THE ASTROPHYSICAL JOURNAL, 561 : 244È253, 2001 November 1
( 2001. The American Astronomical Society. All rights reserved. Printed in U.S.A.
EVALUATING THE N/O CHEMICAL NETWORK : THE DISTRIBUTION OF N O AND NO IN THE
2
SAGITTARIUS B2 COMPLEX
D. T. HALFEN, A. J. APPONI, AND L. M. ZIURYS
Department of Astronomy, Department of Chemistry, and Steward Observatory, University of Arizona, 933 North Cherry Avenue Tucson, AZ 85721-0065
Received 2001 April 30 ; accepted 2001 June 18
ABSTRACT
Mapping observations of the J \ 6 ] 5 transition of N O and the %` , J \ 3/2 ] 1/2 line of NO in
2
1@2
the 2 mm band toward the core region of the Sagittarius B2 complex have been carried out using the
Kitt Peak 12 m telescope. Emission from NO was found to be extended over a region 2@ ] 5@ in size that
includes the Sgr B2 (N), Sgr B2 (M), and Sgr B2 (OH) positions, very similar to the distribution found
for HNO. In contrast, N O emission was conÐned to a source approximately 1@ in extent, slightly elon2
gated in the north-south direction and centered on the Sgr B2 (N) core. A virtually identical distribution
was found for the J \ 14 ] 14
E transition of methanol, which lies 255 K above ground state and
Kq
0
~1
samples very hot gas. Excitation conditions are favorable for the J \ 6 ] 5 line of N O over the entire
2
NO region ; hence, the conÐned nature of this species is a result of chemistry. The J \ 3 ] 2 and
J \ 9 ] 8 lines of N O at 75 and 226 GHz, respectively, were also detected at Sgr B2 (N). Combined
with the J \ 6 ] 5 2data, these transitions indicate a column density for this molecule of N D 1.5
tot
] 1015 cm~2 at this position and an abundance of f (N O/H ) D 1.5 ] 10~9. This fractional abundance
2
2
is almost 2 orders of magnitude higher than predicted by low-temperature chemical models. The N O
2
observations suggest that this molecule is preferentially formed in high-temperature gas ; a likely mechanism is the neutral-neutral reaction NO ] NH ] N O ] H, which has an appreciable rate only at
2 Sgr B2 cloud was N D (0.8È1.5) ] 1016 cm~2,
T [ 125 K. The column density of NO found over the
tot
corresponding to a fractional abundance of f (NO/H ) D (0.8È1.5) ] 10~8, which
is about 1 order of
2
magnitude less than model predictions. The similar distributions of NO and HNO suggest a chemical
connection. It is likely that the major route to HNO is from NO via the ion-molecule process
NO ] HNO` ] NO` ] HNO, which occurs readily at low temperatures. The NO molecule thus
appears to be the main precursor species in the N/O chemical network.
Subject headings : astrochemistry È Galaxy : center È ISM : abundances È ISM : molecules È
molecular processes È radio lines : ISM
1.
INTRODUCTION
cloud [Sgr B2 (N), Sgr B2 (M), and Sgr B2 (NW)] and found
interesting chemical di†erences between the northern and
middle sources. Such di†erentiation has also been investigated by Snyder and collaborators, who discovered that the
distribution of the large organic molecules VyCN, EtCN,
and HCOOCH was conÐned to the Sgr B2 (N) position
3 ; Miao & Snyder 1997 ; Liu & Snyder
(Miao et al. 1995
1999). No emission from these three species was found at
Sgr B2 (M). Snyder thus called this region the ““ Large Molecule Heimat ÏÏ or Sgr B2 (LMH).
Chemical variation has also been found in nitrogencontaining molecules. Ammonia, for example, has an anomalously high abundance of f (NH /H ) Z 10~5 in the Sgr B2
3 2 et al. 1993). In fact,
(N) and Sgr B2 (M) cores (HuŽttemeister
almost all the atomic nitrogen is contained in NH in these
two regions. In contrast, HNCO emission appears3 to peak
at an unusual position approximately 2@ north of Sgr B2
(M), called Sgr B2 (2N) (Wilson et al. 1996 ; Minh et al.
1998). There are no known embedded sources, UCH II
regions, or gas density peaks at Sgr B2 (2N). Wilson et al.
proposed this emission peak to be the ““ northern nitrogen
core.ÏÏ In addition, the Sgr B2 complex is the only source in
which the three species containing an N-O bond have been
detected, namely, NO, HNO, and N O (Ziurys et al. 1994a ;
2
Ziurys, Hollis, & Snyder 1994b).
The complex structure of the Sgr B2 cloud is illustrated in
Figure 1, which is a qualitative picture showing the positions of Sgr B2 (N), Sgr B2 (M), and Sgr B2 (OH), marked
by stars, and that of Sgr B2 (2N), indicated by an open
Predicting chemical abundances in molecular clouds is
problematic. The presence of young stars, outÑows, and
small-scale clumping in such objects brings a high level of
complexity to the problem such that simple, quiescent cloud
models are no longer applicable. One such complicated
source is the Sagittarius B2 cloud. The di†use envelope of
Sgr B2 extends over 40 pc in diameter, while the denser gas
covers a region D5 ] 10 pc in size, elongated in the northsouth direction, and contains D106 M of material (see,
_
e.g., Goldsmith et al. 1987 ; Lis et al. 1993).
Embedded in
this denser gas are at least three compact sources, Sgr B2
(N), Sgr B2 (M), and Sgr B2 (S), which are thought to have
densities of n(H ) D 107 cm~3 (see, e.g., Vogel, Genzel, &
Palmer 1987 ; Lis2 & Goldsmith 1991 ; Lis et al. 1993). These
cores contain ultracompact H II regions as well as H O and
2
OH masers (see, e.g., Reid et al. 1988 ; Gaume & Claussen
1990). Hence, massive star formation is likely occurring in
these sources.
The chemistry of Sgr B2 is naturally quite complex and
spatially dependent. Over 50 di†erent molecules have been
detected toward this cloud, as documented in spectral
surveys of this region (Cummins, Linke, & Thaddeus 1986 ;
Turner 1989 ; Sutton et al. 1991). Many of these surveys
focused on the strong OH maser position Sgr B2 (OH),
while others examined the compact cores. For example, a
very recent band scan in the 1 mm region by Nummelin et
al. (2000), done with the Swedish-ESO Submillimeter Telescope (h D 23@@), surveyed three separate positions in the
244
N/O CHEMICAL NETWORK : N O AND NO IN SGR B2
2
245
Array. Although HNO was found to be extended over a
several arcminute region, these authors found Ðve major
concentrations of this molecule near the northern, middle,
and southern cores. The emission of HNO seemed slightly
o†set from the core positions, however, suggesting chemical
processing in these regions.
Surveys of HNO and NO (Ziurys et al. 1991, 1994b)
showed that these two species are present in a virtually
identical set of sources. As a result, Ziurys et al. (1994b)
concluded that their chemistry was likely to be related. In
Sgr B2, it probably involves N O as well. Therefore, to
2 Snyder (1994), we have
complement the work of Kuan &
mapped the distribution of NO and N O in the Sgr B2
2 The J \ 6 ] 5
complex using the Kitt Peak 12 m telescope.
transition of N O and the %` , J \ 3/2 ] 1/2 hyperÐne
2 were measured,
1@2 which both occur near
components of NO
150 GHz. In addition, a high-energy transition of methanol
was mapped. In this paper we present these results and
compare them with previous HNO and HNCO studies. We
also compare our observed abundances with those of recent
chemical models and explain the varying spatial distributions in terms of the N/O chemical network.
2.
FIG. 1.ÈSgr B2 complex showing the positions of Sgr B2 (N), Sgr B2
(M), and Sgr B2 (OH), indicated by stars, as well as the major H II regions
( Ðlled circles). The solid line indicates the outer extent of the 1 mm continuum emission (Goldsmith et al. 1987). Molecular emission shows distinctly
di†erent peaks, depending on the chemical species. The dashed contours
represent the N \ 3 ] 2 transition of SO, which has a maximum at Sgr
J extends
2
1over Sgr B2 (N) (Goldsmith et al. 1987). The
B2 (M) but also
10
]9
line of EtCN, on the other hand, is centered exclusively on
Sgr1,10
B2 (N)1,9
(Liu & Snyder 1999), while the 4 ] 3 transition of HNCO
03
peaks at the 2N position (Minh et al. 1998),04the location
of the so-called
northern nitrogen core (Wilson et al. 1996). The (0, 0) position on this map
is Sgr B2 (OH) : a \ 17h44m11s. 0, d \ [28¡22@30@@ (B 1950.0).
circle. (1@ corresponds to 2.6 pc at a distance of 8.5 kpc). Sgr
B2 (S) is indicated by the single H II region directly south of
Sgr B2 (OH). As the Ðgure shows, the 1 mm continuum
encompasses the N, S, and M positions (from Goldsmith et
al. 1987), where H II regions also exist, indicated by Ðlled
circles. Varying molecular distributions are shown by emission contours of SO (N \ 3 ] 2 ), EtCN (J
\
Ka,Kcfor
10
] 9 ), and HNCOJ (J 2 \14 ] 3 ). SO,
1,10
1,9 a maximum at the
Ka,Kcmiddle
04 position
03
example,
has
but is
strong toward Sgr B2 (N) as well (Lis & Goldsmith 1991).
EtCN, on the other hand, exclusively exists at Sgr B2 (N)
(Liu & Snyder 1999), while HNCO shows the unusual peak
at Sgr B2 (2N).
The presence of several compounds contains an N-O
bond, the large abundances of NH , and the uncharacteris3 B2 a prime source for
tic distribution of HNCO makes Sgr
investigating nitrogen chemistry, in particular the N/O
chemical network. Kuan & Snyder (1994) initiated the study
of the N/O chemistry in this cloud by mapping the
1 ] 0 transition of HNO across Sgr B2 using the Kitt
01 1200 m telescope and the Berkeley-Illinois-Maryland
Peak
OBSERVATIONS
The data were taken during 1995 February and 2001
January using the former NRAO1 12 m telescope at Kitt
Peak, Arizona. The receivers used were dual-channel,
cooled SIS mixers covering the 1, 2, and 3 mm bands. Each
mixer was operated in single-sideband mode with image
rejection of Z20 dB. The back ends used were 256 channel
Ðlter banks of 500 kHz and 1 MHz resolution operating in
parallel mode (2 ] 128). The temperature scale was determined by the chopper wheel method, corrected for forward
spillover losses, and is given as T *. The radiation temperature T , assuming the source ÐllsRonly the main beam, is
then T \ RT */g .
R wereR made
c
Maps
of the J \ 6 ] 5 line of N O (X 1&`) at
150,735.0 MHz and of the %` , J \ 3/2 ] 1/22 transition of
1@2These lines were mapped on
NO (X2% ) at 150,176.3 MHz.
r
a 5 ] 11 grid with 30A spacing in right ascension and declination, o†set from the (0, 0) position at Sgr B2 (OH) (a \
17h44m11s. 0, d \ [28¡22@30@@ [B1950.0]). The data were
taken in position-switching mode with the o† position 30@
west in azimuth. The beam size of the 12 m telescope at 150
GHz was 42A such that the maps were oversampled and
g \ 0.75. In addition, the J \ 3 ] 2 and J \ 9 ] 8 tranc
sitions
of N O at 75,369.2 and 226,094.0 MHz, respectively,
2
were measured
at a single position near Sgr B2 (N) (a \
17h44m11s. 0, d \ [28¡21@30@@ [B1950.0] ; the N O peak).
The J \ 2 ] 1 line of NO` at 238,383.2 MHz 2(Bowman,
Herbst, & DeLucia 1982) and the main hyperÐne component of the J
\ 5 ] 6 transition of the NO
Ka,Kc
15
06Godfrey, & Harris 1974)2
radical at 70,589.7
MHz (Baron,
were also searched for toward one position close to the
N O peak (a \ 17h44m11s. 0, d \ [28¡21@20@@ [B1950.0]).
2 efficiencies and beam sizes at these other frequencies
Beam
are given in Table 1.
3.
RESULTS
A summary of the observations for the N/O species at the
Sgr B2 (N) and (M) positions is given in Table 1 as well as
1 NRAO is operated by Associated Universities, Inc., under cooperative
agreement with the National Science Foundation.
246
HALFEN, APPONI, & ZIURYS
Vol. 561
TABLE 1
OBSERVATIONS OF N/O SPECIES AND CH OH AT SELECTED POSITIONSa
3
SGR B2 (N)
V
LSR
(km s~1)
*V
1@2
(km s~1)
J\3/2]1/2
...
...
...
...
...
...
...
F\5/2]3/2
150176.3
42
0.34 ^ 0.02
66.6 ^ 0.8
19.5 ^ 1.6
0.34 ^ 0.03 63.1 ^ 1.6
F\3/2]1/2
150198.5
42
0.09 ^ 0.02
64.1 ^ 2.1
14.3 ^ 4.8
0.10 ^ 0.03 60.0 ^ 6.0
F \ 3 / 2 ] 3 / 2b
150218.6
42
0.11 ^ 0.02
64.8 ^ 2.3
23.6 ^ 4.5
0.07 ^ 0.03 64.4 ^ 6.0
F \ 1 / 2 ] 1 / 2b
150225.5
...
...
...
...
...
N O .......... J\3]2
75369.2
85
0.03 ^ 0.02
71.9 ^ 1.9
16.2 ^ 3.7
...
...
2
J\6]5
150735.0
42
0.07 ^ 0.01
66.0 ^ 2.2
18.8 ^ 6.6
0.04 ^ 0.02 62.5 ^ 3.0
J\9]8
226094.0
28
0.07 ^ 0.04
67.0 ^ 1.5
11.4 ^ 3.0
...
...
NO` . . . . . . . . . J \ 2 ] 1
238383.2
26
\0.016
62.0c
20c
...
...
NO . . . . . . . . . . N
\5 ]6
...
...
...
...
...
...
...
2
Ka, Kc
15
06
J \ 11 / 2 ] 13 / 2
...
...
...
...
...
...
...
F \ 13 / 2 ] 15 / 2
70589.6
90
\0.018
62.0c
20c
...
...
HNOd . . . . . . . . J
\1 ]0
81477.6
...
...
69.55 ^ 5.5 31.3 ^ 28.5
...
65.8 ^ 5.4
Ka, Kc
01
00
CH OH . . . . . . J \ 14 ] 14 E
150141.5
42
0.41 ^ 0.02
64.3 ^ 0.3 12.2 ^ 0.79 0.15 ^ 0.03 64.1 ^ 4.7
3
Kq
0
~1
a Source coordinates (epoch 1950) are Sgr B2 (N) : 17h44m10s. 2, [28¡21@13A ; Sgr B2 (M) : 17h44m10s. 2, [28¡22@03A. All errors are 3 p.
b Blended components.
c Assumed value.
d From Kuan & Snyder 1994.
...
16.9 ^ 2.9
11.8 ^ 6.0
13.4 ^ 6.0
MOLECULE
NO . . . . . . . . . . .
TRANSITION
l
(MHZ)
h
b
(arcsec)
g
c
...
0.75
0.75
0.75
...
0.93
0.75
0.54
0.48
...
...
0.94
...
0.75
the methanol line data. This table lists the line parameters
measured for NO, N O, and CH OH (T *, *V , and V )
3 intensities
R
1@2
and the upper limits2 to the line
obtained LSR
for
NO` and NO .
2 electronic ground state, and therefore its
NO has a 2%
r
pure rotational spectrum
contains the e†ects of spin-orbit
coupling (i.e., ) \ 1 and 3/2 ladders) and j-doubling
(positive and negative2 parity components). We observed the
lower energy spin-orbit component ) \ 1 and the positive
parity j-doublet, hence, the designation2 %` . This tran1@2
sition, however, consists of four strong hyperÐne
components, indicated by quantum number F, which arise from
the nitrogen spin of I \ 1. As Table 1 illustrates, the four
hyperÐne (HF) lines were detected at the N and M positions. (Two of the HF components, F \ 3/2 ] 3/2 and
F \ 1/2 ] 1/2, are separated in frequency by 7 MHz [14
km s~1] and hence are blended together to form one single
line.) The main hyperÐne component, F \ 5/2 ] 3/2, was in
fact observed at every position studied in the Sgr B2 cloud,
including Sgr B2 (S) and Sgr B2 (OH). The intensity of this
component was T * D 0.34 K at the N and M positions, and
R were roughly a factor of 2 weaker, as
the weaker HF lines
expected in the optically thin, LTE case. The line widths for
NO are *V D 12È20 km s~1 for unblended HF lines, with
velocities in1@2the range D60È64 km s~1 at the M position
and D64È67 km s~1 at the N position. These values are
typical for Sgr B2 (HuŽttemeister et al. 1993 ; Kuan & Snyder
1994).
Observed in the same bandpass as the NO lines was the
J \ 14 ] 14
E transition of CH OH. The energy
Kq
0
~1 for the upper level of
3 this transition is
above
ground
state
255 K. Hence, it traces warm gas. This line was detected at
Sgr B2 (N), with a weaker feature toward Sgr B2 (M). While
the LSR velocity of CH OH at these positions is compara3 width appears to be somewhat
ble to that of NO, the line
narrower [*V D 12.2 vs. 19.5 km s~1 at Sgr B2 (N)]. A
1@2width was found in HC N as a function of
decrease in line
energy above ground state (de Vicente et3 al. 2000). Groundstate lines had *V D 20 km s~1, while those originating
1@2
T*
R
(K)
V
LSR
(km s~1)
SGR B2 (M)
*V
1@2
(km s~1)
T*
R
(K)
...
11.5 ^ 4.8
...
...
...
...
45.7 ^ 55.5
13.2 ^ 5.4
from the v or v modes exhibited line widths as narrow as 7
7
km s~1. 6The narrower
line width in CH OH may be
3
explained by a similar phenomenon.
A spectrum illustrating the di†erence in NO and CH OH
3
line proÐles is shown in Figure 2. These data were obtained
at the northern source. The positions and relative intensities
of the NO HF components are shown by lines underneath
the spectra and clearly indicate low optical depth in this
transition. The CH OH line is the stronger feature on the
3 noticeably narrower than the nearby
right, which appears
NO transitions. There may be a second velocity component
present in the main HF component (F \ 5/2 ] 3/2) of NO
near V D 90 km s~1. A higher velocity component near
85 km LSR
s~1 has been found at Sgr B2 (N) in metastable NH ,
3
FIG. 2.ÈSpectrum of the %` , J \ 3/2 ] 1/2 transition of NO near 150
1@2 12 m telescope toward Sgr B2 (N). This
GHz, observed with the Kitt Peak
transition consists of four main hyperÐne components, labeled by quantum
number F. The lines under the spectrum show their positions and LTE
relative intensities. The strong line to the right of the NO features is the
14 ] 14 E transition of CH OH, which lies 255 K above ground state.
0
~1
Spectral
resolution
is 1 MHz (2 3km s~1).
No. 1, 2001
N/O CHEMICAL NETWORK : N O AND NO IN SGR B2
2
which is thought to trace warm, low-density material.
Another candidate for the identiÐcation of this feature is
dimethyl ether.
The J \ 6 ] 5 transition of N O was detected at Sgr B2
2
(N) but not at Sgr B2 (S), and only a weak feature was
present at the middle core (see Table 1), similar to the
14 ] 14
E line of CH OH. In addition, the J \ 3 ] 2
0
~1
3
and 9 ] 8 lines of this molecule were detected at Sgr B2 (N).
The J \ 3, 6, and 9 levels lie 7.2, 25.3, and 54.2 K above
ground state, respectively. Consequently, for the J \ 9 ] 8
transition of N O to be observed, this molecule must arise
2
from moderately warm gas. The LSR velocities of N O are
2
similar to those of NO and CH OH at the same positions,
3
except for the J \ 3 ] 2 transition, whose velocity is a little
higher than the others (V D 72 vs. 64È67 km s~1).
However, this transition wasLSR
measured with a considerably
larger beam size (85A vs. [42A), so it may be sampling a
slight velocity gradient. Also, the J \ 9 ] 8 transition of
N O has a narrower line width than the other NO and N O
2 more comparable to that of CH OH. Again, this e†ect
2
lines,
3
may result from excitation.
The three transitions of N O are plotted in Figure 3. As
2
the Ðgure shows, the line shapes
are very similar for each
transition, indicating that they indeed arise from the same
molecule. Three unidentiÐed lines are present in the
FIG. 3.ÈSpectra of the J \ 3 ] 2, 6 ] 5, and 9 ] 8 transitions of N O
2
near 75, 150, and 226 GHz, respectively, observed with the 12 m telescope
toward Sgr B2 (N), the N O peak. Spectral resolutions are 500 kHz (2 km
s~1) for the 3 ] 2 line and2 1 MHz for the 6 ] 5 (2 km s~1) and 9 ] 8 (1.3
km s~1) transitions. The N O lines may consist of two velocity components, similar to what has2been observed for EtCN and VyCN at this
position (Miao et al. 1995). Various unidentiÐed features appear in the
J \ 6 ] 5 spectrum.
247
J \ 6 ] 5 spectrum ; these are not likely to arise from other
velocity components of N O because they do not appear in
2
the other transitions. The N O lines themselves may be
2
comprised of two velocity components. Two Gaussian
curves can be Ðtted to the N O J \ 6 ] 5 line proÐle, with
2
V D 63.5 and 71.9 km s~1 and *V D 8.4 and 4.8 km
LSR
1@2
s~1. Similar velocity components are found in VyCN and
EtCN by Miao et al. (1995). However, the supposed
““ velocity components ÏÏ in N O are barely above the noise,
2
and longer integrations need to be carried out for these
spectra before they can be established with any certainty.
Additionally listed in Table 1 are the upper limits for the
searches for NO` and NO (T * [ 0.02 K) and line param2 R
eter data for HNO from Kuan & Snyder (1994). HNO, like
NO, was readily observed at both cloud cores. The LSR
velocities of HNO, NO, and N O agree within the quoted
2 widths of HNO, on the
errors at these positions. The line
other hand, are unusually broad at the middle and northern
positions relative to the other molecules (*V D 31È46 km
s~1 for HNO as opposed 12È19 km s~1 for 1@2
NO and N O).
2
The 3 p errors on the HNO line widths, which were measured with an interferometer, are larger than the numbers
themselves. The single-dish data of HNO from Kuan &
Snyder do not appear to exhibit these broad line widths but
look very similar to the NO spectra. Based on line proÐles,
it can therefore be concluded that HNO, NO, and N O
2
emission arises from similar material.
The mapping data for the J \ 6 ] 5 line of N O, the
2 and
J \ 3/2 ] 1/2 (%` ), F \ 5/2 ] 3/2 transition of NO,
1@2
the J \ 14 ] 14
E line of CH OH is easily sumKq Of 0the 55~1
marized.2
separate positions3 observed, N O was
detected at only seven of these clustered around Sgr 2B2 (N).
The methanol distribution mimics that of N O, although it
2 contrast, NO
was observed in the NO spectral bandpass. In
was detected at every single position ; however, the emission
becomes weaker at the northern and southern edges of the
map. These e†ects are apparent in Figures 4 and 5.
Figure 4 presents the NO/CH OH spectra as a function
3 of each spectrum is the
of position. The line in the middle
strongest HF component of NO, the weaker lines to the left
are the other HF lines of this molecule, and the other strong
feature to the right is the CH OH line (see Fig. 2). The (0, 0)
position on this map is 3that of Sgr B2 (OH) (a \
17h44m11s. 0, d \ [28¡22@30@@ [B1950.0]), while the star
marks the position of Sgr B2 (N). The NO lines are clearly
visible in every panel, and there appears to be slight northsouth and east-west velocity gradients. To the south and
east, velocities are lower, typically V D 55 km s~1, but
LSRV D 65 km s~1.
increase to the north and west, with
LSR molecules such
These gradients were also observed in other
as HC N (de Vicente et al. 2000) and HNO (Kuan & Snyder
1994). 3The line widths are relatively constant in value across
the map, with *V D 15È20 km s~1. In contrast to NO,
1@2 is sharply conÐned to the Sgr B2 (N)
the CH OH emission
3
position. There is some extended emission 30A south and
west of Sgr B2 (N), but the methanol line disappears 1@ away
from this region.
In Figure 5, spectra of the J \ 6 ] 5 transition of N O
2
are presented as a function of spatial position (see middle
panel of Fig. 3). Again, the strongest line intensity is at Sgr
B2 (N), the panel marked by the asterisk, with weaker emis-
2 Gaussian Ðts to the line proÐles or intensity upper limits for all positions can be obtained from the authors.
248
HALFEN, APPONI, & ZIURYS
Vol. 561
FIG. 4.ÈPosition-spectrum map of the J \ 3/2 ] 1/2 transition of NO (%` ) and the J \ 14 ] 14 E transition of CH OH in the Sgr B2 complex
~1 spectra are plotted3on the same scale as Fig. 2 :
(see Fig. 2). The (0, 0) o†set is at the Sgr B2 (OH) position, and the star marks 1@2
the position ofKqSgr B20 (N). The
LSR velocity for the x-axis and T * (K) for the y-axis. It is evident from the spectra that NO is extended over the entire region, while the CH OH transition
3
sharply peaks near Sgr B2 (N). TheRspectra are spaced in position by 30A.
sion 30A west and south of this core ; the line is not visible at
any other position.
In Figures 6 and 7, contour maps of the peak line intensities, in T * (K), of NO, N O, and CH OH are shown.
R
2 of the strongest
3
Figure 6 shows
a contour map
HF component of NO (F \ 5/2 ] 3/2). Positions of Sgr B2 (N), (M),
and (OH) are indicated, as well as the (2N) peak where
HNCO shows a maximum (Wilson et al. 1996 ; Minh et al.
1998). Overlaid on the NO contours are those of HNO,
indicated by dashed lines, from the interferometer maps of
Kuan & Snyder (1994). Although extended over the entire
region, NO appears to show local maximum near Sgr B2
(M) and Sgr B2 (2N). The peak near the middle position is
coincident with an HNO maximum, but there are no other
small-scale correlations between these two molecules.
The left- and right-hand panels of Figure 7 present the
contour maps for N O (J \ 6 ] 5) and CH OH (J \
2
3 two Kq
14 ] 14
E). The source
distributions of these
lines
0
~1
are extremely similar. Both transitions show a distinct
maximum at Sgr B2 (N), with weaker emission near Sgr B2
FIG. 5.ÈPosition-spectrum map of the J \ 6 ] 5 transition of N O in the Sgr B2 region (see middle panel of Fig. 3). The (0, 0) o†set is at Sgr B2 (OH), and
2 in Fig. 3. The N O line is only observed near the Sgr B2 (N) position. The spectra are
the star marks the position of Sgr B2 (N). The axes are identical to those
2
spaced in position by 30A.
No. 1, 2001
N/O CHEMICAL NETWORK : N O AND NO IN SGR B2
2
249
and 40 K from the extended component of NH over this
3 n D 105
region. Moreover, Lis & Goldsmith (1991) found
cm~3 over the same 10 pc (4@) region, while HuŽttemeister et
al. (1993) derived n D 104 cm~3. Hence, there appears to be
sufficient density and gas hot enough to readily excite the
J \ 6 ] 5 transition of N O over at least 4@ of the Sgr B2
2
cloud.
Further evidence for sufficient excitation conditions
comes from observations of the J \ 12 ] 11 transition of
HC N (Lis & Goldsmith 1991). This line is extended over 4@
3
in the Sgr B2 cloud complex, encompassing the major cores.
The energy above ground state of the J \ 12 level is 28
KÈcomparable to the J \ 6 level of N O. However, HC N
2
3
has a dipole moment of 3.6 DÈover an order of magnitude
larger than that of N O. If this transition is excited over an
2
extended region, then the J \ 6 ] 5 line of N O should be
2
also, provided that the molecule is present. The conÐned
nature of N O emission must therefore be a chemical e†ect.
2
FIG. 6.ÈContour map of the peak line intensity (T *) of the %` , J \ 3/
R
1@2 lowest
2 ] 1/2 ; F \ 5/2 ] 3/2 transition of NO (1 MHz resolution).
The
contour level is at 0.10 K and increases in increments of 0.02 K. The (0, 0)
position is Sgr B2 (OH). The three stars mark the positions of Sgr B2 (N),
Sgr B2 (M), and Sgr B2 (OH), and the open circle indicates the 2N source,
the so-called northern nitrogen core. The beam size for the observations is
shown in the bottom left-hand corner. NO appears to be extended
throughout the complex, with local maxima near the M and 2N cores. The
dashed contours show the four major HNO peaks (from Kuan & Snyder
1994).
(M) and no evidence of emission at Sgr B2 (OH) or Sgr B2
(2N). The maximum in these maps is very close to the HNO
(N) source (Kuan & Snyder 1994). The HNO (S) and HNO
(NW) peaks have no counterparts in either NO or N O.
2
4.
ANALYSIS
4.1. A ConÐned N O Source : Excitation or Chemistry ?
2
The conÐned distribution of N O emission could be a
2
result of either excitation or preferential
chemistry. Excitation does not explain this result. First of all, N O has a
2
rather small dipole moment of 0.16 D. The Einstein
Acoefficient for the J \ 6 ] 5 transition is therefore
4.7 ] 10~7 s~1. Consequently, the critical density needed to
equate the A-coefficient downward with the collisional rate
upward is n D 4 ] 104 cm~3, assuming a collisional cross
section of p cD 10~15 cm2 and a gas kinetic temperature of
20 K. Such a kinetic temperature is likely to be a lower limit
for the Sgr B2 complex. Lis & Goldsmith (1991) for
example, found T D 20È40 K over 10 pc (or 4@) of the
K
cloud, which includes
Sgr B2 (M), (N), and (OH).
HuŽttemeister et al. (1993) derived temperatures between 30
4.2. Column Densities and Abundances of N / O Species
The column density of N O at its peak near Sgr B2 (N)
2
was determined from a rotational
temperature diagram (see
Turner 1991), as shown in Figure 8. This diagram is a plot
of upper-state column density per detected transition versus
energy of the upper state. The three observed transitions of
N O plotted here follow a reasonably straight line and yield
T 2 D 40 K and N D 1.5 ] 1015 cm~2. No correction was
rot for beam sizetotin this plot. (With about a 1@ source, the
made
beam should be nearly Ðlled for every transition.)
For the other molecules observed, only one rotational
transition was measured, and hence the column densities
had to be calculated assuming a rotational temperature
from a single formula, which is
3k105T *V f
R 1@2 rot
N \
.
(1)
tot 8n3lk 2S e~*Egd@Trot R
0 ij
HF
In the expression, l is the frequency of the transition, k is
0
the permanent dipole moment, f is the rotational partirot
tion function, S is the line strength, *E is the energy of
ij ground state, and T gd
the Jth level above
and *V are the
R km s~1),
1@2 respecradiation temperature and line widths (in
tively, of the transition J ] 1 ] J. R is the relative HF
HF NO ) ; otherwise,
intensity, if present in the transition (NO,
2
R \ 1. For the diatomic or linear species involved
(NO,
HF
NO`, N O), an exact value was calculated for the partition
function 2for J ¹ 100. In the case of NO , an asymmetric top
2
with C symmetry, the partition function
was estimated
2v
using the equation (see Turner 1991)
f \ 1 [(kT /h)3n/ABC]1@2 .
(2)
rot 2
Here A, B, and C are the rotational constants of the molecule. Rotational temperatures were chosen based on the
N O result for Sgr B2 (N) and on kinetic and excitation
2
temperatures
derived from measurements of other molecules for the other positions (Lis & Goldsmith 1991 ;
HuŽttemeister et al. 1993). These values are given in Table 2.
It should be noted that the dipole moments of NO and NO
are comparable to that of N O (k B 0.2È0.3 D ; see Table2
2 has
0 not been measured ; it
2). The dipole moment of NO`
was estimated to be D1 D, based on dipole moments of
other molecular ions.
Resulting column densities for NO and N O at the core
positions in Sgr B2 are tabulated in Table 2, 2along with the
250
HALFEN, APPONI, & ZIURYS
Vol. 561
FIG. 7.ÈContour maps of the peak line intensity T * (K) of the J \ 6 ] 5 transition of N O (left) and the J \ 14 ] 14
E line of CH OH (right). The
R in increments of 0.005 K. For CH OH,
2 the lowest contour
Kq is 0.04
0 K ~1
lowest contour of the N O map is 0.010 K and increases
with increments3of 0.04 K. Beam
2
sizes are shown in the lower
left-hand corners, and the three stars mark the positions of Sgr3 B2 (N), (M), and (OH). The northern nitrogen core, Sgr B2 (2N), is
marked by an open circle. The (0, 0) position is that of Sgr B2 (OH). The emission of both molecules shows a distinct peak at Sgr B2 (N), with some
north-south elongation.
upper limits for NO and NO`. Additionally listed are
column density values2for HNO derived by Kuan & Snyder
(1994). Fractional abundances, relative to H , are also
2
listed, which assume N(H ) D 1024 cm~2 (see Nummelin
et
2
al. 2000). As the table shows, the most abundant N/O molecule is NO itself, which has a fairly constant column
density of N D (0.8È1.5) ] 1016 cm~2 across several arctot f (NO/H ) D (0.8È1.5) ] 10~8. N O has
minutes with
2 near Sgr B2 (N), which
2 correN D 1015 cm~2 at its peak
tot
sponds to f D 10~9. HNO, on the other hand, has a column
density and abundance almost 2 orders of magnitude
smaller than that of NO across the north-south ridge
TABLE 2
COLUMN DENSITIES AND ABUNDANCES OF N/O MOLECULES IN SGR B2
f (X / H )
2
MOLECULE
k
0
(D)
N O........
2
NO . . . . . . . . .
0.16
0.153
NO` . . . . . . .
NO . . . . . . . .
2
HNOc . . . . . .
D1.0
0.32
1.67
T (K)
rot
37
25
37
25
25
37
37
18
29
20
POSITION
N
tot
(cm~2)
Observeda
Early-Times Model
(3.2 ] 105 yr)b
Steady-State Model
(1 ] 108 yr)b
N
2N
N
M
S
N
N
N
N
N
1.50 ] 1015
1.32 ] 1016
1.54 ] 1016
9.33 ] 1015
7.72 ] 1015
\4.5 ] 1012
\3.3 ] 1015
2.5 ] 1014
3.6 ] 1014
1.1 ] 1014
1.50 ] 10~9
1.32 ] 10~8
1.54 ] 10~8
9.33 ] 10~9
7.22 ] 10~9
\4.5 ] 10~12
\3.3 ] 10~9
2.5 ] 10~10
3.6 ] 10~10
1.1 ] 10~10
2.926 ] 10~11
2.189 ] 10~7
...
...
...
3.808 ] 10~11
2.072 ] 10~11
7.483 ] 10~10
...
...
2.757 ] 10~11
3.267 ] 10~6
...
...
...
1.503 ] 10~10
1.156 ] 10~10
4.133 ] 10~9
...
...
a Assumes N(H ) \ 1 ] 1024 cm~2 ; Nummelin et al. 2000.
2
b Millar et al. 1997.
c Kuan & Snyder 1994.
No. 1, 2001
N/O CHEMICAL NETWORK : N O AND NO IN SGR B2
2
251
temperature-sensitive gas-phase reactions. A search
through the most recent versions of the UMIST reaction
rate catalog (see, e.g., Le Tue†, Millar, & Markwick 2000)
suggests that the only viable processes leading to N O are
2 were
neutral-neutral reactions. No ion-molecule processes
found in the data set that produce N O. Of the neutralneutral reactions, the only ones with a2 rate-constant k Z
10~12 cm3 s~1 were
FIG. 8.ÈRotational diagram for N O, based on the J \ 3 ] 2, 6 ] 5,
2
and 9 ] 8 transitions observed. The three
data points lie approximately
along a straight line and suggest N D 1015 cm~2 and T D 40 K.
rot
[N D 1È4 ] 1014
cm~2
and
f (HNO/H ) D (1È4)
2
] tot
10~10]. The upper limit for NO` places its abundance
over a factor of 1000 less than that of NO, while that for
NO indicates N [ 1015 cm~2, or f [ 10~9.
2
tot
5.
DISCUSSION
5.1. High-T emperature Gas-Phase Production of N O
2
The most striking result of this study is the limited spatial
extent of N O in the Sgr B2 complex as compared to NO
and HNO. 2This molecule appears to exist solely in the
vicinity of Sgr B2 (N), with a deconvolved source size of
h [ 45@@. It is not present at the so-called northern nitrogen
s
core
of Wilson et al. (1996) ; in fact, the emission drops
sharply to the north, avoiding this region altogether. Moreover, the line widths of the N O transitions toward their
2 as opposed to *V D 30
peak are *V D 11È19 km s~1,
1@2
1@2
km s~1 found in HNCO (Wilson et al. 1996).
Some chemical e†ect is limiting the production of N O
2
across the Sgr B2 complex. Miao et al. (1995) also found
such molecules as EtCN, VyCN, and HCOOCH conÐned
3 authors
to a small region centered on Sgr B2 (N). These
attributed this e†ect to grain surface chemistry in the dusty
core of this source followed by subsequent evaporation
caused by the high temperatures of this material. Indeed,
both NH observations (HuŽttemeister et al. 1993) and
3
HC N measurements
(de Vicente et al. 2000) indicate gas
3
temperatures near 200È350 K at Sgr B2 (N). However, this
chemical scenario probably does not apply to N O, whose
2
structure involves two unsaturated bonds, i.e., NxNxO.
If
N O were formed on grain surfaces, these bonds would
2 be saturated.
likely
Another alternative for the synthesis of this molecule is
k1
NO ] N ÈÈÈÕ N O ] O ,
(3)
2
2
k2
NO ] NH ÈÈÈÕ N O ] OH ,
(4)
2
2
k3
(5)
NO ] NH ÈÈÈÕ N O ] H .
3
The Ðrst two processes may not be important because the
abundance of NO is not high, given our failure to detect
2
this species. Furthermore, the rate of the Ðrst process is
k D 3 ] 10~12 cm3 s~1. The second rate is not well
1
known.
It has an inverse temperature dependence between
200 and 300 K, but at lower temperatures the rate is highly
uncertain. At 200 K, k D 4 ] 10~11 cm3 s~1.
The third reaction 2may be more probable because both
reactants are known interstellar molecules. One reactant,
NO, is abundant throughout the Sgr B2 complex, as this
work has demonstrated. The other reaction partner, NH, is
a simple hydride that has been detected optically (Meyer &
Roth 1991 ; the pure rotational transitions of NH lie in the
far-infrared and thus pose difficulties for ground-based
detections). NH is likely to be widespread in molecular
clouds similar to OH and CH. The third reaction additionally obeys a rate law that increases rapidly with temperature (Mallard et al. 1998) :
k \ 1.16 ] 10~10(T /300)~1.03 exp ([420/T ) .
(6)
3
In Figure 9, this rate is plotted against temperature. As this
diagram shows, the rate is negligible near 50 K but increases
to k D (1È3) ] 10~11 cm3 s~1 between 125 and 300 K.
3 this reaction becomes important for T [ 125 K.
Hence,
Temperatures are known to exceed this value Ktoward Sgr
B2 (N), as demonstrated by other molecular measurements
(HuŽttemeister et al. 1993 ; de Vicente et al. 2000) and as
indicated by the detection of the 14 ] 14 E transition of
0
~1N O at Sgr B2
CH OH. The distribution and existence
of
(N) 3 can therefore be explained by the 2 reaction of
NO ] NH, which proceeds only at elevated temperatures.
The importance of high-temperature chemistry in the
production of N O is additionally supported by chemical
2
models. Millar, Farquhar,
& Willacy (1997) have calculated
the abundance of this species in their most recent model, at
both ““ early times ÏÏ (D3.2 ] 105 yr) and steady state (108
yr). Their calculations are for a 10 K cloud. The calculated
abundances are f D 3 ] 10~11 at both epochsÈalmost 2
orders of magnitude less than the observed value of
f D 2 ] 10~9 (see Table 2). Therefore, 10 K gas is not likely
to contain a signiÐcant amount of N O.
2
5.2. Relationship of NO and HNO
NO appears to be an important precursor molecule to
N O. Is it also linked to HNO ? It is clear from this work
2 the HNO study of Kuan & Snyder (1994) that both
and
species are present in the same bulk gas throughout Sgr B2.
Examining the UMIST rate tables, the fastest reaction
252
HALFEN, APPONI, & ZIURYS
FIG. 9.ÈPlot of the rate of the reaction NO ] NH ] N O ] H vs.
2 a value
temperature. The rate increases rapidly for T Z 50 K and has
k Z 10~11 cm3 s~1 for T Z 125 K.
leading to HNO is via NO :
k4
NO ] HNO` ÈÈÈÕ HNO ] NO` .
(7)
The rate here is k D 7 ] 10~10 cm3 s~1, i.e., close to the
4 other reactions leading to HNO are
Langevin value. All
neutral-neutral processes and are at least an order of magnitude slower. Furthermore, HNO` is rapidly produced via
the ion-molecule reaction (Le Tue† et al. 2000)
Vol. 561
fore are not feasible even at 200 K. The only possible route
to NO seems to be the reaction
2
k5
O ] HNO ÈÈÈÕ NO ] H .
(9)
2
The rate here is relatively slow, with k D 1 ] 10~12 cm3
5
s~1. All other rates considered for NO and N O are at least
2
an order of magnitude faster. Therefore, the failure to detect
NO does not contradict chemical rate predictions. The
2
Millar et al. (1997) model in fact calculates an NO abun2
dance of f D 2 ] 10~11 at early times (Table 2). The upper
limit found here is f \ 10~9.
In contrast, NO` is rapidly produced from NO through
charge exchange reactions with many di†erent ions (H`,
H`, C`, CH`, CH`, NH`, etc.). Most of these processes
2
3
3
proceed near the Langevin rate (D10~9 cm3 s~1 ; Le Tue†
et al. 2000). However, the reverse processes lead rapidly
back to NO, such that the NO` abundance, as predicted by
Millar et al. (1997), is only f D 4 ] 10~11. (HNO and N O
2
can also produce NO`, but there are many more pathways
from NO with comparable or faster rates.)
The upper limit obtained for NO` from our observations
is f [ 10~12. However, this is based on an estimated dipole
moment of 1 D. (The dipole moment of NO`, to our knowledge, has never been measured.) If this value is smaller by a
factor of 3.5, for example, then the observed upper limit
would exactly match the model predictions.
A schematic summarizing the chemical network linking
NO, N O, HNO, NO , and NO` is shown in Figure 10.
2 scheme appears to be NO, which is
The key2 molecule in this
the main precursor to HNO (via HNO`) as well as N O (by
the temperature-dependent process NO ] NH). It 2also is
the key reactant leading to NO`. The major route to NO
is through HNO, and NO can subsequently form N O2
2 discussed. Finally, NO can
2
through the processes already
2
form NO via a reaction with O (NO ] O ] O ] NO),
2
2
but this process is slow.
Because a fair amount of nitrogen is contained in
ammonia, especially toward the Sgr B2 (M) and Sgr B2 (N)
cores (see, e.g., HuŽttemeister et al. 1993), it is important to
NO ] H` ] HNO` ] H .
(8)
3
2
Hence, provided there is sufficient NO and H`, HNO can
3
be synthesized.
The observed abundance of HNO in the Sgr B2 complex
is f D (1È3) ] 10~10 (see Table 2), while it is f D (0.6È
1.5) ] 10~8 for NO. Therefore, NO is at least a factor of 10
more abundant than HNO and, consequently, could be the
main precursor for HNO via the above reaction scheme.
Curiously, the early-time calculations of the model of Millar
et al. (1997) reproduce the observed HNO abundance very
well (see Table 2). For NO, on the other hand, the calculated value is about a factor of 15 too high, even at early
times.
5.3. T he N/O Chemical Network
Two other molecules searched for in this study are NO`
and NO . The latter species is very similar to N O in that
there are2 no ion-molecule reactions leading to its2 synthesis
(Le Tue† et al. 2000). It seems to be produced only by
neutral-neutral processes. In fact, many of these reactions
have extremely large activation energy barriers and there-
FIG. 10.ÈDiagram illustrating the N/O chemical network. The key
species in this scheme is NO, which can be converted to N O via the
2 to HNO
temperature-dependent process NO ] NH ] N O ] H and also
2 ] HNO ] NO`. NO`
by the fast ion-molecule reaction HNO` ] NO
arises from NO and HNO via charge exchange processes ; NO is pro2 dependuced slowly from HNO ] O ] NO ] H. The network is highly
2
dent on neutral-neutral reactions.
No. 1, 2001
N/O CHEMICAL NETWORK : N O AND NO IN SGR B2
2
consider how the N/O chemical network relates to NH .
3
Although NH is involved in a few charge exchange reac3
tions with NO and NO`, it in general does not participate
in the N/O network (Le Tue† et al. 2000). In fact, it is not
part of any pathways in the formation or destruction of
N O. The chemistry of N/O compounds is therefore almost
2
totally distinct from that of NH .
3
Because NO is the cornerstone of the N/O scheme, it is of
interest to consider what the major routes to its formation
are. There are various ion-molecule reactions of HNO`
that lead to NO, but the reverse processes are also fast,
making this pathway circular. (Rest frequencies for HNO`
are unavailable, so its abundance and distribution are
unknown.) Examination of other pathways suggests that
the most promising reaction leading to the synthesis of NO
is
k6
NH ] O ÈÈÈÕ NO ] H .
(10)
The rate here is k D 1.2 ] 10~10 cm3 s~1, with no activation energy, while6 the reverse process has E D 30,000 K
(Le Tue† et al. 2000) and hence will not occur.actThis neutralneutral reaction is therefore critical to the N/O network. It
is quite interesting to note that many of the important reaction routes in this network are not ion-molecule processes
but rather neutral-neutral reactions.
6.
CONCLUSION
Mapping observations of the NO and N O molecules
across the Sgr B2 complex has lead to a new 2interpretation
of the chemical processes producing these species. The
253
widespread distribution of NO across 5@ of the cloud suggests that it is formed by a low-temperature process, which
is likely to be the reaction NH ] O ] NO ] H. N O, in
2
contrast, is conÐned to a D1@ source centered at Sgr B2
(N)Èa nearly identical distribution as found for the highenergy (E B 255 K) 14 ] 14
E transition of CH OH.
u
0
~1
3
This e†ect cannot arise from excitation. Therefore, N O
2
must be exclusively formed in hot gas near the northern
core. Gas-phase reactions likely account for this hightemperature synthesis, in particular the process NH
] NO ] N O ] H, which becomes efficient only at tem2
peratures T Z 125 K. Indeed, the so-called Large Molecule
K
Heimat at Sgr B2 (N) may actually be the ““ Hot Molecule
Heimat,ÏÏ or Sgr B2 (HMH). HNO, on the other hand, is
probably produced by the ion-molecule reaction
HNO` ] NO ] HNO ] NO`. Its distribution is qualitatively similar to NO across the Sgr B2 cloud. Also, emission
from N O was not present at the so-called northern nitro2
gen core, although NO may have a weak maximum at this
position. Finally, these observations demonstrate that
neutral-neutral reactions, even with activation energy barriers, may play an important role in the synthesis of simple
molecules in complex cloud cores like those found in Sgr
B2.
This research was supported by NSF Grant AST 9820576. The authors thank John P. Schaefer and the
Research Corporation for providing the funding to keep the
Kitt Peak 12 m telescope operational so that this project
could be completed.
REFERENCES
Baron, P. A., Godfrey, P. D., & Harris, P. O. 1974, J. Chem. Phys., 60, 3723
Miao, Y., Mehringer, D., Kuan, Y.-J., & Snyder, L. E. 1995, ApJ, 445, L59
Bowman, W. C., Herbst, E., & De Lucia, F. C. 1982, J. Chem. Phys., 77,
Miao, Y., & Snyder, L. E. 1997, ApJ, 480, L67
4261
Millar, T. J., Farquhar, P. R. A., & Willacy, K. 1997, A&AS, 121, 139
Cummings, S. E., Linke, R. A., & Thaddeus, P. 1986, ApJS, 60, 819
Minh, Y. C., Haikala, L., Hjalmarson, A., & Irvine, W. M. 1998, ApJ, 498,
de Vicente, P., Mart• n-Pintado, J., Neri, R., & Colom, P. 2000, A&A, 361,
261
1058
Nummelin, A., Bergman, P., Hjalmarson, AŽ ., Friberg, P., Irvine, W. M.,
Gaume, R. A., & Claussen, M. T. 1990, ApJ, 351, 538
Millar, T. J., Ohishi, M., & Saito, S. 2000, ApJS, 128, 213
Goldsmith, P. F., Snell, R. L., Hasegawa, T., & Ukita, N. 1987, ApJ, 314,
Reid, M. J., Schneps, M. H., Moran, J. M., Gwinn, C. R., Genzel, R.,
525
Downes, D., & RoŽnnaŽng, B. 1988, ApJ, 330, 809
HuŽttemeister, S., Wilson, T. L., Henkel, C., & Mauersberger, R. 1993,
Sutton, E. C., Jaminet, P. A., Danchi, W. C., & Blake, G. A. 1991, ApJS, 77,
A&A, 276, 445
255
Kuan, Y.-J., & Snyder, L. E. 1994, ApJS, 94, 651
Turner, B. E. 1989, ApJS, 70, 539
Le Tue†, Y. H., Millar, T. J., & Markwick, A. J. 2000, A&AS, 146, 157
ÈÈÈ. 1991, ApJS, 76, 617
Lis, D. C., & Goldsmith, P. F. 1991, ApJ, 369, 157
Vogel, S. N., Genzel, R., & Palmer, P. 1987, ApJ, 316, 243
Lis, D. C., Goldsmith, P. F., Carlstrom, J. E., & Scoville, N. Z. 1993, ApJ,
Wilson, T. L., Snyder, L. E., Comoretto, P. R., Jewell, P. R., & Henkel, C.
402, 238
1996, A&A, 314, 909
Liu, S.-Y., & Snyder, L. E. 1999, ApJ, 523, 683
Ziurys, L. M., Apponi, A. J., Hollis, J. M., & Snyder, L. E. 1994a, ApJ, 436,
Mallard, W. G., Westley, F., Herron, J. T., Hampson, R. F., & Frizzell,
L181
D. H. 1998, NIST Chemical Data Base, Ver. 2Q98 (Gaitnersburg :
Ziurys, L. M., Hollis, J. M., & Snyder, L. E. 1994b, ApJ, 430, 706
NIST)
Ziurys, L. M., McGonagle, D., Minh, Y., & Irvine, W. M. 1991, ApJ, 373,
Meyer, D. M., & Roth, K. C. 1991, ApJ, 376, L49
535