THE ASTROPHYSICAL JOURNAL, 481 : 800È808, 1997 June 1
( 1997. The American Astronomical Society. All rights reserved. Printed in U.S.A.
NEW OBSERVATIONS OF THE [HCO`]/[HOC`] RATIO IN DENSE
MOLECULAR CLOUDS
A. J. APPONI1 AND L. M. ZIURYS1
Department of Chemistry, Arizona State University, Tempe, AZ 85287
Received 1996 August 19 ; accepted 1996 December 20
ABSTRACT
New measurements of the millimeter-wave transitions of HOC` toward dense molecular clouds have
been carried out using the NRAO 12 m telescope. The J \ 1 ] 0 transition of HOC` at 89 GHz has
been detected for the Ðrst time toward DR21 (OH), W51M, W3 (OH), NGC 7538, Orion (3N, 1E), NGC
2024, and G34.3. The J \ 2 ] 1 and J \ 3 ] 2 lines at 179 GHz and 268 GHz have also been observed
in W51 and Orion KL. In conjunction with past observations of HOC` toward Orion KL and SgrB2
(OH) by Ziurys & Apponi, these data conÐrm the presence of this ion in the interstellar medium. The
column densities of HOC` were found to be N D (7 ] 1010)È(3 ] 1012) cm~2, which correspond to
tot relative to H . Observations of the J \ 1 ] 0 tranfractional abundances of fD(2 ] 10~13)È(3 ] 10~12)
2 in [HCO`]/[HOC`] ratios of 360È
sition of HC18O` at 85 GHz have also been carried out, resulting
6000. These values are in good agreement with recent modeling by Herbst & Woon and conÐrm that the
reaction of HOC` with H indeed has a substantial barrier at low (T \ 50 K) temperatures. The wide2 additionally suggests that there is not a strong preference for synthesis of
spread abundance of HOC`
this ion in photon-dominated regions, although there may be a small enhancement toward such gas.
Finally, conÐrmation of interstellar HOC` has been accomplished through observations of weak emission (T [ 50 mK) at 179 GHz, proving that measurements near the atmospheric water line at 183 GHz
are notRas difficult as previously thought.
Subject headings : ISM : abundances È ISM : clouds È ISM : molecules È radio lines : ISM
1.
INTRODUCTION
product was HOC` because the proton affinity of the
carbon atom of CO is higher than that of the oxygen atom
(e.g., Dixon, Komornicki, & Kraemer 1984 ; Freeman et al.
1987). Such a distribution results in an [HCO`]/[HOC`]
ratio of D17, suggesting that HOC` might be easily
observable in the interstellar medium.
Prior to 1995, only one astronomical study was performed on HOC`, that by Woods et al. (1983). Although
their searches produced primarily negative results, these
authors detected a line near the J \ 1 ] 0 transition of
HOC` towards SgrB2 (OH), which suggested an
[HCO`]/[HOC`] ratio of D375. At that time, this detection was questioned because of possible contamination by a
nearby line of formic acid and because it contradicted the
current theory. Woods et al. (1983) suggested that this ion
was depleted by the reaction HOC` ] H ] HCO` ] H.
This idea was also adopted separately by Green (1984) and
DeFrees et al. (1984), but was soon discounted due to the
very slow reaction rate for this process and the uncertain
availability of atomic hydrogen in dense interstellar clouds.
A new destruction pathway for HOC` was subsequently
introduced by Jarrold et al. (1986) where this ion is converted to HCO` by the reaction
One of the most fundamental discoveries in astrophysics
to date is the detection of HCO` (Buhl & Snyder 1970 ;
Klemperer 1970). The presence of this species in the interstellar medium has validated chemical models which
incorporate ion-molecule mechanisms as the dominant synthetic scheme. Since this time it has been generally accepted
that most of the compounds found in the dense interstellar
medium are synthesized through ion-molecule reactions,
and complex chemical models involving such processes
have been able to accurately predict observed abundances,
with only a few exceptions (e.g., Millar et al. 1991). One of
the most stringent checks on the theory used in these
models has been the observation of simple closed systems
involving two isomeric forms of a species, such as HCN and
HNC (e.g., Ziurys & Turner 1986, Goldsmith et al. 1981).
In the past 15 years there has been a substantial interest
in the simple system involving HCO` and its metastable
isomer, HOC`. The HOC` ion lies D16,600 K higher in
energy than its more stable isomeric form, HCO`, with a
barrier to internal conversion of D20,128 K (DeFrees,
McLean, & Herbst 1984). Because interstellar chemistry is
kinetically controlled, the relative production and destruction rates govern the abundances of these two species. It is
widely recognized that HCO` is formed via reaction of H`
3
with CO ; however, this process also results in the production of HOC`. Early theoretical calculations by Herbst
et al. (1976) suggested that both pathways are equally probable and should result in an equal distribution of these two
species. Illies, Jarrold, & Bowers (1982) later studied this
reaction in the laboratory, using collision-induced dissociation techniques, and concluded that only 6% of the
HOC` ] H ] HCO` ] H .
2
2
(1)
Although this process was Ðrst calculated to have a high
activation energy barrier (DeFrees et al. 1984), further
studies suggested it was so fast that little HOC` should be
found in the interstellar medium. In fact, Jarrold et al. (1986)
predicted that [HCO`]/[HOC`] [ 10,000 at typical cloud
temperatures and densities, and that the ratio increased
with n(H ). Additional laboratory measurements by
2
McEwan (1992)
indicated that this reaction was relatively
fast (k D 10~10 cm3 s~1) at 300 K as well.
1 Current address : Departments of Astronomy and Chemistry and
Steward Observatory, University of Arizona, Tucson, AZ 85721.
800
[HCO`]/[HOC`] IN DENSE MOLECULAR CLOUDS
Very recently, new astronomical observations for HOC`
have been conducted by Ziurys & Apponi (1995), which
conÐrm the existence of this ion in the dense interstellar
medium. In addition to the J \ 1 ] 0 transition, these
authors detected the J \ 2 ] 1 and J \ 3 ] 2 lines of
HOC` towards SgrB2 (OH). They also observed the
J \ 1 ] 0 transition towards Orion KL. Their work
removed any doubt of contamination of the J \ 1 ] 0
emission by formic acid because the linewidths in Orion KL
are sufficiently narrow to unambiguously resolve the
HOC` line from that of HCOOH, which lies a few MHz
away in frequency. Ziurys & Apponi reevaluated the
[HCO`]/[HOC`] ratio by measuring the 13C and 18O isotopomers of HCO`, reporting values for this ratio of 140È
360 for SgrB2 (OH) and D1800 for Orion KL, respectively.
In that paper they not only concluded that the original
[HCO`]/[HOC`] abundance ratio of D375 reported by
Woods et al. (1983) was correct, but also that HOC` must
exist in gas densities near 106 cm~3, found from the readily
observed J \ 3 ] 2 transition.
As an extension of our initial work on HOC`, we have
conducted a survey of this ion toward additional dense
interstellar clouds, including several star-forming regions
and a few sources with known photo-dissociation regions.
We have detected J \ 1 ] 0 transitions in all but one
source, the dark cloud L134N. We also have conducted
searches for the J \ 2 ] 1 and J \ 3 ] 2 transitions
towards several of these clouds. Furthermore, we have
carried out measurements of the J \ 1 ] 0 line of HC18O`
as well. From these observations, we have determined
[HCO`]/[HOC`] ratios that typically range from 900 to
6000. As we were carrying out our survey, Herbst & Woon
(1996) conducted additional modiÐcations to the ab initio
calculations for the reaction of H and HOC`. These
authors have found that this reaction2 does have a substantial activation energy barrier and is far less efficient
than previously thought. They, in fact, predict an
[HCO`]/[HOC`] abundance ratio of 1500È4000 for dense
gas, in agreement with our observations.
In this paper, we present our measurements and the new
[HCO`]/[HOC`] ratios. We discuss these results in light
of the recent Ðndings of Herbst & Woon (1996) and examine
competing mechanisms associated with photon-dominated
regions (e.g., Sternberg & Dalgarno 1995).
2.
OBSERVATIONS
The measurements were carried out at the NRAO2 12 m
telescope located at Kitt Peak, Arizona. The majority of the
data was taken in three observing sessions, 1994 February,
JanuaryÈApril 1995, and 1996 March. Three transitions of
HOC` were measured, the J \ 1 ] 0 line at 89,487 MHz,
the J \ 2 ] 1 line at 178,972 MHz, and the J \ 3 ] 2 transition at 268,451 MHz. In addition, one transition of
HC18O`, the J \ 1 ] 0 line at 85,162 MHz, was observed.
The telescope efficiencies, g (corrected for forward spillc 0.41 at 89 (and 85) GHz, 179
over losses) were 0.89, 0.63, and
GHz, and 268 GHz, respectively, with beam sizes of 69A,
36A, and 24A. The three receivers used were helium-cooled
dual-channel SIS mixers, operated in single sideband mode
2 The National Radio Astronomy Observatory (NRAO) is operated by
the Associated Universities, Inc., under cooperative agreement with the
National Science Foundation.
801
with at least 20 dB rejection of the image. The temperature
scale, determined by the chopper wheel method (corrected
for g ), is given by T *, where T \ T */g . The back ends
fss
R
R
R c
used were two 256-channel Ðlter banks and a hybrid
spectrometer. For each observation the Ðlter banks were
conÐgured in parallel mode (2 ] 128 channels) so that two
di†erent frequency resolutions could be utilized. With the
exception of L134N, all 3 mm data were taken with 250 kHz
and 500 kHz resolutions, all 2 mm data were taken with 500
kHz and 1 MHz resolutions, and all 1.2 mm data were
taken with 1 MHz and 2 MHz resolutions. The 2 ] 768
channel hybrid spectrometer was used with 600 MHz bandwidth, resulting in a resolution of 781 kHz. For the L134N
data, the Ðlter bank resolutions were 100 kHz and 250 kHz
and the hybrid spectrometer was set to 49 kHz per channel.
The data were taken in position-switching mode, with a
selected o† position of 30@ west in azimuth. A list of sources
and coordinates is given in Table 1.
3.
RESULTS
Table 1 summarizes the data obtained for HOC` and
HC18O`, including transitions observed and line parameters determined for each species. The individual spectra
were processed by removal of a Ðrst-order baseline and then
each feature was Ðtted with a Gaussian proÐle. Multiple
Gaussian Ðts and data modeling were used for the 1 mm
spectra of Orion KL and W51M because of contaminating
features. The line parameters determined are the antenna
temperature (T *), the velocity (V ), and the FWHM line
R listed in TableLSR
width (*V ). Also
1 are the Ðlter resolutions
1@2
used in determining the Ðtted parameters.
Figure 1 presents the spectra of the J \ 1 ] 0 transition
of HOC` and HC18O` toward all of the sources listed in
Table 1 (except L134N). The top panel shows the HC18O`
data and the bottom panel shows the HOC` measurements. As the Ðgure illustrates, the J \ 1 ] 0 transition of
HOC` has been clearly detected in every cloud that was
searched (a total of eight) with the exception of L134N,
where it might be observed given more integration time. As
seen in Table 1, the antenna temperatures for the J \ 1 ] 0
transition range from D0.05 K in Orion (3N, 1E) to D0.01
K in W3 (OH). By comparison, this transition was found by
Ziurys & Apponi (1995) to be somewhat stronger (T * \
0.072 K) towards SgrB2 (OH). As also shown in Table 1,Rfor
the clouds studied here the line widths determined are not
very broad, ranging from 1.8 km s~1 to 9.2 km s~1, suggesting that this species does not arise from hot shocked gas or
from outÑow sources. Moreover, in each case, the velocity
and proÐle for HOC` and HC18O` are quite consistent,
with the possible exception of G34.3 and NGC 7538 (Figs.
1b and 1d). For G34.3, the J \ 1 ] 0 transition of HC18O`
has *V \ 5.7 km s~1, which is somewhat wider than the
1@20 line of HOC` (*V \ 2.7 km s~1). This result is
J\1]
consistent with observations1@2
of the Galactic center, where
many velocity components are known to exist in common
molecules. Although there may be several components to
the HC18O` emission, there deÐnitely is a velocity that
matches what is seen for the HOC` J \ 1 ] 0 transition. A
similar situation was found for SgrB2 (OH) (see Ziurys &
Apponi). In the case of NGC 7538, a line width of *V D 5
1@2 for
km s~1 was found for HOC`, as opposed to 2 km s~1
HC18O`. A poor signal-to-noise ratio can readily account
for these di†erences. (The larger HOC` width was used for
the column density calculations for NGC 7538, but not in
802
APPONI & ZIURYS
TABLE 1
OBSERVATIONS OF HOC` AND HC18O`a
HOC`
V
J \ 1 ] 0b
J \ 2 ] 1c
J \ 3 ] 2c
T * (K)
R
0.031 ^ 0.007
\0.023
\0.030
G34.3 . . . . . . . . . . . . . . . .
L134N . . . . . . . . . . . . . . .
NGC 2024 . . . . . . . . . .
NGC 7538 . . . . . . . . . .
Orion (3N, 1E) . . . . . .
Orion KL . . . . . . . . . . .
J \ 1 ] 0b
J \ 1 ] 0d
J \ 1 ] 0b
J \ 1 ] 0b
J \ 1 ] 0b
J \ 1 ] 0b,f
J \ 2 ] 1c,g
J \ 3 ] 2c,h
0.021 ^ 0.007
\0.02
0.047 ^ 0.010
0.015 ^ 0.005
0.051 ^ 0.007
0.028 ^ 0.005
D0.05
0.087 ^ 0.007
W3 (OH) . . . . . . . . . . . .
W51M . . . . . . . . . . . . . . .
J \ 1 ] 0b
J \ 1 ] 0i
J \ 2 ] 1i
J \ 3 ] 2c,j
0.008 ^ 0.003
0.041 ^ 0.005
0.057 ^ 0.005
D0.02
SOURCE
Transition
DR21 (OH) . . . . . . . . .
(km s~1)
LSR
[2.9 ^ 0.84
[2.9
[2.9
HC18O`
V
J \ 1 ] 0b
...
...
T * (K)
R
0.130 ^ 0.010
...
...
2.7 ^ 0.84
...
3.1 ^ 0.84
D5
1.8 ^ 0.84
3.8 ^ 0.84
...
2.8 ^ 1.12
J \ 1 ] 0b
J \ 1 ] 0e
J \ 1 ] 0b
J \ 1 ] 0b
J \ 1 ] 0b
J \ 1 ] 0b
...
...
0.186 ^ 0.015
0.150
0.065 ^ 0.015
0.072 ^ 0.010
0.154 ^ 0.010
0.091 ^ 0.010
...
...
58.1 ^ 0.88
2.1
10.9 ^ 0.88
[56.9 ^ 0.88
9.3 ^ 0.88
8.3 ^ 0.88
...
...
5.7 ^ 0.88
0.5
2.8 ^ 0.88
1.8 ^ 0.88
1.6 ^ 0.88
3.6 ^ 0.88
...
...
4.5 ^ 0.84
9.2 ^ 1.68
7.4 ^ 1.68
...
J \ 1 ] 0b
J \ 1 ] 0i
...
...
0.065 ^ 0.015
0.106 ^ 0.010
...
...
[46.5 ^ 0.88
56.7 ^ 1.76
...
...
3.9 ^ 0.88
8.5 ^ 1.76
...
...
*V
(km s~1)
1@2
4.7 ^ 0.84
...
...
58.0 ^ 0.84
2.5
10.7 ^ 0.84
[55.7 ^ 0.84
9.3 ^ 0.84
7.5 ^ 0.84
D9
9.5 ^ 1.12
[46.8 ^ 0.84
56.3 ^ 1.68
57.0 ^ 1.68
D57
Transition
(km s~1)
LSR
[3.4 ^ 0.88
...
...
*V
(km s~1)
1@2
3.7 ^ 0.88
...
...
a Observations toward DR21 (OH) : a \ 20h37m14s. 0, d \ 42¡12@00A. 0 ; G34.3 : a \ 18h50m46s. 4, d \ 01¡11@14A. 0 ; L134N : a \ 15h51m24s. 0, d \
[02¡43@31A. 0 ; NGC 2024 : a \ 05h39m13s. 0, d \ [01¡57@04A. 0 ; NGC 7538 : a \ 23h11m36s. 6, d \ 61¡11@47A. 0 ; Orion (3N, 1E) : a \ 0.5h32m50s. 8, d \
[05¡21@23A. 0 ; Orion KL : a \ 05h32m46s. 8, d \ [05¡24@23A. 0 ; W3 (OH) : a \ 02h23m17s. 0, d \ 61¡38@54A. 0 ; W51M : a \ 19h21m26s. 2, d \ 14¡24@43A. 0. All
coordinates are epoch 1950. Quoted errors are 3 p.
b Filter resolution of 250 kHz was used.
c Filter resolution of 1000 kHz was used.
d Correlator resolution of 49 kHz was used.
e From Swade (1989).
f Observation previously published by Ziurys and Apponi 1995.
g Contaminated by EtCN.
h Values obtained using a 10 component model of the observed spectrum.
i Filter resolution of 500 kHz was used.
j Blended with other lines.
determining the [HCO`]/[HOC`] abundance ratio for
that source.)
Figure 2a shows the spectra of the J \ 2 ] 1 and
J \ 3 ] 2 transitions of HOC` obtained towards Orion
KL and W51M. The line parameters for these features are
also listed in Table 1. The spectrum of the J \ 2 ] 1 transition in Orion KL is contaminated by the 21
] 20
transition of EtCN at 178,975.1 MHz, as the 0,21
top panel1,20
of
Figure 2a illustrates. The HOC` feature is seen only as a
shoulder on the EtCN peak. There are three other favorable
transitions of this molecule lying in the same 2 mm
bandpass (7 ] 6 , 20
] 19
, and 20
] 19
2,5 detected
6,14
6,15 Ziurys
6,14
lines) which 3,4
were easily
in6,13
Orion KL (see
&
Apponi 1995).
The bottom panel of Figure 2a shows data at the
J \ 3 ] 2 frequency of HOC` toward Orion KL. There
appears to be a broad feature in this spectrum upon which
several narrow lines are superimposed, as shown in the inset
on the upper right-hand corner of the panel. The J \ 3 ] 2
line lies almost exactly coincident with the 7 ] 6 transition of 33SO , which is at 268,450.3 MHz. 2,6
The SO1,5line is
2
2
a composite of six hyperÐne components whose separation
is only a few MHz. None of the hyperÐne components individually match any of the observed narrow spectral features, but they may account for the broad one. It is expected
that if 33SO is present in Orion KL, it would have line
parameters 2similar to the main isotopomer. Therefore,
assuming that this species (although not yet conÐrmed in
Orion KL) arises from the plateau and has line width of 30
km s~1 and a V
of 8 km s~1, it can be modeled and
LSR raw data. The model of the six hypersubtracted from the
Ðne components of 33SO is seen as a smooth curve overlay
2
on the raw data in the inset. The result after subtraction is
shown in the main part of the panel in Figure 2a, in which
Ðve separate, narrow lines are visible. One feature is identiÐed as HOC`. (A high-energy transition of VyCN is close in
frequency to this feature, but it lies D290 K above ground
state and hence is unlikely to be so narrow). Another line is
possibly the 8 ] 9
EE transition of (CH ) O at
8,0 other
7,3features could not be identiÐed
32
268,461 MHz. The
in
this bandÈnot unusual for Orion KL, as we are well into
the confusion limit of this source at T * \ 30 mK, peak to
R
peak. The V
for the J \ 3 ] 2 transition
of HOC`
LSR
toward Orion KL was determined to be 9.5 km s~1, somewhat higher than that in the J \ 1 ] 0 transition (V \
LSRbe a
7.5 km s~1) but within the errors. The di†erence could
consequence of the complicated Ðt used in determining the
line parameters. Due to the apparent success of this Ðt, the
Gaussian proÐle for the central feature was used in determining the column density for HOC`. However, this
deconvolved line was not considered in determining the
[HCO`]/[HOC`] ratio.
Figure 2b shows these two transitions observed toward
W51M. As in Orion, the J \ 2 ] 1 line of HOC` (top panel)
could possibly su†er from contamination by the
2
] 20
line of EtCN, which lies 97 K above ground
0,21 a line0,20
with
strength of 17.0. However, a comparable transition of this molecule (20
] 19
) was not observed in
3,18
this source down to a level
of \503,17
mK, although this transition is 100 K above ground with a line strength of 19.5.
Moreover, the line width of 7.4 ^ 2.1 km s~1 of the
J \ 2 ] 1 feature is consistent with the J \ 1 ] 0 transition, which has *V \ 9.2 ^ 2.1 km s~1. Hence, we con1@2
clude that this line primarily
arises from HOC`.
FIG. 1a
FIG. 1b
FIG. 1c
FIG. 1d
FIG. 1.ÈSpectra of the J \ 1 ] 0 transitions of HC18O` (top panel) at 85 GHz and newly-detected HOC` (bottom panel) at 89 GHz observed with the
12 m telescope towards (a) DR21 (OH), (b) G34.3, (c) NGC 2024, (d) NGC 7538, (e) Orion (3N, 1E), ( f ) Orion KL, (g) W3 (OH), and (h) W51M. The
resolutions used are listed in Table 1. The LSR velocities and linewidths are consistent between the two species throughout the molecular clouds surveyed
here (see text for details), suggesting that both molecules arise from the same gas.
FIG. 1e
FIG. 1f
FIG. 1g
FIG. 1h
[HCO`]/[HOC`] IN DENSE MOLECULAR CLOUDS
FIG. 2a
805
FIG. 2b
FIG. 2.ÈSpectra of the J \ 2 ] 1 (top) and J \ 3 ] 2 (bottom) transitions of HOC` near 179 GHz and 268 GHz, respectively, observed with the 12 m
telescope toward Orion KL (a) and W51M (b). In the Orion spectrum, the J \ 2 ] 1 transition appears as an unresolved shoulder on the 21
] 20
0,21 ) O, 1,20
transition of EtCN. The J \ 3 ] 2 Orion data has been modeled and a possible contaminating 33SO line subtracted. The remaining HOC`, (CH
and
2
3
2
unidentiÐed lines are shown in the main panel. The raw data for the J \ 3 ] 2 transition is shown in the inset at the upper right. The dotted line is a model
of
the six hyperÐne components of 33SO . For the W51M spectra, the J \ 2 ] 1 transition appears to be uncontaminated (see text for details). However, the
2
J \ 3 ] 2 transition (bottom) has an abnormal
proÐle and line width which can only be explained as a blend of several lines. The solid line drawn through the
data is a representation of three features seen in Orion KL (a), modeled using typical line proÐles for W51M.
As in the Orion KL spectra, the J \ 3 ] 2 transition in
W51 (bottom panel) shows a very wide proÐle. We were
unable to model this spectral feature with the six hyperÐne
components of 33SO , as we were in the Orion KL spectrum. The W51 line is2much broader (D35 km s~1) than the
expected frequency range of the hyperÐne lines would
suggest. (The line widths in W51M do not seem to vary
much from species to species ; they are typically around
5È10 km s~1, 8 km s~1 for HC18O`, for example). It cannot
be modeled with two velocity components separated by 7
km s~1 either, as seen in CO toward our position (Mufson
& Liszt 1979). Additionally, this line is much too Ñat across
the top to Ðt with a Gaussian model, even when we assume
an atypical line width of 30 km s~1 for each of the hyperÐne
components in the Ðt. Since contamination by 33SO is very
2 of the
unlikely in this source, we are left with the composite
three other features seen in the Orion KL spectra. A model
of these three possible lines is shown as a smooth curve
overlay in Figure 2b and Ðts the observed spectra well. The
model was constructed using the same center frequencies
for the three peaks observed in Orion KL : 268,445 MHz,
268,451 MHz, and 268,463 MHz. The line widths were held
Ðxed at 10 km s~1 and the line temperatures were varied
until the best Ðt was determined. Because we were able to
reproduce this conglomerate with the same three lines
observed in Orion KL, it is reasonable to assume that part
of this feature is due to the J \ 3 ] 2 emission of HOC`.
Although the Ðt appears to be quite accurate, the derived
intensity for the J \ 3 ] 2 transition of HOC` toward this
source was not used in the column density calculations.
This model was used only as a representation of the result-
ant spectrum in Figure 2b merely to point out that the
J \ 3 ] 2 transition was observed towards W51M.
4.
DISCUSSION
4.1. HOC` Column Densities and Abundances
The HOC` column densities for a given state, N , were
l
calculated using the following formula for a linear molecule,
assuming low optical depth :
3kT *V
R 1@2
,
(2)
N\
l 8n3lk2[1 [ (J /J )]
ij
bg rot
where the dipole moment matrix element is expressed as
k2 \ ( J ] 1)k2/(2J ] 1) and the Rayleigh-Jeans coefficients
ij expressed as
0 J \ (hl/k)[1/(ehl@kT [ 1)], substituting the
are
T
background temperature,
T (2.73 K), and the rotational
temperature, T , to obtainbgJ and J , respectively. In
rot l is the frequency
bg
rotof the transition
these expressions,
J ] 1 ] J, k is the permanent dipole moment (2.8 D), T is
R
the radiation0 temperature, and *V
is the FWHM line1@2
width. The total column density, N , can then be extracted
tot
from the following relation :
(2J ] 1) exp ([*E /kT )
N
gd rot ,
l \
(3)
f
N
rot
tot
where *E is the energy of the Jth level above ground and
gd
f is the rotational
partition function.
rotTable 2 lists the derived column densities for each source
using the calculations described above. In most cases an
excitation temperature T \ 15 K, was used, and it was
ex that the source Ðlled the largest
assumed that T \ T and
ex
rot
806
APPONI & ZIURYS
TABLE 2
COLUMN DENSITIES AND FRACTIONAL ABUNDANCES
Source
N (HOC`)
tot
(cm~2)
T
ex
(K)
N(H )
2
(cm~2)
Sgr B2 (OH) . . . . . . . .
DR21 (OH) . . . . . . . . .
G34.3 . . . . . . . . . . . . . . . .
L134N . . . . . . . . . . . . . . .
NGC 2024 . . . . . . . . . .
NGC 7538 . . . . . . . . . .
Orion (3N, 1E) . . . . . .
Orion KL . . . . . . . . . . .
W3 (OH) . . . . . . . . . . . .
W51M . . . . . . . . . . . . . . .
3.0 ] 1012 a
1.4 ] 1011
1.2 ] 1011
\1.3 ] 1010
3.1 ] 1011
1.6 ] 1011
2.0 ] 1011
2.0 ] 1011 c
7.7 ] 1010
5.4 ] 1011 c
15
15
15
7
15
15
15
15
15
15
1 ] 1024 b
1 ] 1023 d
5 ] 1023 e
2 ] 1022 f
2 ] 1023 g
1 ] 1023 h
4 ] 1023 i
1 ] 1024 j
1 ] 1023 k
1 ] 1024 l
f (HOC`/H )
2
3 ] 10~12
1 ] 10~12
2 ] 10~13
\7 ] 10~13
2 ] 10~12
2 ] 10~12
5 ] 10~13
2 ] 10~13
8 ] 10~13
5 ] 10~13
a Ziurys & Apponi 1995, best Ðt of three observed transitions.
b Sutton et al. 1991.
c Best Ðt of two observed transitions.
d J. Mangum 1996, private communication.
e Churchwell, Walmsley, & Wood 1992.
f Swade 1989.
g Barnes & Crutcher 1992.
h Wilson et al. 1983.
i Harris et al. 1983.
j Batrla et al. 1983.
k Wilson, Gaume, & Johnston 1993.
l Ja†ee, Becklin, & Hildebrand 1984.
telescope beam of D70A at 85 GHz. For the two sources
where more than one transition was observed (Orion KL
and W51M), a rotational diagram analysis was used to
determine the total column density and the rotational temperature. A rotational temperature of D5 K was found for
each of these sources. These results are consistent with those
reported by Ziurys & Apponi (1995) for SgrB2 (OH).
Because of the high densities required for a molecule with a
large permanent dipole moment to exhibit thermalized
emission, it is expected that T \ T , where T is the gas
rot &k Apponi (1995)
k
kinetic temperature. In fact, Ziurys
calculated a rotational temperature of 5 K towards SgrB2 (OH)
for HOC`, which is lower than the gas kinetic temperature
of 25 K derived from CO observations by Lis & Goldsmith
(1991). The values reported in Table 2 range from N \
7.7 ] 1010 cm~2 for W3 (OH) to 5.4 ] 1011 cm~2totfor
W51M. The largest column density for HOC` is 3.0 ] 1012
cm~2 in SgrB2 (OH), as found previously by Ziurys &
Apponi (1995).
The molecular hydrogen column densities, taken from
the literature, are also listed in Table 2 and were used to
calculate the fractional abundance of HOC` for each
source. The fractional abundances range from 2 ] 10~13 in
Orion KL and G34.3 to 3 ] 10~12 for SgrB2 (OH), with an
average value of about 1 ] 10~12. The upper limit to the
one dark cloud studied, L134N, is f \ 7 ] 10~13. This
upper limit therefore is not stringent enough to single out
the dark cloud as having an atypical HOC` abundance.
Hence, the HOC` fractional abundance varies about 1
order of magnitude among the dense interstellar clouds
observed in this work.
4.2. [HCO`]/[HOC`] Abundance Ratio in
Molecular Clouds
The [HCO`]/[HOC`] abundance ratios were calculated in the manner outlined by Ziurys & Apponi (1995), i.e.,
through measurements of the 18O isotopomer of HCO`
and correcting the relative antenna temperatures by the
appropriate 16O/18O ratio. [As discussed by Woods et al.
Vol. 481
(1983), the antenna temperatures can be directly compared.]
The resulting ratio values are given in Table 3. The 16O/18O
ratio was calculated as a function of the distance from the
galactic center, D
(kpc), from a relationship given in
GC
Wilson & Rood (1994), obtained by observations of H CO.
2
Their Ðt resulted in the expression
16O/18O \ (58.8 ^ 11.8)D
] 37.1 ^ 82.6 .
(4)
GC
Listed also in Table 3 are D (the distance from the galacGC
tic center) and the calculated 16O/18O ratio used. As the
table shows, the [HCO`]/[HOC`] values range from 360
in SgrB2 (OH) from Ziurys & Apponi (1995) to 6000 in W3
(OH), with an average value of 2500. The SgrB2 (OH) ratio
is the lowest and clearly deviates from the other values,
which lie between 1000 and 6000. [The value of 2100 reported for Orion KL di†ers from 1800, reported in Ziurys &
Apponi (1995), only because a di†erent 16O/18O ratio was
used].
The conÐrmation of the existence of HOC` in interstellar
gas by Ziurys & Apponi (1995) prompted a more complete
ab initio study by Herbst & Woon (1996) of the destruction
of this ion by reaction with H (see eq. [1]). Their new study
di†ers signiÐcantly from that2 of Jarrold et al. (1986), who
found little or no activation energy barrier for the process.
Herbst & Woon found that a positive energy was associated
with the transition state when the zero-point energy was
included and the e†ects of tunneling under that transition
state. Furthermore, they produced a ““ new standard model ÏÏ
(Bettens, Lee, & Herbst 1995 ; Lee, Bettens, & Herbst 1996)
that yielded an [HCO`]/[HOC`] abundance ratio of
1500È4000 under normal conditions found in the dense
interstellar medium. This model is in very good agreement
with the observations in this work, where we Ðnd typical
values of 400È6000 throughout the galaxy. Also, it is interesting that the model Ðnds little dependence of the
[HCO`]/[HOC`] abundance ratio on the cloud density.
4.3. HOC` in Photon-dominated Regions ?
Studies over the past several years have suggested that
certain chemical species, such as C`, CO`, and CH , are
2
enhanced in H II-molecular cloud interfaces, or photondominated regions (PDRs) (e.g., Schenewerk et al. 1988 ;
Sternberg & Dalgarno 1995). Another such molecule is
HCO. In an idea Ðrst proposed by Snyder, Hollis, & Ulich
(1976) and later observationally conÐrmed by Schenewerk
et al. (1988), HCO appears to be produced near H II regions
TABLE 3
[HCO`]/[HOC`] RATIOS
Source
D
GC
(kpc)a
16O/18Ob
[HCO`]/[HOC`]c
SgrB2 (OH) . . . . . . . . .
DR21 (OH) . . . . . . . . .
G34.3 . . . . . . . . . . . . . . . .
L134N . . . . . . . . . . . . . . .
NGC 2024 . . . . . . . . . .
NGC 7538 . . . . . . . . . .
Orion (3N, 1E) . . . . . .
Orion KL . . . . . . . . . . .
W3 (OH) . . . . . . . . . . . .
W51M . . . . . . . . . . . . . . .
0.2
10.0
7.0
9.5
10.5
12.0
10.5
10.5
12.0
8.0
250
630
450
600
650
740
650
650
740
510
360
2600
4000
[4500
900
3500
2000
2100
6000
1300
a Wilson et al. 1979.
b Wilson & Rood 1994.
c Ratio calculated by T * (HC18O`)/T *(HOC`) ] (16O/18O).
R
R
No. 2, 1997
[HCO`]/[HOC`] IN DENSE MOLECULAR CLOUDS
via secondary reactions involving C`, CH , and O. Thus
2
the formation of this radical seems to be linked to PDRs.
Another possible species that could be enhanced in these
regions through secondary processes like the ones that form
HCO is HOC`. Although this ion is primarily formed by
the reaction of H` and CO, only 6% of the products are
3
HOC` (Illies et al. 1982). There are two other synthetic
pathways more efficient in producing HOC` that may
become important in PDRs. Both involve species that are
known to have elevated PDR abundances, C` and CO`
(Sternberg & Dalgarno 1995) :
HCO` ] H 16% ,
ˆ
C` ] H O
2 >
HOC` ] H 84% .
HCO` ] H 52% ,
ˆ
CO` ] H
2>
HOC` ] H 48% .
(5)
(6)
Reactions (5) and (6) have been studied by Freeman et al.
(1987) using selected-ion Ñow-tube mass spectroscopy. In
their work they found that reaction (5) produces 84%
HOC` while reaction (6) produces 48% HOC`, and that
both reactions proceed at the Langevin rate of (1È2) ] 10~9
cm3 s~1. If in fact these processes are signiÐcant in the
production of HOC` towards H II regions, the relative formation rates of this ion and HCO` would be altered. Consequently, a decrease in the [HCO`]/[HOC`] ratio
relative to normal cloud conditions would be expected.
Schenewerk et al. (1988) have identiÐed many possible
PDR regions through observations of HCO, three of which
overlap in this study : SgrB2, NGC 7538, and NGC 2024.
Two of these sources show a signiÐcant decrease in this
ratio, namely SgrB2 (OH) ([HCO`]/[HOC`] \ 360) and
NGC 2024 ([HCO`]/[HOC`] \ 900), but NGC 7538 does
not ([HCO`]/[HOC`] \ 3500). NGC 7538, on the other
hand, has an inconsistency in the line widths between the
HOC` emission (D5 km s~1) and the HC18O` emission
(1.8 km s~1), attributable to poor S/N. It is unlikely that
better S/N would change the [HCO`]/[HOC`] ratio very
much, however.
To date there are no chemical models available that
attempt to predict the relative abundance of [HCO`]/
[HOC`] in PDRs. Sternberg & Dalgarno (1995) postulate
that extensive material exists, with a great deal of overlap
between the distributions of C` and H O, which may be
e†ective reactants in the production of 2HOC`. Also, they
predict large abundances of CO` at the edges of PDRs.
Comparison with observations of the reactant molecules
in PDR regions may o†er some additional insight. The
importance of the H O ] C` process is difficult to evalu2 water is a common constituent of
ate, however, because
molecular clouds and is certainly not conÐned to PDRs
alone (e.g., Gensheimer, Mauersberger, & Wilson 1996).
Furthermore, observations of C` at 158 km generally
concern unresolved spectra unsuitable for detailed comparison with HOC` millimeter wave lines (e.g., Stacey et al.
1993). The signiÐcance of reaction (6) remains unknown
because of the lack of measurements of CO` in the objects
that contain HOC`. CO` has only been detected in three
sources, NGC 7027, M17SW, and the Orion Bar (Latter,
Walker, & Maloney 1993 ; StoŽrzer, Stutzki, & Sternberg
1995), none which have been studied in this work. It is clear
807
that the distribution of HOC` appears to be much larger
than any of the exclusive tracer molecules of PDRs ; therefore it is certainly not conÐned to these regions alone. Furthermore, the model of Bettens et al. (1995), which ignores
the e†ects of PDRs, accurately predicts the [HCO`]/
[HOC`] abundance ratio, suggesting that these species
predominantly exist in extended gas and are largely unaffected by H II interfaces associated with star formation. This
fact is evident in the Orion KL spectra, where narrow linewidths of D3 km s~1 and velocities near 8 km s~1 are
found, typical of the extended ridge gas of OMC-1. Clearly,
more observational data is necessary in order to understand
the e†ect PDRs have on the chemistry of HOC`.
4.4. Detection of the J \ 2 ] 1 line of HOC` :
Observations at 179 GHz
Another important result of this project has been the
detection of HOC` emission near 179 GHz, done at Kitt
Peak using the 12 m telescope. Prior to these measurements,
it has often been postulated that observations within several
GHz of the atmospheric water line at 183 GHz were
extremely difficult, even for strong emission, given the
atmospheric opacity. However, we have clearly been able to
detect weak features with intensities as little as 50 mK at
179 GHz for low declination sources such as SgrB2 (OH)
[see Ziurys & Apponi (1995) and Figure 2]. Although
observations at this frequency were made best when the
water vapor content at zenith was 1È2 mm, we were still
able to obtain reasonable data at levels of 4 mm. Typical
system temperatures obtained were T D 1200 K at elevation 30¡ with an atmospheric watersysvapor content of 4
mm (or an atmospheric opacity at 225 GHz of 0.2).
We make this point because observations in supposedly
inaccessible regions of the millimeter/submillimeter spectrum should at least be attempted before being declared
impossible, even at nonideal sites such as Kitt Peak. We, for
example, had to do our J \ 2 ] 1 observations during
unassigned time because telescope referees maintained that
sensitive observations at 179 GHz could not be carried out.
5.
CONCLUSION
Our initial observations of conÐrming transitions of
HOC` in SgrB2 (OH) and Orion KL suggested a much
smaller [HCO`]/[HOC`] ratio than originally predicted
by chemical models. Here we have extended this work by
observing HOC` in a larger sample of molecular clouds,
detecting the J \ 1 ] 0 transition in seven new sources, all
regions with high mass star formation. Comparison with
measurements of HC18O` strongly imply that HOC`
arises from extended molecular material. A general
[HCO`]/[HOC`] ratio of 360È6000 has also now been
determined in dense gas. This ratio is in very good agreement with new calculations by Herbst and Woon that
suggest that the reaction of HOC` ] H is much slower
2
than previously thought at low temperatures.
The widespread distribution of HOC` found from these observations additionally indicates that there is not a strong
preferential production of this ion in PDRs.
This research was supported by NSF grant AST 9503274. Special thanks are due to Antonio Perfetto for
helping us test the 12 m telescope at an observing frequency
of 179 GHz. We also thank the 12 m operators for their
great assistance with the observations, especially Paul Hart.
808
APPONI & ZIURYS
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