The Astrophysical Journal, 725:561–570, 2010 December 10
C 2010.
doi:10.1088/0004-637X/725/1/561
The American Astronomical Society. All rights reserved. Printed in the U.S.A.
OBSERVATIONS OF THE [HNCS]/[HSCN] RATIO IN Sgr B2 AND TMC-1: EVIDENCE FOR
LOW-TEMPERATURE GAS-PHASE CHEMISTRY
G. R. Adande1 , D. T. Halfen1 , L. M. Ziurys1 , D. Quan2 , and E. Herbst3
1
Departments of Chemistry and Astronomy, Arizona Radio Observatory, and Steward Observatory, University of Arizona,
933 N. Cherry Avenue, Tucson, AZ 85721, USA
2 The Chemical Physics Program, The Ohio State University, Columbus, OH 43210, USA
3 Departments of Physics, Chemistry, and Astronomy, The Ohio State University, Columbus, OH 43210, USA
0,7
→ 60,6 and 80,8 → 70,7 transitions
of both HNCS and HSCN were detected in TMC-1, the first identification of either molecule in a cold, dark cloud.
Emission from HNCS and HSCN was found to be extended over the Sgr B2 cloud, with a single velocity component
and a linewidth of ∼20–25 km s−1 . Column densities derived for HSCN in Sgr B2 are typically Ntot ∼ (0.2–1) ×
1013 cm−2 , with Ntot ∼ (0.8–5) × 1013 cm−2 for the more stable isomer, HNCS. In TMC-1, these species have similar
column densities of (6–8) × 1010 cm−2 . The [HNCS]/[HSCN] abundance ratio ranges from 2 to 7 in Sgr B2, with a
value of ∼1 in TMC-1. In contrast, the [HNCO]/[HOCN] ratio in Sgr B2 is ∼110–250. Gas-grain chemical models
do not reproduce the observed abundances of the sulfur isomers in either source. Given the energy difference of over
3200 K between HNCS and HSCN, these observations suggest that both molecules are produced from gas-phase,
ion–molecule chemistry with a common precursor, HNCSH+ . The oxygen analogs, in contrast, probably have a
more complex chemical network, perhaps involving the H2 NCO+ precursor, which preferentially leads to HNCO.
Key words: astrochemistry – ISM: abundances – ISM: individual objects (Sgr B2, TMC-1) – ISM: molecules –
radio lines: ISM
(2010) recalculated rate coefficients for the rotational excitation
of HNC, pointing out that [HCN]/[HNC] ratios in dark clouds
may be higher than the previous Large Velocity Gradient (LVG)
estimates. In a similar fashion, measurements of the relative
HCO+ and HOC+ abundances have led to a better understanding of the dynamics of the various formation, destruction, and
isomerization pathways for these species (Herbst & Woon 1996;
Li et al. 2008; Savage & Ziurys 2004).
The sulfur-containing molecules HNCS and HSCN constitute
a new isomer pair. HNCS, which is the most stable of the
possible CHNS isomers, and is slightly bent with a 1 A ground
state. The species was first detected by Frerking et al. (1979) in
Sgr B2(OH). HSCN, which lies over 3200 K higher in energy
than HNCS and also has a bent structure (Wierzejewska &
Moc 2003), is a new interstellar molecule recently identified in
Sgr B2(N) by Halfen et al. (2009). HSCN is highly unstable, and
has been only detected on Earth when formed by UV-photolysis
of HNCS (Wierzejewska & Mielke 2001) or in a low-pressure
discharge (Brünken et al. 2009b). In Sgr B2, in contrast, HSCN is
about a factor of 3 less abundant than HNCS (Halfen et al. 2009),
illustrating the dominance of non-equilibrium, kinetically driven
chemistry in dense clouds. The other possible isomers HCNS
and HSNC lie 17,300 and 18,100 K above HNCS in energy
(Wierzejewska & Moc 2003); however, these two species have
yet to be detected in the ISM.
Shortly after its spectrum was measured by millimeterwave laboratory techniques (Brünken et al. 2009a), HOCN,
the oxygen analog of HSCN, was identified in Sgr B2(N),
dark clouds and low-mass protostars (Brünken et al. 2009a;
Marcelino et al. 2010). HOCN is one of the higher-energy
(12,300 K; Schuurman et al. 2004) isomers of HNCO, an
1. INTRODUCTION
The study of the relative abundances of metastable isomers
can give important insight into the chemical processes occurring in the interstellar medium (ISM; Green & Herbst 1979).
Observations of abundance ratios between isomer pairs provide
direct, quantitative tests of chemical models. Kinetic information can also be retrieved from these data, such as branching ratios in a given reaction. Certainly much has been learned about
chemical processes in molecular clouds from measurements of
the [HCN]/[HNC] or [HCO+ ]/[HOC+ ] ratios (Goldsmith et al.
1981; Schilke et al. 1992; Apponi et al. 1999; Apponi & Ziurys
1997; Savage & Ziurys 2004).
Observations of HCN and HNC (Goldsmith et al. 1981;
Schilke et al. 1992; Hirota et al. 1998) illustrate what can be
deduced from examining abundance ratios. HNC lies 7400 K
higher in energy than the more stable form HCN, with a barrier
to conversion of 24,100 K (van Mourik et al. 2001). Studies of
this isomer pair have confirmed the importance of gas-phase dissociative recombination reactions in the ISM, and the existence
of a common ionic precursor HNCH+ , which was subsequently
detected in molecular clouds (Ziurys & Turner 1986). Moreover,
the interpretation of the temperature dependence of the [HCN]/
[HNC] ratio is still the topic of theoretical studies. Ishii et al.
(2006), for example, have used quantum chemical calculations
to estimate a branching ratio of 57%:43% (HCN:HNC) from the
dissociative recombination of HNCH+ . Talbi & Herbst (1998)
have postulated that the high abundance of HNC in dark clouds
cannot be explained uniquely by ion–molecule processes; rather,
additional neutral–neutral reactions involving hydrogen transfer must be taken into account. More recently, Sarrasin et al.
561
562
ADANDE ET AL.
Vol. 725
Table 1
Molecular Transitions Observed in Sgr B2 and TMC-1
Species
Frequency (MHz)
HSCN
80283.16
91750.63
103217.47
82101.80
93830.05
105558.07
83900.57
87925.18
HNCS
HOCN
HNCO
Transition JKa,Kc
→ JKa,Kc
70,7
80,8
90,9
70,7
80,8
90,9
40,4
40,4
→ 60,6
→ 70,7
→ 80,8
→ 60,6
→ 70,7
→ 80,8
→ 30,3
→ 30,3
abundant and widespread interstellar molecule (Martı́n et al.
2009). The other two isomers, HCNO and HONC, lie at energies
34,500 K and 42,200 K above that of HNCO (Schuurman et al.
2004). An [HNCO]/[HOCN] ratio of 120–350 was found for
selected positions across Sgr B2 (Brünken et al. 2010; Marcelino
et al. 2010), in marked contrast to the [HNCS]/[HSCN] ratio
of ∼3 (Halfen et al. 2009). The [HNCO]/[HOCN] ratio in
dark clouds and low-mass protostars has been measured to be
∼20–60, suggesting a temperature dependence analogous to the
[HCN]/[HNC] system. It should also be noted that HCNO has
recently been detected in molecular clouds, as well (Marcelino
et al. 2009).
It is possible that several formation schemes are affecting the
isomer ratios in different regions of any given molecular cloud,
as chemistry is often spatially dependent. In order to gain a better
understanding of the formation and destruction mechanisms of
HNCS and HSCN, we have conducted mapping observations
of the JKa,Kc = 80,8 → 70,7 and 90,9 → 80,8 transitions of these
molecules across Sgr B2, using the 12 m telescope of the Arizona
Radio Observatory (ARO). We also detected both sulfur species
in TMC-1. In addition, we have mapped the JKa,Kc = 40,3 →
30,3 transition of both HNCO and HOCN over a similar region
in Sgr B2. From these measurements, abundance ratios have
been established for these isomer systems, which were then
compared with gas-grain chemical models. Here we report our
observations, their analysis, and the implications of the results
for interstellar chemistry.
2. OBSERVATIONS
The data were taken during 2009 April and 2010 February at
the ARO 12 m telescope on Kitt Peak, Arizona. A dual-channel
ALMA-type Band 3 receiver (83–116 GHz) using sidebandseparating mixers was primarily used for this study. Image rejection was 16 dB, inherent in the mixer architecture. Measurements below 83 GHz were conducted with a dual channel,
SIS receiver (“3 mm LOW”), where the mixer backshorts were
tuned to suppress the image sideband, with typical rejection of
>20 dB. Local oscillator shifts of 20 MHz were performed at
every frequency to check for image sideband contamination.
The temperature scale at the 12 m is determined by the chopper
wheel method, corrected for spillover losses, and given as TR∗ .
The main beam brightness temperature TR is then TR = TR∗ /ηc ,
where ηc is the corrected beam efficiency. The backends used
were 256 channel filter banks of 1 and 2 MHz, respectively,
operating in parallel mode (2 × 128) to accommodate the two
perpendicular polarizations. For the TMC-1 observations, filter banks with 100 and 250 kHz resolution were employed, as
well as an autocorrelator (MAC) with a spectral resolution of
97.6 kHz.
El (K)
μ2 S (D2 )
ηc
θ b (arcsec)
11.57
15.42
19.83
11.83
15.77
20.28
6.04
6.33
85.75
98.00
110.25
18.83
21.52
24.21
54.72
10.27
0.92
0.88
0.85
0.92
0.88
0.85
0.91
0.90
78
68
61
77
67
60
75
72
The frequencies for each transition measured toward Sgr B2
and TMC-1 are listed in Table 1, as well as the line strength and
the energy above ground state. Additionally, the beam size and
main beam efficiencies ηc at the respected frequencies are
provided. In Sgr B2, the JKa,Kc = 80,8 → 70,7 and 90,9 → 80,8
transitions of HNCS and HSCN and the JKa,Kc = 40,3 → 30,3
line of HNCO and HOCN were mapped on a 6 × 3 grid with
60 spacing in right ascension and declination, offset from the
(0,0) position, centered at α = 17h 44m 11.s 0, δ = −28◦ 22 00
(B1950.0), near Sgr B2(M). Position-switching mode with a 30
offset in azimuth was used. The data were slightly undersampled
in this study, as the 12 m beam size ranges from 60 to 75
from 84 to 105 GHz (see Table 1). Toward TMC-1, HNCS and
HSCN were observed at a single position at α = 04h 38m 38.s 6,
δ = 25◦ 35 45 (B1950.0).
3. RESULTS
A summary of the observation of the JKa,Kc = 80,8 →
70,7 and 90,9 → 80,8 transitions of HNCS and HSCN at each
map position in Sgr B2 is given in Table 2. Table 3 lists the
mapping observations for the JKa,Kc = 40,4 → 30,3 transition
of HNCO and HOCN. These tables present the line parameters
(TR∗ , ΔV1/2 , and VLSR ) for each species at a given position.
Typically, the line widths are between 19.0 and 29.0 km s−1 ,
with an average value of 24 ± 3 km s−1, while the LSR
velocities change systematically from north to south across the
Sgr B2 cloud. This effect is obvious in Figure 1, which displays
the spectra of the JKa,Kc = 40,4 → 30,3 transitions of HNCO
(left panels) and HOCN (right panels) along the north–south
axis. The VLSR values are ∼64–68 km s−1 near Sgr B2(2N)
and Sgr B2(N), decrease to ∼58–64 km s−1 near Sgr B2(M),
falling to ∼51–58 km s−1 in the southern regions. This gradient
has been seen previously in other molecules, such as N2 O
and HC3 N, and the LSR velocities of HNCS and the other
three species agree well with the past observations (de Vicente
et al. 2000; Halfen et al. 2001). This gradient is indicative of
internal motion in the cloud, perhaps rotation or multiple cloud
structure.
Figure 2 shows the spectra taken at each position for the
JKa,Kc = 80,8 → 70,7 transition for HNCS (a) and HSCN (b).
The dashed line indicates VLSR = 62 km s−1 . The (0,0) position
on these maps is at α = 17h 44m 11.s 0, δ = −28◦ 22 00 (B1950.0),
near Sgr B2(M). From these data, it is quite obvious that
both sulfur-containing isomers have emission at almost every
position, except at the map edges. Given longer integrations,
the molecules are likely to be detected at those positions and
beyond. There is also an obvious velocity shift from Sgr B2(S)
to Sgr B2(2N), as previously discussed.
As illustrated in Table 2, the linewidths and velocities at each
position agree very well between HNCS and HSCN, clearly
No. 1, 2010
OBSERVATIONS OF THE [HNCS]/[HSCN] RATIO IN Sgr B2 AND TMC-1
563
Table 2
Summary of Mapping Observations for HNCS and HSCN
Δα
Δδ
Transition
J
HSCN
TR∗
(mK)
ΔV1/2
(km s−1 )
VLSR
(km s−1 )
(mK)
→ 70,7
→ 80,8
→ 70,7
→ 80,8
→ 70,7
→ 80,8
→ 70,7
→ 80,8
→ 70,7
→ 80,8
→ 70,7
→ 80,8
→ 70,7
→ 80,8
→ 70,7
→ 80,8
→ 70,7
→ 80,8
→ 70,7
→ 80,8
→ 70,7
→ 80,8
→ 70,7
→ 80,8
→ 70,7
→ 80,8
→ 70,7
→ 80,8
→ 70,7
→ 80,8
→ 70,7
→ 80,8
→ 70,7
→ 80,8
→ 70,7
→ 80,8
21 ± 4
14 ± 5
30 ± 5
44 ± 5
27 ± 4
33 ± 5
36 ± 4
41 ± 5
36 ± 4
53 ± 5
27 ± 4
42 ± 4
25 ± 4
30 ± 4
59 ± 3
58 ± 3
52 ± 4
75 ± 4
42 ± 4
42 ± 4
76 ± 3
86 ± 4
76 ± 4
81 ± 5
62 ± 4
67 ± 4
108 ± 3
110 ± 4
76 ± 4
59 ± 4
28 ± 4
∼18
46 ± 5
40 ± 5
33 ± 4
22 ± 5
22 ± 6
28 ± 6
25 ± 6
23 ± 6
26 ± 6
18 ± 6
20 ± 6
21 ± 6
22 ± 6
21 ± 6
27 ± 6
27 ± 6
27 ± 6
28 ± 6
28 ± 6
25 ± 6
28 ± 6
29 ± 6
28 ± 6
20 ± 6
26 ± 6
28 ± 6
25 ± 6
24 ± 6
25 ± 6
25 ± 6
26 ± 6
21 ± 6
21 ± 6
24 ± 6
22 ± 6
∼20
24 ± 6
22 ± 6
21 ± 6
25 ± 6
52 ± 6
54 ± 6
56 ± 6
55 ± 6
56 ± 6
53 ± 6
59 ± 6
62 ± 6
61 ± 6
59 ± 6
62 ± 6
59 ± 6
56 ± 6
59 ± 6
62 ± 6
63 ± 6
62 ± 6
64 ± 6
62 ± 6
62 ± 6
65 ± 6
64 ± 6
67 ± 6
67 ± 6
62 ± 6
62 ± 6
63 ± 6
64 ± 6
64 ± 6
62 ± 6
61 ± 6
∼61
61 ± 6
60 ± 6
62 ± 6
60 ± 6
∼12
9±5
24 ± 4
31 ± 5
17 ± 5
17 ± 5
21 ± 4
17 ± 5
48 ± 4
41 ± 5
30 ± 4
<15
29 ± 4
20 ± 4
72 ± 4
39 ± 5
70 ± 4
40 ± 5
27 ± 4
<15
75 ± 4
60 ± 4
63 ± 3
34 ± 5
31 ± 4
16 ± 5
60 ± 4
24 ± 5
35 ± 4
14 ± 5
15 ± 5
<15
20 ± 5
<12
<10
<10
70,7 → 60,6
80,8 → 70,7
8±2
8±2
Ka,Kc
→ JKa,Kc
HNCS
TR∗
ΔV1/2
(km s−1 )
VLSR
(km s−1 )
Sgr B2a
1
−2
0
−2
−1
−2
1
−1
0
−1
−1
−1
1
0
0
0
−1
0
1
1
0
1
−1
1
1
2
0
2
−1
2
1
3
0
3
−1
3
80,8
90,9
80,8
90,9
80,8
90,9
80,8
90,9
80,8
90,9
80,8
90,9
80,8
90,9
80,8
90,9
80,8
90,9
80,8
90,9
80,8
90,9
80,8
90,9
80,8
90,9
80,8
90,9
80,8
90,9
80,8
90,9
80,8
90,9
80,8
90,9
∼20
29 ± 6
21 ± 6
18 ± 6
21 ± 6
20 ± 6
26 ± 6
24 ± 6
21 ± 6
27 ± 6
21 ± 6
...
20 ± 6
23 ± 6
23 ± 6
23 ± 6
26 ± 6
27 ± 6
27 ± 6
...
26 ± 6
26 ± 6
28 ± 6
23 ± 6
24 ± 6
26 ± 6
26 ± 6
26 ± 6
20 ± 6
29 ± 6
20 ± 6
∼55
51 ± 6
51 ± 6
54 ± 6
53 ± 6
59 ± 6
56 ± 6
63 ± 6
60 ± 6
58 ± 6
60 ± 6
...
60 ± 6
58 ± 6
62 ± 6
62 ± 6
62 ± 6
67 ± 6
62 ± 6
...
68 ± 6
67 ± 6
66 ± 6
67 ± 6
62 ± 6
62 ± 6
65 ± 6
64 ± 6
65 ± 6
60 ± 6
62 ± 6
28 ± 6
63 ± 6
TMC-1b
0.75 ± 0.37 5.80 ± 0.37
0.64 ± 0.32 5.80 ± 0.32
15 ± 4
10 ± 3
0.74 ± 0.37 5.80 ± 0.37
0.99 ± 0.33 6.00 ± 0.33
Notes.
a (0,0) position: α = 17h 44m 11.s 0 ; δ = −28◦ 22 00 (B1950.0).
b α = 04h 38m 38.s 6; δ = 25◦ 35 45 (B1950.0).
indicating that they arise from the same gas. The intensities of
both the JKa,Kc = 80,8 → 70,7 and 90,9 → 80,8 transitions of
HNCS are similar at any given position. For HSCN, in contrast,
the JKa,Kc = 80,8 → 70,7 transition has generally a slightly higher
intensity (∼20 mK) than the JKa,Kc = 90,9 → 80,8 line. This
difference is likely an excitation effect; HSCN has a higher
dipole moment than HNCS (μa = 3.5 D versus 1.64 D: Durig
et al. 2006; Szalanski et al. 1978), and thus a faster radiative
decay rate between rotational levels.
Additional lines are present in these spectra, as well, arising from CCS, (CH3 )2 O, C2 H5 CN, HCOOCH3 , CH3 CH2 OH,
CH3 OH, and other molecules, as shown in Figure 3. Here spectra of both transitions of HNCS and HSCN at the Sgr B2(N)
position are displayed. Some lines, such as that arising from
CH3 CH2 OH, are only present at the N and M positions. Other
features, arising from CCS and HCOOCH3 , for instance, have
more extended emission (cf. Figures 2 and 3). These differences
likely result in part from molecular excitation—the densities and
temperatures are known to be higher at M and N (de Vicente
et al. 1997).
Contour maps of the peak intensity of the JKa,Kc = 80,8 →
70,7 transition of HNCS (a) and HSCN (b) are given in Figure 4.
For HSCN, the line intensity has a maximum at Sgr B2(N).
Interestingly for HNCS, the peak intensity lies further north at
the Sgr B2(2N) position. However, both molecules are present
well beyond the hot core boundaries, unlike other species such
as C2 H5 CN (Liu & Snyder 1999), which is only observed in
Sgr B2(N), or N2 O, which was solely detected in the regions
near Sgr B2(N) and Sgr B2(M) (Halfen et al. 2001).
For comparison, contour maps of the peak intensity of HNCO
(a) and HOCN (b) are given in Figure 5. As is evident from the
figures, HNCO and HOCN have their maximum intensity at
564
ADANDE ET AL.
Vol. 725
Table 3
Summary of Mapping Observations for HOCN and HNCO toward Sgr B2a
Δα
Δδ
HOCN
TR∗
ΔV1/2
(km s−1 )
(K)
−2
−2
−2
−1
−1
−1
0
0
0
1
1
1
2
2
2
3
3
3
1
0
−1
1
0
−1
1
0
−1
1
0
−1
1
0
−1
1
0
−1
0.04
0.15
0.07
0.08
0.18
0.12
0.06
0.16
0.20
0.11
0.20
0.25
0.17
0.28
0.21
0.06
0.08
0.06
HNCO
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
28.6
17.0
22.6
21.4
19.4
20.1
24.8
18.4
22.4
21.8
21.3
20.9
20.7
21.2
18.7
19.5
19.9
20.3
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
VLSR
(km s−1 )
3.6
3.6
3.6
3.6
3.6
3.6
3.6
3.6
3.6
3.6
3.6
3.6
3.6
3.6
3.6
3.6
3.6
3.6
55.6
52.2
56.9
57.7
58.2
60.2
59.1
62.4
64.3
62.5
68.1
68.3
63.8
66.3
66.7
60.3
59.9
59.0
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
Note. a (0,0) position: α = 17h 44m 11.s 0; δ = −28◦ 22 00 (B1950.0).
6
Sgr B2(2N)
HNCO
40,4
30,3
4
HOCN
30,3
40,4
Sgr B2(2N)
0.2
0.1
2
0.0
0
6
Sgr B2(N)
Sgr B2(N)
0.2
4
0.1
2
0.0
0
6
Sgr B2(M)
Sgr B2(M)
0.2
4
0.1
2
0.0
0
Sgr B2(S)
Sgr B2(S)
6
0.2
4
0.1
2
0
-18
0.0
22
62
VLSR (km/s)
102
142
-18
22
62
VLSR (km/s)
TR∗
102
142
3.6
3.6
3.6
3.6
3.6
3.6
3.6
3.6
3.6
3.6
3.6
3.6
3.6
3.6
3.6
3.6
3.6
3.6
ΔV1/2
(km s−1 )
(K)
1.94
4.02
2.59
3.10
4.23
3.49
2.75
4.22
5.18
3.85
6.40
7.27
5.08
7.65
6.14
2.45
2.82
2.53
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
0.03
0.03
0.03
0.03
0.03
0.03
0.03
0.03
0.03
0.03
0.03
0.03
0.02
0.03
0.03
0.02
0.02
0.02
25.1
20.0
26.1
22.4
20.8
25.8
24.2
22.4
27.9
24.6
25.3
23.6
26.5
24.9
23.9
25.8
25.0
20.7
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
3.4
3.4
3.4
3.4
3.4
3.4
3.4
3.4
3.4
3.4
3.4
3.4
3.4
3.4
3.4
3.4
3.4
3.4
VLSR
km s−1 )
54.3
54.0
57.0
57.1
58.2
60.6
58.1
63.0
65.8
60.4
66.3
68.3
60.9
63.8
63.5
60.4
60.6
59.4
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
3.4
3.4
3.4
3.4
3.4
3.4
3.4
3.4
3.4
3.4
3.4
3.4
3.4
3.4
3.4
3.4
3.4
3.4
OBSERVATIONS OF THE [HNCS]/[HSCN] RATIO IN Sgr B2 AND TMC-1
HNCS: J = 80,8
(a)
70,7
SgrB2(N)
Sgr B2
0.1
CCS
0
HNCS
0.1
M
TR* (K)
0.0
0.1
0.0
TR* (K)
0.0
0.1
-2
U
80,8
U
0.0
0.1
S
-1
U
CH3CHO
0.1
CH3C3N
1
c-H2C3O
0.0
N
J = 90,9
CH3OH
HNCS
J=
90,9
80,8
(CH2OH)2
0.1
2N
2
0.2
CH3CH2OH
HSCN
0.0
HNCS
3
Δδ (arcmin)
565
HSCN
No. 1, 2010
CCS
0.2
HNCS
J = 80,8
70,7
HSCN
J = 80,8
70,7
200
-100
0
100
200
-100
0
100
200
VLSR (km/s)
1
0
0.1
-1
Δα (arcmin)
HSCN: J = 80,8
(b)
70,7
Sgr B2
0.10
U
0.0
0.05
3
-200
0.00
2N
2
0.05
1
M
HCOOCH3
0.05
TR* (K)
0.00
0
0.00
S
-1
0.05
0.00
0.05
-2
0.00
-200
-100
0
100
200
-100
0
100
200
-100
0
100
200
VLSR (km/s)
1
0
0
100
200
-100
0
100
200
VLSR (km/s)
0.00
HSCN
-100
0.05
N
Δδ (arcmin)
HSCN
100
HCOOCH3
0
HNCS
-100
U
-200
(CH3)2O
C2H5CN
0.0
-1
Δα (arcmin)
Figure 2. Position-spectrum map of the JKa,Kc = 80,8 → 70,7 transition of
HNCS (a) and HSCN (b) at 93 and 91 GHz, respectively, obtained with the
ARO 12 m. Spectral resolution is 2 MHz. The dotted line on the data assumes
VLSR = 62.0 km s−1 for the given HNCS or HSCN frequency. The center map
position is at α = 17h 44m 11.s 0, δ = −28◦ 22 00 (B1950.0) near Sgr B2(M).
The spectra are spaced in position by 1 arcmin (θ b ∼ 67–68 arcsec). All spectra
for HNCS or HSCN are plotted on the same temperature scale. Emission from
both molecules is present at almost every position observed.
Previous studies found that strong chemical differences exist between the different regions of Sgr B2. While large
organic molecules, or molecules whose formation process requires higher temperatures (>100 K), are concentrated towards Sgr B2(N) (Miao et al. 1995), some smaller nitrogencontaining molecules, such as HNCO, have emission maxima
at Sgr B2(2N) (Wilson et al. 1996). Surface chemistry on grains,
Figure 3. Spectra of the JKa,Kc = 90,9 → 80,8 and 80,8 → 70,7 transitions
of HNCS (left panels) and HSCN (right panels) measured at the Sgr B2(N)
position using the ARO 12 m. The data are plotted on the same TR∗ scale,
and show transitions of other molecules which appear to be more confined (cf.
Figure 2). Spectral resolution is 2 MHz.
followed by a warm-up phase and subsequent grain mantle evaporation, is believed to play a major role in the chemical composition of such hot cores (Garrod et al. 2008).
The chemical concentrations seen in Sgr B2 are not particularly discernable for either HNCS or HSCN, or in their oxygen
analogs, HNCO and HOCN. Both isomer pairs have extended
spatial distributions over a 6 × 3 region, where the gas temperature probably does not exceed 50 K—well beyond the hot
cores (de Vicente et al. 1997). The line intensities appear to
follow the gas density gradient from the central H ii regions
to the outer envelope, although there are local maxima at the
N or 2N positions. This distribution sharply contrasts with that
of some larger organic molecules like EtCN and VyCN, which
are quite concentrated at the Sgr B2(N) hot core (Miao et al.
1995). Moreover, very similar line shapes are exhibited by all
four molecules at each position (see Figure 2). There is no sign
of broadening in the respective line profiles due to outflows or
shocks. Finally, Halfen et al. (2009) measured numerous transitions of HNCS and HSCN toward Sgr B2(N), finding a fairly
low rotational temperature of about 20 K for both species.
4.2. HNCS and HSCN Column Densities
The column densities of HNCS, HSCN, HNCO, and HOCN
were calculated using the following formula:
Ntot =
3k105 TR ΔV1/2 ζrot
.
8π 3 νμ2 Se−ΔEgd /Trot
(1)
566
ADANDE ET AL.
(a)
Vol. 725
(b)
Figure 4. Contour maps of Sgr B2 of the peak intensity TR∗ (K) of the JKa,Kc = 80,8 → 70,7 transition of HNCS (a) and HSCN (b) measured with 2 MHz resolution.
The (0,0) position of the map is at α = 17h 44m 11.s 0, δ = −28◦ 22 00 (B1950.0) near Sgr B2(M). The lowest contour for the HNCS map is 0.010 K and increases in
increments of 0.010 K up to 0.110 K. For HSCN, the lowest contour is 0.010 K with increments of 0.010 K up to 0.080 K. Darker shade contours represent higher
antenna temperatures. Beam sizes are shown in the lower left-hand corners, and the stars indicate the positions of Sgr B2(2N), (N), (M), (OH), and (S), respectively,
from top to bottom. The maps show that HNCS emission has one maximum at the northern nitrogen core, Sgr B2(2N), while HSCN has a single emission peak at
Sgr B2(N).
(a)
Figure 5. Contour maps of Sgr B2 of the peak intensity TR∗ (K) of the JKa,Kc = 40,4 → 30,3
(b)
No. 1, 2010
OBSERVATIONS OF THE [HNCS]/[HSCN] RATIO IN Sgr B2 AND TMC-1
567
Table 4
Column Densities in Sgr B2 and TMC-1
Δδ
Ntot (HNCS) (cm−2 )
−2
−2
−2
−1
−1
−1
0
0
0
1
1
1
2
2
2
3
3
3
8.0
1.7
1.2
1.5
1.8
1.9
1.6
2.9
3.6
1.9
4.2
3.6
3.1
4.7
2.8
9.0
1.9
1.2
Δα
Ntot (HSCN) (cm−2 )
Ntot (HNCO) (cm−2 )
Ntot (HOCN) (cm−2 )
Sgr B2
1
0
−1
1
0
−1
1
0
−1
1
0
−1
1
0
−1
1
0
−1
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
3.8
0.6
0.5
0.6
0.6
0.5
0.5
1.2
1.0
0.8
1.4
1.4
1.4
2.0
1.2
4.9
0.8
0.6
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
1012
1013
1013
1013
1013
1013
1013
1013
1013
1013
1013
1013
1013
1013
1013
1012
1013
1013
1.9
4.1
2.7
3.6
8.4
3.6
3.9
9.5
1.1
4.0
1.3
9.4
4.3
8.1
4.1
2.5
3.3
1.7
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
0.9
1.7
1.3
1.7
3.2
2.1
1.9
5.0
0.5
2.2
0.6
5.0
2.3
4.6
2.3
1.2
1.8
0.8
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
1012
1012
1012
1012
1012
1012
1012
1012
1013
1012
1013
1012
1012
1012
1012
1012
1012
1012
7.0
1.1
9.7
9.9
1.3
1.3
9.5
1.4
2.1
1.4
2.3
2.5
1.9
2.7
2.1
9.0
1.0
7.5
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
1.7
0.3
2.3
2.5
0.3
0.3
2.3
0.3
0.5
0.3
0.6
0.6
0.5
0.7
0.5
2.2
0.2
1.9
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
1014
1015
1014
1014
1015
1015
1014
1015
1015
1015
1015
1015
1015
1015
1015
1014
1015
1014
3.6
8.0
5.0
5.4
1.1
7.6
4.7
9.3
1.4
7.6
1.3
1.6
1.1
1.9
1.2
3.7
5.0
3.8
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
1.9
2.6
1.9
1.9
0.3
2.4
1.9
2.9
0.4
2.4
0.4
0.5
0.3
0.5
0.4
1.5
1.8
1.6
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
1012
1012
1012
1012
1013
1012
1012
1012
1013
1012
1013
1013
1013
1013
1013
1012
1012
1012
TMC-1
8.3 ± 4.4 × 1010
6.4 ± 2.1 × 1010
4.0 × 1012 a
< 1.9 × 1010 b
Notes.
a Marcelino et al. (2009).
b Marcelino et al. (2010).
0.02
HSCN
70,7
Jth level above the ground state, TR and ΔV1/2 are the radiation
temperature and line width of the line, respectively, Trot is the
assumed rotational temperature, and ζ rot is the partition function.
This formula assumes that the lines are optically thin, and the
source fills the main telescope beam. The partition function
simplifies to the formula (Turner 1991)
TMC-1
60,6
U
0.00
80,8
TR* (K)
0.01
U
c-C3H
70,7
U
U
ζrot
0.00
0.01
HNCS
70,7
60,6
80,8
70,7
0.00
0.01
0.00
-14.2
-4.2
5.8
15.8
25.8
VLSR (km/s)
Figure 6. Spectra of the JKa,Kc = 70,7 → 60,6 and JKa,Kc = 80,8 → 70,7
transitions of HSCN (top panels) and HNCS (lower panels) near 80–82 and
91–92 GHz, respectively, measured with the ARO 12 m toward TMC-1. Spectral
resolution is 100 kHz. Both HNCS and HSCN are clearly present in this dark
cloud.
Here ν is the frequency of the transition, μ is the permanent
dipole moment, S is the line strength, ΔEgd is the energy of the
π (kT )3
=
h3 ABC
12
,
(2)
where A, B, and C are the rotational constants. A rotational
temperature of 20 K was assumed in these calculations, based
on previous observations by Halfen et al. (2009), who derived
rotation temperatures of 18 ± 3 K for HNCS and 19 ± 2 K
for HSCN in Sgr B2(N). In the case of HNCS and HSCN, two
transitions were observed (JKa,Kc = 80,8 → 70,7 and 90,9 → 80,8 ),
and column densities were calculated individually from each,
and then averaged. Typically, the Ntot values did not vary more
than a factor of 2 from each other. For HNCO and HOCN, Ntot
was derived from the one transition measured. Uncertainties in
the columns densities were calculated based on the errors in
line intensity and linewidth, as given in Table 2, as well as a
±3 K error on the rotational temperature, as determined from
the analysis of Halfen et al. (2009).
The column densities derived in this manner are listed in Table 4 for all positions observed in Sgr B2. Typical uncertainties
for the column densities are 40%–50%. For HNCS, the values
across the cloud fall in the range (0.8–4.7) × 1013 cm−2 , while
for HSCN, Ntot ∼ (0.2–1.3) × 1013 cm−2 . A similar spread
of values was found for the oxygen analogs, with (0.7–2.7) ×
1015 cm−2 for HNCO, and (0.4–1.9) × 1013 cm−2 for HOCN.
Therefore, the individual column densities of all four species
vary by no more than an order of magnitude across Sgr B2,
and, within these uncertainties, the relative column densities
for HNCS and HSCN vary by no more than a factor of 2.
568
ADANDE ET AL.
Table 5
Isomer Ratios in Sgr B2 and TMC-1
Δα
Δδ
HNCS/HSCN
HNCO/HOCN
Sgr B2
1
0
−1
1
0
−1
1
0
−1
1
0
−1
1
0
−1
1
0
−1
−2
−2
−2
−1
−1
−1
0
0
0
1
1
1
2
2
2
3
3
3
4.2
4.1
4.6
4.1
2.2
5.4
4.0
3.1
3.3
4.7
3.1
3.8
7.0
5.8
6.8
3.7
5.7
6.8
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
2.9
2.2
2.9
2.5
1.1
3.3
2.4
2.0
1.8
3.3
1.8
2.5
4.7
4.1
4.9
2.6
4.1
4.8
193
143
194
184
114
169
203
146
146
179
172
149
173
145
169
245
201
195
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
114
58
86
80
45
68
96
59
55
73
68
59
66
52
69
118
88
97
TMC-1
1.4 ± 0.7
...
While HSCN and its oxygen analog have similar abundances,
HNCS is clearly about two orders of magnitude less prevalent
that HNCO.
Brünken et al. (2010) and Marcelino et al. (2010) observed
the N, M, and S positions in Sgr B2 in HOCN and HNCO, using
the IRAM 30 m telescope. The column densities they found for
their “extended” component in both molecules agree very well
with our values. For example, at the N position, they derived
Ntot = 1.6 × 1013 cm−2 for HOCN and Ntot = 2.9 × 1015 cm−2
for HNCO. Our values are Ntot (HOCN) = 1.3 × 1013 cm−2 and
Ntot (HNCO) = 2.3 × 1015 cm−2 , about a 20% difference. The
agreement between column densities is further evidence that
these two molecules are extended and have a near unity filling
factor across the cloud.
Toward TMC-1, the column densities were calculated in an
identical manner, except a small correction (16%–18%) was
made to account for the 2.7 K cosmic background. Trot was
assumed to be 10 K. In this source, the column density for
HSCN was determined to be 6 ×
Vol. 725
No. 1, 2010
OBSERVATIONS OF THE [HNCS]/[HSCN] RATIO IN Sgr B2 AND TMC-1
569
+
NCS via H-atom transfer reactions with molecular hydrogen:
NCS + H2 → HNCS + H
+
+
10-1010
-10
-11
(3)
10
-10
10
HNCS
HNCS obs. near hot cores
-11
10
HSCN obs. near hot cores
HSCN
10
10-12
-12
-12
10
+
+
(4)
The assumed existence of two structures for protonated HNCS
is based on calculated structures and energies for the analogous
O-containing species (Schuurman et al. 2004), and a 1:1 product
branching ratio is assumed. The two neutral isomers can
be formed by the dissociative recombination of the cationic
precursors and free electrons:
-13
10
10
10
10-14
-14
10
-15
10
X
HNCS + H2 → HNCSH /H2 NCS + H.
+
[n ] / [H 2 ]
-13
-14
-15
10
-16
10
10-16
-16
10
-17
-17
10
10
10
10-18
-18
-18
10
---- HNCS(g),
warm up model
HNCS(g), warm up model
HSCN(g),
model
.… HSCN(g),
warmwarm
up up
model
-19
10
-20
-20
10
10-20
10
2
−
HNCSH + e → HNCS/HSCN + H
+
(5)
-12
(6)
[n ]X / [H 2 ]
10
10-12
[n ] / [H 2 ]
The 1:1 branching ratio for the products of the HNCSH+
recombination is once again assumed. There are also possible
neutral–neutral formation reactions for the CHNS isomers, such
as
NS + CH2 → HCNS + H.
(7)
4
10
5
10
6
10
10
8
10
7
10
t/yr
10
H2 NCS+ + e− → HNCS + H.
3
10
-10
10-1010
-11
-10
10
HNCS obs. cold halo
HNCS
10
HSCN
10
-11
HSCN obs. cold halo
-12
-13
-13
10
10
-14
10
10-14
-14
10
-15
-15
10
10
-16
10
10-16
-16
10
-17
-17
10
10
10
10-18
-18
While these processes can be exothermic, they likely have
energy barriers. The rates for such reactions cannot typically
compete with those of ion–molecule mechanisms (Woodall et al.
2007); to a good approximation, neutral–neutral reactions for
this isomer pair can be neglected.
On grain surfaces, the formation of the CHNS isomers starts
from accretion of the neutral radical NCS, which subsequently
can acquire one hydrogen atom at either end of the molecule.
The hydrogenated NCS surface species then enter the gas phase
via non-thermal desorption or thermal desorption (evaporation)
to produce HNCS and HSCN. In the cold model, gas-phase
formation is always dominant because non-thermal desorption
is not efficient. In the warm-up model, on the other hand,
evaporation becomes much more important during the heating
and subsequent phases, and the synthesis on grain surfaces plays
a major role.
Once formed, the CHNS isomers can be destroyed by abundant cations (H+ , He+ , C+ , H3 + , etc.). In the case of the CHNO
isomers, destruction can also occur by well-characterized reactions with atomic carbon and oxygen with either zero or small
activation energy barriers (Quan et al. 2010). We have included
some of these processes with similar energetics for the CHNS
isomers, although analogous reaction rate coefficients do not exist. The effect of adding the atomic reactions is only important
at very early times, before atomic carbon is converted mainly
into CO.
Our modeling results are depicted in Figure 7. Panel (a) shows
the calculated HNCS and HSCN fractional abundances with
respect to H2 as functions of time for the warm environment
case. Fractional abundances observed towards Sgr B2(M) are
also shown, calculated assuming an H2 column density of
1024 cm−2 (Marcelino et al 2010; Lis & Goldsmith 1990). The
calculated abundances for HNCS and HSCN both peak around
3 × 105 yr, after the warm-up is finished. The HNCS peak value
fits the observations well, while the HSCN peak is an order of
magnitude too low. Thus, the model overestimates the [HNCS]/
[HSCN] ratio by about a factor of 10.
Panel (b) compares the cold-core model and the observed
HNCS and HSCN abundances towards the Sgr B2 position
(Δα,Δδ) = (1,−2), which should be part of the “cold halo.”
-18
10
---- HNCS(g),
cold model
HNCS(g), cold model
.… HSCN(g),
cold model
HSCN(g),
cold model
-19
10
10-20
10
-19
10
-20
-19
10
-20
2
10
102
3
10
4
10
104
5
10
t/yr
6
10
106
7
10
10
8
10
108
Years
Figure 7. Comparison of calculated CHNS isomers abundances as a function
of time (dotted and dashed lines) vs. observed values (straight, solid lines): (a)
“warm-up” model with observed Sgr B2(M) abundances, assuming N(H2 ) =
1024 cm−2 ; (b) cold model with abundances from the position (Δα, Δδ) =
(1, −2) in Sgr B2, assuming N(H2 ) = 1024 cm−2 .
In this case, calculated peak abundances for both HNCS and
HSCN, which also occur at a time of 3 × 105 yr, are a factor of
ten lower than observed values. After tpeak , both HNCS and
HSCN maintain a comparable ratio. The calculated ratio is
[HNCS]/[HSCN] ∼ 10, however, in reasonable agreement with
the observed value of ∼4.
The so-called cold halo results can also be applied to
TMC-1. Assuming an H2 column density of 1022 cm−2 (Kaifu
et al. 2004), the observed fractional abundances for HNCS
and HSCN are 8.3 × 10−12 and 6.0 × 10−12 . Such values
are significantly higher than the predicted peak abundances for
HNCS and HSCN of 1 × 10−12 and 1.6 × 10−13 . Furthermore,
the theoretical ratio is still about 10, not near 1, as observed.
We also constructed cold models with higher sulfur abundances
(1.5 × 10−6 wrt H, about 18 times of the normal value, 8.0 ×
10−8 ), but without success because this element, at least with
our parameters, is easily depleted onto dust particles. Clearly,
the details of the chemistry of these two isomers are not yet
understood, and/or the simple assumptions made here need
modification. The simplest change would be to lower the product
branching fraction of the H2 NCS+ ion in Equation (2), which
would push the gas-phase formation rate toward equality for
HNCS and HSCN.
4.5. [HNCS]/[HSCN] Versus [HNCO]/[HOCN]
As discussed by Marcelino et al. (2010), the CHNO isomers
have a puzzling chemistry in molecular clouds. The abundance
ratio [HCNO]/[HOCN] varies substantially between cold, quiescent clouds, where it is near unity, and warmer objects, where
570
ADANDE ET AL.
the values are typically 0.1 or less. Our current gas-grain model
(Quan et al. 2010) manages to reproduce this temperature dependence for selected times in warmer objects. The [HNCO]/
[HOCN] ratio, however, is usually ∼50–150 in most sources,
warm or cold. Our ratios in Sgr B2 fall in the range ∼114—245
and never approach the low values of [HNCS]/[HSCN] ∼2–7.
HOCN has yet to be detected in TMC-1, as well.
HNCO and HOCN share with their sulfur analogs an extended
distribution and homogeneous line widths through the observed
Sgr B2 region. Both species peak at the same position, about
1 arcmin north of Sgr B2(N). The fact that HNCO is far more
abundant than any of its higher energy isomers, unlike HNCS,
must be a vital chemical clue to the differences between these
two isomer systems. As postulated by Marcelino et al. (2010),
H2 NCO+ may be an important precursor to HNCO that does not
lead to any of the higher energy isomers. The sulfur equivalent,
H2 NCS+ , does not seem to be a key reactant ion. HNCSH+
appears to be the dominant precursor, leading to relatively equal
amounts of HNCS and HSCN.
Recent work (Ishii et al. 2006) supports the hypothesis that
HNCH+ is the main precursor of HNC and HCN in interstellar
clouds. Other destruction and isomerization processes are then
needed to explain the wide variation of the [HCN]/[HNC] ratio.
The neutral–neutral reaction, H + HNC ↔ HCN + H, for
example, could account for the enhanced abundance of HCN
in warmer regions (Talbi et al. 1996), but the activation barrier
for the forward reaction of 2,100 K is sufficiently large that
temperatures higher than those in hot cores would be needed for
it to be important given the low abundance of atomic hydrogen.
By analogy, it is unlikely that a similar reactive isomerization of
HSCN to HNCS will occur efficiently at hot core temperatures.
Therefore, once HSCN and HNCS are formed, they do not
interconvert. The [HNCS]/[HSCN] ratio thus probably reflects,
to a first approximation, the nearly equal branching ratio of the
reaction, HSCNH+ + e− .
5. CONCLUSIONS
A new isomer pair, HNCS and its metastable form, HSCN,
has been studied for the first time in interstellar gas using
millimeter-wave observations. Toward Sgr B2, both species
have extended emission at least 6 × 3 across the cloud.
Moreover, both molecules have been identified in TMC-1. The
[HNCS]/[HSCN] ratio in Sgr B2 is ∼2–7, with the lower values
near the warm hot cores, and a value near unity in TMC-1.
These data indicate that the chemistry of HNCS and HSCN
primarily involves gas-phase reactions in colder material. The
electron recombination of HNCSH+ is a plausible formation
pathway to both isomers with a nearly equal branching ratio.
The possible precursor ion, H2 NCS+ , which leads solely to
HNCS, does not appear to play a significant role. Detailed
chemical modeling cannot yet produce the results in either
Sgr B2 or TMC-1, but branching ratios and reaction rates of
almost all relevant pathways are unknown. More laboratory
data are needed for this system. Detection of the precursor
molecules NCS, NCS+ , HNCSH+ , and H2 NCS+ would also
further elucidate the chemical processes that lead to these
metastable isomers.
This research was supported by the Center for Chemical
Innovation program through NSF grant CHE 08-47919, as
well as AST 09-06534. E.H. acknowledges support for his
research program in astrochemistry from the NSF Division of
Astronomical Sciences, for his research program in chemical
Vol. 725
kinetics from the Center for the Chemical Universe (NSF
Chemistry), and for his program in the evolution of pre-planetary
matter (NASA NAI).
REFERENCES
Apponi, A. J., Pesch, T. C., & Ziurys, L. M. 1999, ApJ, 519, L89
Apponi, A. J., & Ziurys, L. M. 1997, ApJ, 481, 800
Brünken, S., Belloche, A., Martı́n, S., Verheyen, L., & Menten, K. M.
2010, A&A, 516, A109
Brünken, S., Gottlieb, C. A., McCarthy, M. C., & Thaddeus, P. 2009a, ApJ, 697,
880
Brünken, S., Yu, Z., Gottlieb, C. A., McCarthy, M. C., & Thaddeus, P.
2009b, ApJ, 706, 1588
de Vicente, P., Martı́n-Pintado, J., Neri, R., & Colom, P. 2000, A&A, 361,
1058
de Vicente, P., Martı́n-Pintado, J., & Wilson, T. L. 1997, A&A, 320, 957
Durig, J. R., Zheng, C., & Deeb, H. 2006, J. Mol. Struct., 784, 78
Frerking, M. A., Linke, R. A., & Thaddeus, P. 1979, ApJ, 234, 143
Garrod, R. T., Widicus Weaver, S. L., & Herbst, E. 2008, ApJ, 682, 283
Gaume, R. A., & Claussen, M. J. 1990, ApJ, 351, 538
Gaume, R. A., Claussen, M. J., De Pree, C. G., Goss, W. M., & Mehringer,
D. M. 1995, ApJ, 449, 663
Goldsmith, P. F., Irvine, W. M., Hjalmarson, A., & Ellder, J. 1986, ApJ, 310,
383
Goldsmith, P. F., Langer, W. D., Ellder, J., Kollberg, E., & Irvine, W. 1981, ApJ,
249, 524
Goldsmith, P. F., Snell, R. L., Hasegawa, T., & Ukita, N. 1987, ApJ, 314, 525
Green, S., & Herbst, E. 1979, ApJ, 229, 121
Halfen, D. T., Apponi, A. J., Woolf, N., Polt, R., & Ziurys, L. M. 2006, ApJ,
639, 237
Halfen, D. T., Apponi, A. J., & Ziurys, L. M. 2001, ApJ, 561, 244
Halfen, D. T., Ziurys, L. M., Brünken, S., McCarthy, M. C., Gottlieb, C. A., &
Thaddeus, P. 2009, ApJ, 702, L124
Herbst, E., & Woon, D. E. 1996, ApJ, 463, L113
Hirota, T., Yamamoto, S., Mikami, H., & Ohishi, M. 1998, ApJ, 503, 717
Hocking, W. H., Gerry, M. C. L., & Winnewisser, G. 1975, Can. J. Phys., 53,
1869
Hollis, J. M., Lovas, F. J., Remijan, A. J., Jewel, P. R., Ilyushin, V. V., & Kleiner,
I. 2006, ApJ, 643, L25
Ishii, K., Tajima, A., Taketsugu, T., & Yamashita, K. 2006, ApJ, 636, 927
Kaifu, N., et al. 2004, PASJ, 56, 69
Li, H., Hirano, T., Amano, T., & Le Roy, R. J. 2008, J. Chem. Phys., 129, 244306
Lis, D. C., & Goldsmith, P. F. 1990, ApJ, 356, 195
Lis, D. C., Goldsmith, P. F., Carlstrom, J. E., & Scoville, N. Z. 1993, ApJ, 402,
238
Liu, S. Y., & Snyder, L. E. 1999, ApJ, 523, 683
Marcelino, N., Brünken, S., Cernicharo, J., Quan, D., Roueff, E., Herbst, E., &
Thaddeus, P. 2010, A&A, 516, A105
Marcelino, N., Cernicharo, J., Tercero, B., & Roueff, E. 2009, ApJ, 690, L27
Martı́n, S., Martı́n-Pintado, J., & Mauersberger, R. 2009, ApJ, 694, 610
Miao, Y., Mehringer, D. M., Kuan, Y. J., & Snyder, L. E. 1995, ApJ, 445, L59
Minh, Y. C., Haikala, L., Hjalmarson, A., & Irvine, W. M. 1998, ApJ, 498, 261
Quan, D., Herbst, E., Osamura, Y., & Roueff, E. 2010, ApJ, submitted
Sarrasin, E., Ben Abdallah, D., Wernli, M., Faure, A., Cernicharo, J., & Lique,
F. 2010, MNRAS, 404, 518
Savage, C., & Ziurys, L. M. 2004, ApJ, 616, 966
Schilke, P., Walmsley, C. M., Pineau des Forets, G., Roueff, E., Flower, D. R.,
& Guilloteau, S. 1992, A&A, 256, 595
Schuurman, M. S., Muir, S. R., Allen, W. D., & Schaefer, H. F., III 2004,
J. Chem. Phys., 120, 11586
Szalanski, L. B., Gerry, M. C. L., Winnewisser, G., Yamada, K. M. T., &
Winnewisser, M. 1978, Can. J. Phys., 56, 1297
Talbi, D., Ellinger, Y., & Herbst, E. 1996, A&A, 314, 688
Talbi, D., & Herbst, E. 1998, A&A, 333, 1007
Turner, B. E. 1991, ApJS, 76, 617
van Mourik, T., Harris, G. J., Polyansky, O. L., Tennyson, J., Császár, A. G., &
Knowles, P. J. 2001, J. Chem. Phys., 115, 3706
Wierzejewska, M., & Mielke, Z. 2001, Chem. Phys. Lett., 349, 227
Wierzejewska, M., & Moc, J. 2003, J. Phys. Chem. A, 107, 11209
Wilson, T. L., Snyder, L. E., Comoretto, G., Jewell, P. R., & Henkel, C. 1996,
A&A, 314, 909
Woodall, J., Agundez, M., Markwick-Kemper, A. J., & Millar, T. J. 2007, A&A,
466, 1197
Ziurys, L. M., & Turner, B. E. 1986, ApJ, 302, L31