ∼ 0.24–1.3 × 10−7 .
In the supergiant NML Cyg, the abundance of HCO+ is
f0 ∼ 1.5–2.8 × 10−8 , considering both red- and blueshifted
flows, with an average value of 2.2 × 10−8 . These abundances
are somewhat lower than those found in TX Cam and IK Tau,
and are comparable to those measured in the red- and blueshifted
outflows of VY CMa (1–2 × 10−8 ).
NML Cyg is one of the largest (40 M ) and most luminous M stars in the Galaxy, with a mass-loss rate of 0.6–7 ×
10−4 M yr−1 (Schuster et al. 2009; Zubko et al. 2004), very
similar in characteristics to VY CMa. Neither NML Cyg nor
VY CMa are thought to have spherically symmetric envelopes
(Schuster et al. 2006; Ziurys et al. 2007). Infrared studies have
suggested a bean-like shape for the envelope of NML Cyg on an
5. DISCUSSION
5.1 HCO+ in Oxygen-rich Environments:
Supergiants versus AGB Stars
The derived abundances for the four oxygen-rich envelopes
fall in the range f0 ∼ 1.5 × 10−8 –1.3 × 10−7 , a variation of
about a factor of 10. As shown in Table 3, the sources with the
highest HCO+ abundance are TX Cam and IK Tau, with f0 ∼
0.4–1.3 × 10−7 . These objects are both thought to be in the early
asymptotic giant branch (E-AGB) phase of stellar evolution
and have not yet undergone so-called third dredge-up (Herwig
2005). They also have similar mass-loss rates of 4.4–4.6 ×
10−6 M yr−1 . The only substantial difference between these
stars is that TX Cam is thought to have a spherical shell, while
that of IK Tau is asymmetric (Decin et al. 2010a). W Hya,
also thought to be an E-AGB star, has a somewhat lower HCO+
6
The Astrophysical Journal, 743:36 (8pp), 2011 December 10
Pulliam, Edwards, & Ziurys
Table 5
Comparison of Observations and Theoretical Predictions
Source
VY CMa
NML Cyg
TX Cam
IK Tau
W Hya
IRC + 10216
f [HCO+ ]/[H2 ]
Observations
1–2 × 10−8 b
1.5–2.8 × 10−8 b
1.3 × 10−7
4.4 × 10−8
2.4 × 10−8
4.1 × 10−9
rHCO+ (cm)a
Observations
f [HCO+ ]/[H2 ]
Models
rHCO+ (cm)a
Models
1–2 × 1017
9.5 × 1016
5.8 × 1016
3.9 × 1016
1.1 × 1016
7.6 × 1016
2 × 10−8 c
2 × 10−8 c
0.7–2 × 10−7 c,d
0.4–2 × 10−7 c,d
10−6 c
0.45–3.1 × 10−9 e,f
4 × 1017 c
4 × 1017 c
3–4 × 1016 c,d
3 × 1016 c
1016 c
3 × 1016 e
Notes.
a Radius at peak abundance, r
shell .
b For red and blue-shifted flows.
c Mamon et al. (1987).
d Willacy & Millar (1997).
e Agúndez & Cernicharo (2006).
f Cordiner & Millar (2009).
the bulk of the emission lies within 50 (Guelin et al. 1997). To
our knowledge, HCO+ has not been detected in any other C-rich
shell to date.
Identification of HCO+ in IRC + 10216 indicates that
ion–molecule reactions play a role in the chemistry of the
outer shell. This possibility has long been debated (Agúndez
& Cernicharo 2006). Theoretical calculations in the past have
suggested that ion–molecule reactions lead to the carbon chain
compounds in IRC + 10216, via such species as C2 H+2 and C2 H+3
(e.g., Nejad & Millar 1987). Other models have focused on
radical–radical processes producing such chains (Cordiner &
Millar 2009). If HCO+ is an indicator of the relative amounts
of positive ions in the outer shell of IRC + 10216, their abundances are likely to be small. Free radicals, in contrast, have
concentrations that are considerably higher. The column density of CCH, for example, is 5 × 1015 cm−2 , while that of C4 H
is 3 × 1015 cm−2 (Cernicharo et al. 2000). Atomic carbon is
prevalent as well, with 4–7 × 1015 cm−2 in IRC + 10216, with
emission extending out to r ∼ 45 (Keene et al. 1993). The
peak column density of HCO+ is ∼4 × 1012 cm−2 –orders of
magnitude lower. This comparison would seem to indicate that
radical processes dominate over ion–molecule reactions in the
outer shell chemistry of IRC + 10216.
arcsecond scale, possibly caused by a nearby OB association,
Cyg OB2. More recent observations show IR emission correlating with the bipolar axis traced by water and OH masers
(Schuster et al. 2009). VY CMa also has a very irregularly shaped envelope, resulting from random mass-loss events
(Smith et al. 2001; Humphreys et al. 2005). The shell morphology of both stars varies considerably from those of AGB stars.
In all cases, the distribution of HCO+ extends out to large
stellar radii in the O-rich shells, typically r ∼ 1–9 × 1016 cm (see
Table 5). These radii correspond to approximately 350–2000 R∗ .
HCO+ is clearly an outer envelope molecule.
As described by Mamon et al. (1987), HCO+ is likely formed
in the outer shell from two primary parent species: CO and
H2 O. In order to examine possible correlations, the abundances
of these two species are tabulated in Table 3, along with their
spatial extents, as best as can be determined from the current
literature. As the table shows, the abundance of water is highest
in VY CMa and W Hya, with f ∼ 10−3 , although it is quite
prevalent (∼10−4 ) in all the O-rich shells studied here. Clearly
water impacts the HCO+ production. CO is somewhat more
abundant in the three AGB stars, with f ∼ 1–5 × 10−4 —about
a factor of 5–10 higher than in the supergiants. The spatial
correlation of HCO+ with CO is quite striking, as well: see
Table 3. For water, such a relationship is more difficult to define
because its transitions are not nearly as accessible as CO, and
hence spatial maps are generally not available (e.g., Justtanont
et al. 2005; Zubko et al. 2004). Based on this limited sample,
the abundance of HCO+ appears to loosely track both H2 O and
CO. HCO+ may be somewhat more abundant in the envelopes of
AGB stars TX Cam and IK Tau because they have additional CO.
5.3. Chemical Pathways to Circumstellar HCO+
Chemical models suggest that HCO+ is formed in the outer
envelopes of stars via two primary synthetic routes (Mamon
et al. 1987; Willacy & Millar 1997; Agúndez & Cernicharo
2006). One pathway is from the simple protonation reaction of
CO, as is thought to occur in molecular clouds; see Equation (1).
Another pathway is via H2 O:
5.2. HCO+ in IRC + 10216
C+ + H2 O → HCO+ + H.
The detection of the J = 2 → 1 and J = 3 → 2 transitions of
HCO+ in IRC + 10216 ends the debate concerning the presence
of this molecule in this C-rich shell. The abundance of HCO+ in
this source was determined to be 4.1 × 10−9 —about a factor of
10–30 less prevalent than in IK Tau and TX Cam, also AGB stars.
The water abundance in IRC + 10216 is also correspondingly
low at f ∼ 2 × 10−6 (Hasegawa et al. 2006), although CO is
very prevalent with f ∼ 10−3 (Teyssier et al. 2006). Such results
suggest that water is an important contributor to the HCO+
abundance in circumstellar envelopes. In IRC + 10216, HCO+
does appear to spatially correlate with CO, with r ∼ 45 . CO
has been observed out to a radius of 120 in this envelope, but
(4)
The destruction of HCO+ is thought to occur via electron
dissociative recombination, or proton transfer to water to create
H3 O + (Mamon et al. 1987). Based on these pathways, fractional
abundances for HCO+ have been computed by several models
and are listed for comparison in Table 5.
Willacy & Millar (1997) have calculated abundances for
HCO+ in TX Cam and IK Tau of f ∼ 6.6 × 10−8 and 3.9 ×
10−8 , respectively—within a factor of two of the observed
abundances (see Table 5). These authors also predict an outer
radius, as defined here, of about 3 × 1016 cm, close to the
observed values of 3.9–5.8 × 1016 cm. Mamon et al. (1987)
7
The Astrophysical Journal, 743:36 (8pp), 2011 December 10
Pulliam, Edwards, & Ziurys
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also derived abundances for HCO in O-rich stars, but as a
function of mass-loss rate. These authors suggest that as mass
loss of a star increases, shielding of CO also increases, leading
to less C + and other ions. They predict that stars with lower
mass-loss rates have higher HCO+ abundances. For stars with
rates ∼4 × 10−6 M yr−1 , such as TX Cam and IK Tau, Mamon
et al. calculated f (HCO+ ) ∼ 2 × 10−7 ; for higher rates near
10−4 M yr−1 , as applies to supergiants, f (HCO+ ) ∼ 2 × 10−8 .
For a mass-loss rate of 10−7 M yr−1 , similar to W Hya, an
HCO+ abundance of ∼10−6 was computed.
With the exception of W Hya, the predictions of Mamon
et al. (1987) follow the observational trend, as shown in Table 5.
W Hya, in contrast, has an abundance that is about a factor of
50 lower than the model calculation. It could be at very low
mass-loss rates (<10−6 M yr−1 ), such as that of W Hya, the
lack of self-shielding is overwhelmed by the lack of material,
and molecular production is generally reduced. A larger sample
of stars needs to be studied to fully explore this correlation,
although the current observations are certainly suggestive.
Agúndez & Cernicharo (2006) predict an abundance of
f ∼ 4.5 × 10−10 for HCO+ in IRC + 10216—a factor of 10 less
than the observed value. Cordiner and Millar (2009) estimate
f ∼ 3.1 × 10−9 , in very good agreement with the observations.
The outer radius calculated by Agúndez & Cernicharo (2006) is
within a factor of two of that observed, as well. Overall, there
is good agreement between theory and observation in this case.
Other C-rich envelopes need to be investigated, however, before
the chemistry of HCO+ in these types of objects can be fully
evaluated.
6. CONCLUSION
This study has shown that HCO+ appears to be a common
species in oxygen-rich circumstellar shells, both of supergiants
and AGB stars. It is also present in the carbon-rich envelope of
IRC + 10216, but in much lower concentration. The abundance
of HCO+ appears to be best correlated with that of water in
stellar envelopes; however, its production from CO must also
play a significant role. The abundance of HCO+ also appears to
be inversely proportional to the mass-loss rate, as long as this
rate is >10−6 M yr−1 . Additional sources warrant investigation
to further examine such trends. These observations also suggest
that ion–molecule reactions must be occurring in circumstellar
gas at some level. Other ions such as H3 O + may be detectable
in O-rich shells.
This work is supported by NSF grant AST-09-06534. The
authors thank the staff of the ARO for assistance in conducting
these observations.
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