We discuss airborne observations of rotational transitions

of various hydride molecules in the interstellar medium, including H_2^(18)O

and HCI. The detection of these transitions is now feasible with a new,

sensitive submillimeter receiver which has been developed for the NASA

Kuiper Airborne Observatory (KAO) over the past several years

Blake, G. A.

Carlstrom, J.

Keene, J.

Miller, D.

Schilke, P.

Ugras, N. G.

Zmuidzinas, J.

English

Caltech Authors - Main

Airborne Astronomy Symposium on the Galactic Ecosystem 
ASP Conference Series, Vol. 73, 1995 
M.R. Haas, J.A. Davidson, and E.F. Erickson {eds.) 
33 
SUB MILLIMETER SPECTROSCOPY OF INTERSTELLAR HYDRIDES 
J. ZMUIDZINAS, G. A. BLAKEI, J. CARLSTROM, J. KEENE, D. 
MILLER, P. SCHILKE, N. G. UGRAS 
Division of Physics, Mathematics, and Astronomy, and !Division of 
Geological and Planetary Sciences, California Institute of Technology, 320-
47, Pasadena, CA 91125 
ABSTRACT We discuss airborne observations of rotational transitions 
of various hydride molecules in the interstellar medium, including H2 180 
and HCI. The detection of these transitions is now feasible with a new, 
sensitive submillimeter receiver which has been developed for the NASA 
Kuiper Airborne Observatory (KAO) over the past several years. 
INTRODUCTION 
Atmospheric water vapor is responsible for the generally poor atmospheric 
transmission in the submillimeter band (300-3000 GHz). In particular, even 
for observatories located on dry high-altitude sites such as Mauna Kea, the 
atmosphere is essentially opaque between 500-600 GHz and 700-800 GHz because 
of the very strong water lines at 547 GHz and 752 GHz. In addition, ground-
based submillimeter observations are impossible above 900 GHz. Nonetheless, 
these portions of the spectrum are very interesting astronomically because they 
contain many fundamental rotational transitions of hydride molecules, such as 
CHat 532 and 536 GHz, H2 180 at 547 and 745 GHz, NH3 at 572 GHz, HCl at 626 
GHz, CH2 at 945 GHz, and H30+ at 985 GHz. Observations of these molecules 
are important for testing our understanding of the chemistry of molecular clouds, 
since the hydrides are the initial products of the ion-molecule reaction network 
which is thought to be responsible for the synthesis of more the more complex 
molecules commonly observed at radio and millimeter wavelengths. 
In addition, hydrides may play a crucial role in the energetics of molecular 
clouds, particularly at the high densities found in star-forming cores, because 
their large rotational level spacings and large dipole moments make them very 
effective coolants. In fact, it is possible that cooling due to hydrides, especially 
HzO, plays a decisive role in establishing the rate at which molecular clouds 
collapse, and thus ultimately controls the efficiency of star formation. 
The Kuiper Airborne Observatory (KAO) is the only platform at present 
which allows observations of the transitions listed above (with perhaps the 
34 J. Zmuidzinas et a/. 
exception of HCl, which can be observed from Manna Kea in extremely good 
weather) . Although some of these molecules can also be observed through 
transitions at radio wavelengths, airborne observations of the fundamental 
rotational transitions can often give essential information regarding the 
abundance and excitation of these species. 
In this paper, we discuss studies of interstellar H2 180 and HCl we have been 
pursuing over the past two years. We have also detected the CH transitions at 
532 and 536 GHz in Orion, W51 , M17, and other sources, but these observations 
will not be discussed in this paper due to the limited space. The observations 
which we present were obtained with a new submillimeter heterodyne receiver 
developed for the KAO, which at present operates over the 500-750 GHz band, 
which is described in a paper elsewhere in this volume (Zmuidzinas et al. 1994b) . 
KAO 1993 June 626 GHz HCI(l-0) Sgr B2 
0.5 
0 50 100 
Velocity (km/ s) 
Figure 1. Solid line: detection of the 626 GHz HCl line in 
absorption toward Sgr B2. Dashed line: spectrum predicted 
by our radiative transfer model for a fractional abundance of 
HCJ/ H2 = 1.1 x w-9 . 
HCl IN ABSORPTION IN SGR B2 
The abundance of atomic chlorine in diffuse interstellar clouds has been studied 
using ultraviolet absorption lines by Jura & York (1978), Harris & Bromage 
(1984), and Harris, Gry, and Bromage (1984). These studies have shown that 
the depletion of chlorine (on dust grains) in diffuse clouds is not severe, with 
1.2 Submillimeter Spectroscopy of Interstellar Hydrides 35 
abundances at most a factor of 2-3 below the solar value of Cl/H ~ 2 x w-7 
(Anders & Grevesse 1989), although there is some indication that the depletion 
increases with density. It would be very interesting to study the depletion 
in denser regions; however, ultraviolet studies of dense clouds are obviously 
impossible. The submillimeter rotational transitions of HCl may be a useful 
probe of the chlorine depletion in dense molecular clouds, since gas-phase 
chemical models predict that a substantial fraction (> 10%) of chlorine will be 
in the form of HCl (Jura 1974; Dalgarno et al. 1974; Blake, Anicich, & Huntress 
1986; Schilke, Phillips, & Wang 1994). 
The 626 GHz J = 1 _, 0 line was first detected in OMC-1 from the KAO by 
Blake, Keene , and Phillips (1985). The observed emission line (T.A ~ 0.7 K) was 
interpreted to give HCI/ H2 ,...., 0.5-5.0 x w-8 , which corresponds to a probable 
range of chlorine depletion of 3-30 (below solar). The lower end of this range 
could be consistent with the chlorine depletion in diffuse clouds, but Neufeld 
and Green (1994) have recently reanalyzed this data using improved collisional 
excitation rates and a more detailed radiative transfer model, and argue that 
in fact the abundance may be only 2 X w-9 . Hence, it is likely that chlorine is 
either heavily depleted or the fraction of chlorine in HCl is much smaller than 
the predictions of the chemical models. 
During a June 1993 KAO deployment to Hickam AFB in Honolulu, HI, we 
detected the 626 GHz line in absorption against the submillimeter continuum 
from the Sgr B2 (N) and (M) dust cores (Figure 1). It is not entirely surprising 
that the line is in absorption, given the fact that this is a ground-state transition 
which has a very large critical density of ncr= 4 x 107 cm-3. Furthermore, Sgr 
B2 has an extended envelope of moderate density, relatively cool molecular gas 
which could produce the absorption. However , Sgr B2 is also known to have 
very massive, dense, and hot cores, which might have been expected to produce 
an emission line instead. 
To clarify the situation, we created a detailed radiative transfer model. The 
model assumes a spherical cloud whose density and temperature (dust and gas) 
vary with radius as 
and 
n(r) = 2.2 x 103 + 8.6 x 104 (-r-) -2 cm-3 
1 pc 
T (r) = 40 (__:_) -o.s K . 
lpc 
This simple cloud model is based on the work of Lis, Goldsmith and 
collaborators, and is consistent with a map of C180{1 - 0) (Lis & Goldsmith 
1989) and with a large variety of dust continuum observations ranging from 4" 
OVRO interferometer measurements at A = 1.3 mm to the continuum intensity 
we observe in our 2' KAO beam at A= 479 J.Lm (Zmuidzinas et al. 1994a). Our 
radiative transfer program subdivides this model cloud into 200 radial shells, 
and self-consistently solves for the level populations and radiation intensities in 
each shell using an accelerated lambda-iteration method (Rybicki & Hummer 
1991). This model treats collisional and radiative excitation of the molecular 
levels up to J = 11, and includes emission and absorption of radiation by both 
molecules and dust. 
36 J. Zmuidzinas et al. 
The model is successful in predicting our observed absorption line (Figure 
1). The abundance needed to obtain the measured line to continuum ratio 
is HCI/ H2 :::::; 1.1 x 10-9 ; hence, it appears that the chlorine is depleted by a 
factor of 50-180 below the solar abundance, if in fact HCl represents 10-30% 
of the gas-phase chlorine as predicted by the chemical models. This conclusion 
is fairly robust, since the same HCl fradional abundance can be obtained by 
comparing an HCl column density of 1.6 x 1014 cm-2 derived from the integrated 
optical depth (determined from the line to continuum ratio), to an H2 column 
density of 1.5 x 1023 cm-2 for the absorbing envelope (determined from the C180 
map of Lis & Goldsmith 1989). The radiative transfer model verifies that 
the assumptions inherent in this simple calculation are correct , and that the 
absorption line is indeed formed in t he cool , presumably quiescent outer layers 
of the cloud, corresponding to r > 5 pc, n ,..., 6 x 103 em - 3 , and Tdust ,..., 20 K. An 
interesting prediction of our model is that dust continuum radiation dominates 
the excitation of the rotational levels of HCl throughout the cloud; this is quite 
plausible since Sgr B2 is a very bright continuum source and since the critical 
density of the 626 GHz transition is very large, ncr = 4 x 107 cm-3 . 
KAO 1993 June Sgr B2 
0 .8 
0 .6 
0.4 
0 .2 
0 50 100 
Velocity (km/s) 
Figure 2 . The 547 GHz H2 180 transition detected in absorption 
in Sgr B2. The dashed line is the prediction of a simple model 
consisting of a background continuum source (T.A :::::; 0. 7 K ) and a 
foreground cloud with two velocity components. The minimum 
H2 180 column density capable of producing this absorption is 
about 1.1 x 1014 cm-2 . 
1.2 Submillimeter Spectroscopy of Interstellar Hydrides 37 
AIRBORNE STUDIES OF OXYGEN-18 WATER 
Oxygen is the third most abundant element, and yet surprisingly little is 
understood about the chemistry of oxygen in molecular clouds. Potentially 
abundant oxygen-bearing species include 0 , 0 2, and H20 , but ground-based 
observations of these species are obviously difficult. Although the 22 GHz H20 
masers are very well known, the energy levels of the 616 -+ 523 transition lie 
about 640 K above the ground state. In fact , this is why the 22 GHz transition 
is observable from the ground, and partially explains why the masers are thought 
to arise from very dense and hot clumps of gas rather than the more ordinary 
regions of molecular clouds. If it is abundant, water may also be the major 
coolant of dense star-forming regions, since the water molecule has a strong 
asymmetric rotor spectrum with transitions throughout the submillimeter and 
far-infrared. 
0.4 
g 
0.2 
..... :< 
0 
- 50 0 50 100 
Velocity (km/s) 
Figure 3. Detection of the 110 - 101 547 GHz ground-state 
t ransition of ortho-H2 180 in Orion-KL. 
To probe the abundance of water in a more general way, one must observe 
lower lying transitions which are more likely to be populated under the typical 
conditions found in interstellar clouds. However, absorption by atmospheric H2 0 
precludes this approach, unless one takes advantage of the fact that the lines of 
H2180 are slightly shifted in frequency due to the somewhat larger moments of 
inertia. However, the isotopic ratio is 160jl8 0:::::: 500, so the H2 180 lines are weak. 
Nonetheless, we have recently detected the 547 GHz ground-state transition and 
the 745 GHz transition (Blower :=:::: 100 K) in Orion-KL using our SIS receiver on 
38 J. Zmuidzinas et al. 
the KAO. We have also detected the 547 GHz line in absorption toward Sgr B2. 
Using the Caltech Submillimeter Observatory (CSO), we have followed up the 
airborne observations with detections of the 391 GHz transition {Eiower::::: 300 K) 
in Orion-KL, Sgr B2, and several other sources as well. 
The 547 GHz H2 180 absorption line in Sgr B2 is shown in Figure 2. This 
detection can be analyzed in the same manner as the HCl absorption line, since 
the critical densities and A-coefficients of these two ground-state transitions are 
similar. The integrated optical depth determined from the line to continuum 
ratio implies a minimum H2 180 column density of 1.1 x 1014 cm-2 . As in the 
case of HCl, we suspect that this is the actual column density in the low 
excitation molecular envelope that we are sampling. Thus, we arrive at a 
fractional abundance of H2 180 / H2 ::::: 7 x w- 10 , again taking the relevant H2 
column density to be 1.5 x 1023 cm-2 for the absorbing envelope. Since the 
isotopic ratio near the center of the galaxy is thought to be 160jl8 0 ::::: 250, we 
obtain H20/ H2::::: 1.7 X w-7 . This value is low, but within the range predicted 
by current chemical models. 
ORION-KL 
H2
160 745 GHz 
0.5 
g 
0 
- 0 . 5 '--'--'--'----''"--'L........JL........JL........JL........JL......JL........JL........JL........J--lL........JL........JL......JL......J---I__J 
-50 0 50 100 
Velocity (km/s) 
Figure 4. Detection of the 2u - 202 745 GHz transition of 
para-H2 180 in Orion-KL. The energy of the lower level in this 
transition is about 100 K. 
H2180 may have previously been detected in Orion-KL through the 203 
GHz and 321 GHz transitions using ground-based telescopes. However, these 
observations are badly confused by strong nearby 802 lines which hampers their 
interpretation. As Figures 3 and 4 show, we have unambigously detected the 
547 and 745 GHz lines from the KAO. In addition, we have obtained a solid 
1.2 Submillimeter Spectroscopy of Interstellar Hydrides 39 
detection of the 391 GHz line from the CSO. All of these H2180 observations of 
Orion-KL are summarized in the table below. 
H 2180 LINES DETECTED IN OMC-1 
Telescope v Line Elower Bbeam r· A (GHz) (K) (arcsec) (K) 
NRAO a 203 3t3 _, 22o 194 30 0.9 
cso b 322 5ts _, 422 452 20 0.6 
cso c 391 414 _, 321 303 17 2.5 
KAO c 547 lto _, lot 0 150 0.55 
KAO c 745 2u -+ 2o2 101 llO 0.65 
a Jacq et al. 1988; blended with 802 12o,l2 -+ 111,11 · 
b Menten et al. 1990; blended with 802 3~.30 _, 33s,29· 
c Zmuidzinas et al. 1994. 
We have attempted a simple rotation diagram analysis of these lines, which 
yields H20 abundances in the w- 7 range, but the large variation in beamwidths 
makes the results fairly sensitive to the assumed source size. In addition, the 547 
GHz line appears to be weaker than what one would extrapolate from the higher-
lying transitions, and furthermore, the line profile of the 547 GHz transition 
does not seem to be consistent with either the 745 or 391 GHz profiles. Thus, 
we speculate that there may be foreground lower excitation gas which partially 
absorbs the 54 7 GHz radiation and distorts the line shape. We hope to learn 
more about the spatial distribution of H2180 by mapping the 391 GHz t ransition 
at the CSO, and perhaps by mapping the 203 GHz transition with the OVRO 
interferometer. This could give us the source size and therefore reduce the 
ambiguity in the rotation diagram analysis. In addition, these maps might tell 
us whether the "hot core" or "low-velocity outflow" component is primarily 
responsible for the H2 180 emission, and therefore whether evaporation from 
radiatively heated dust grains or shock chemistry is leading to a substantial 
enhancement of the H2180 abundance. We have some hint of the answer from 
the linewidths of the H2 180 transitions that we observe, which seem to be more 
consistent with the ·'low-velocity outflow" source than with the "hot core", but 
the inconsistency in the observed line profiles points to several emission (or 
absorption) components. 
ACKNOWLEDGEMENTS 
'vVe are deeply grateful to the staff of the Kuiper Airborne Observatory for their 
invaluable assistance with this new project, and commend their enthusiasm and 
dedication to airborne astronomy. We also thartk T. Phillips, D. Neufeld, W. 
Langer, and G. Melnick for illuminating discussions. We are also indebted to 
A. Betz and R. Boreiko for the loan of their AOS. This work was supported by 
a grant from the NASA Airborne Astronomy program (NAG2-744). SIS mixer 
development at Caltech is supported by NASA grant NAGW-107, NASA/ JPL, 
and a NSF PYI grant to J. Zmuidzinas. 
40 J. Zmuidzinas et al. 
REFERENCES 
Anders, E. , & Grevesse, N. 1989, Geochim. Cosmochim. Acta, 53, 197 
Blake, G. A., Keene, J ., & Phillips, T. G. 1985, ApJ, 295, 501 
Blake, G. A. , Anicich, V. G. , & Huntress, W. T. 1986, ApJ, 300, 415 
Dalgarno, A., de Jong, T. , Oppenheimer, M., & Black, J. H. 1974, ApJL, 192, 
L37 
Harris, A. W. , & Bromage, G. E. 1984, MNRAS, 208, 941 
Harris, A. W. , Gry, C., & Bromage, G. E. 1984, ApJ, 284, 157 
Jacq, T. , Jewell, P.R., Henkel, C., Walmsley, C. M. , & Baudry, A. 1988, A&A, 
199, L5. 
Jura, M.1974, ApJL, 190, L33 
Jura, M., & York, D. G. 1978, ApJ, 219, 861 
Lis, D. C., & Goldsmith, P. 1989, ApJ, 337, 704 
Menten, K. M., Melnick, G. J ., Phillips, T. G., & Neufeld, D. A. 1990, ApJL, 
363, L27. 
Neufeld, D. A., & Green, S. 1994, ApJ, in press. 
Rybicki, G. B. , & Hummer, D. G. 1991, A&A, 245, 171 
Schilke, P. , Phillips, T. G. , & Wang, N. 1994, in prep. 
Zmuidzinas, J ., Blake, G. A., Carlstrom J ., Keene, J. , Miller, D. , & Schilke, P. 
1994a, in prep. 
Zmuidzinas, J. , Carlstrom, J. , Miller, D., & Ugras, N. G. 1994b, in Proc of the 
Airborne Astronomy Symp on the Galactic Ecosystem: From Gas to Stars 
to Dust, ed. M. R. Haas, J . A. Davidson, & E. F . Erickson (San Francisco: 
ASP), paper 710. 


Submillimeter spectroscopy of interstellar hydrides

Airborne Astronomy Symposium on the Galactic Ecosystem 
ASP Conference Series, Vol. 73, 1995 
M.R. Haas, J.A. Davidson, and E.F. Erickson {eds.) 
33 
SUB MILLIMETER SPECTROSCOPY OF INTERSTELLAR HYDRIDES 
J. ZMUIDZINAS, G. A. BLAKEI, J. CARLSTROM, J. KEENE, D. 
MILLER, P. SCHILKE, N. G. UGRAS 
Division of Physics, Mathematics, and Astronomy, and !Division of 
Geological and Planetary Sciences, California Institute of Technology, 320-
47, Pasadena, CA 91125 
ABSTRACT We discuss airborne observations of rotational transitions 
of various hydride molecules in the interstellar medium, including H2 180 
and HCI. The detection of these transitions is now feasible with a new, 
sensitive submillimeter receiver which has been developed for the NASA 
Kuiper Airborne Observatory (KAO) over the past several years. 
INTRODUCTION 
Atmospheric water vapor is responsible for the generally poor atmospheric 
transmission in the submillimeter band (300-3000 GHz). In particular, even 
for observatories located on dry high-altitude sites such as Mauna Kea, the 
atmosphere is essentially opaque between 500-600 GHz and 700-800 GHz because 
of the very strong water lines at 547 GHz and 752 GHz. In addition, ground-
based submillimeter observations are impossible above 900 GHz. Nonetheless, 
these portions of the spectrum are very interesting astronomically because they 
contain many fundamental rotational transitions of hydride molecules, such as 
CHat 532 and 536 GHz, H2 180 at 547 and 745 GHz, NH3 at 572 GHz, HCl at 626 
GHz, CH2 at 945 GHz, and H30+ at 985 GHz. Observations of these molecules 
are important for testing our understanding of the chemistry of molecular clouds, 
since the hydrides are the initial products of the ion-molecule reaction network 
which is thought to be responsible for the synthesis of more the more complex 
molecules commonly observed at radio and millimeter wavelengths. 
In addition, hydrides may play a crucial role in the energetics of molecular 
clouds, particularly at the high densities found in star-forming cores, because 
their large rotational level spacings and large dipole moments make them very 
effective coolants. In fact, it is possible that cooling due to hydrides, especially 
HzO, plays a decisive role in establishing the rate at which molecular clouds 
collapse, and thus ultimately controls the efficiency of star formation. 
The Kuiper Airborne Observatory (KAO) is the only platform at present 
which allows observations of the transitions listed above (with perhaps the 
34 J. Zmuidzinas et a/. 
exception of HCl, which can be observed from Manna Kea in extremely good 
weather) . Although some of these molecules can also be observed through 
transitions at radio wavelengths, airborne observations of the fundamental 
rotational transitions can often give essential information regarding the 
abundance and excitation of these species. 
In this paper, we discuss studies of interstellar H2 180 and HCl we have been 
pursuing over the past two years. We have also detected the CH transitions at 
532 and 536 GHz in Orion, W51 , M17, and other sources, but these observations 
will not be discussed in this paper due to the limited space. The observations 
which we present were obtained with a new submillimeter heterodyne receiver 
developed for the KAO, which at present operates over the 500-750 GHz band, 
which is described in a paper elsewhere in this volume (Zmuidzinas et al. 1994b) . 
KAO 1993 June 626 GHz HCI(l-0) Sgr B2 
0.5 
0 50 100 
Velocity (km/ s) 
Figure 1. Solid line: detection of the 626 GHz HCl line in 
absorption toward Sgr B2. Dashed line: spectrum predicted 
by our radiative transfer model for a fractional abundance of 
HCJ/ H2 = 1.1 x w-9 . 
HCl IN ABSORPTION IN SGR B2 
The abundance of atomic chlorine in diffuse interstellar clouds has been studied 
using ultraviolet absorption lines by Jura & York (1978), Harris & Bromage 
(1984), and Harris, Gry, and Bromage (1984). These studies have shown that 
the depletion of chlorine (on dust grains) in diffuse clouds is not severe, with 
1.2 Submillimeter Spectroscopy of Interstellar Hydrides 35 
abundances at most a factor of 2-3 below the solar value of Cl/H ~ 2 x w-7 
(Anders & Grevesse 1989), although there is some indication that the depletion 
increases with density. It would be very interesting to study the depletion 
in denser regions; however, ultraviolet studies of dense clouds are obviously 
impossible. The submillimeter rotational transitions of HCl may be a useful 
probe of the chlorine depletion in dense molecular clouds, since gas-phase 
chemical models predict that a substantial fraction (> 10%) of chlorine will be 
in the form of HCl (Jura 1974; Dalgarno et al. 1974; Blake, Anicich, & Huntress 
1986; Schilke, Phillips, & Wang 1994). 
The 626 GHz J = 1 _, 0 line was first detected in OMC-1 from the KAO by 
Blake, Keene , and Phillips (1985). The observed emission line (T.A ~ 0.7 K) was 
interpreted to give HCI/ H2 ,...., 0.5-5.0 x w-8 , which corresponds to a probable 
range of chlorine depletion of 3-30 (below solar). The lower end of this range 
could be consistent with the chlorine depletion in diffuse clouds, but Neufeld 
and Green (1994) have recently reanalyzed this data using improved collisional 
excitation rates and a more detailed radiative transfer model, and argue that 
in fact the abundance may be only 2 X w-9 . Hence, it is likely that chlorine is 
either heavily depleted or the fraction of chlorine in HCl is much smaller than 
the predictions of the chemical models. 
During a June 1993 KAO deployment to Hickam AFB in Honolulu, HI, we 
detected the 626 GHz line in absorption against the submillimeter continuum 
from the Sgr B2 (N) and (M) dust cores (Figure 1). It is not entirely surprising 
that the line is in absorption, given the fact that this is a ground-state transition 
which has a very large critical density of ncr= 4 x 107 cm-3. Furthermore, Sgr 
B2 has an extended envelope of moderate density, relatively cool molecular gas 
which could produce the absorption. However , Sgr B2 is also known to have 
very massive, dense, and hot cores, which might have been expected to produce 
an emission line instead. 
To clarify the situation, we created a detailed radiative transfer model. The 
model assumes a spherical cloud whose density and temperature (dust and gas) 
vary with radius as 
and 
n(r) = 2.2 x 103 + 8.6 x 104 (-r-) -2 cm-3 
1 pc 
T (r) = 40 (__:_) -o.s K . 
lpc 
This simple cloud model is based on the work of Lis, Goldsmith and 
collaborators, and is consistent with a map of C180{1 - 0) (Lis & Goldsmith 
1989) and with a large variety of dust continuum observations ranging from 4" 
OVRO interferometer measurements at A = 1.3 mm to the continuum intensity 
we observe in our 2' KAO beam at A= 479 J.Lm (Zmuidzinas et al. 1994a). Our 
radiative transfer program subdivides this model cloud into 200 radial shells, 
and self-consistently solves for the level populations and radiation intensities in 
each shell using an accelerated lambda-iteration method (Rybicki & Hummer 
1991). This model treats collisional and radiative excitation of the molecular 
levels up to J = 11, and includes emission and absorption of radiation by both 
molecules and dust. 
36 J. Zmuidzinas et al. 
The model is successful in predicting our observed absorption line (Figure 
1). The abundance needed to obtain the measured line to continuum ratio 
is HCI/ H2 :::::; 1.1 x 10-9 ; hence, it appears that the chlorine is depleted by a 
factor of 50-180 below the solar abundance, if in fact HCl represents 10-30% 
of the gas-phase chlorine as predicted by the chemical models. This conclusion 
is fairly robust, since the same HCl fradional abundance can be obtained by 
comparing an HCl column density of 1.6 x 1014 cm-2 derived from the integrated 
optical depth (determined from the line to continuum ratio), to an H2 column 
density of 1.5 x 1023 cm-2 for the absorbing envelope (determined from the C180 
map of Lis & Goldsmith 1989). The radiative transfer model verifies that 
the assumptions inherent in this simple calculation are correct , and that the 
absorption line is indeed formed in t he cool , presumably quiescent outer layers 
of the cloud, corresponding to r > 5 pc, n ,..., 6 x 103 em - 3 , and Tdust ,..., 20 K. An 
interesting prediction of our model is that dust continuum radiation dominates 
the excitation of the rotational levels of HCl throughout the cloud; this is quite 
plausible since Sgr B2 is a very bright continuum source and since the critical 
density of the 626 GHz transition is very large, ncr = 4 x 107 cm-3 . 
KAO 1993 June Sgr B2 
0 .8 
0 .6 
0.4 
0 .2 
0 50 100 
Velocity (km/s) 
Figure 2 . The 547 GHz H2 180 transition detected in absorption 
in Sgr B2. The dashed line is the prediction of a simple model 
consisting of a background continuum source (T.A :::::; 0. 7 K ) and a 
foreground cloud with two velocity components. The minimum 
H2 180 column density capable of producing this absorption is 
about 1.1 x 1014 cm-2 . 
1.2 Submillimeter Spectroscopy of Interstellar Hydrides 37 
AIRBORNE STUDIES OF OXYGEN-18 WATER 
Oxygen is the third most abundant element, and yet surprisingly little is 
understood about the chemistry of oxygen in molecular clouds. Potentially 
abundant oxygen-bearing species include 0 , 0 2, and H20 , but ground-based 
observations of these species are obviously difficult. Although the 22 GHz H20 
masers are very well known, the energy levels of the 616 -+ 523 transition lie 
about 640 K above the ground state. In fact , this is why the 22 GHz transition 
is observable from the ground, and partially explains why the masers are thought 
to arise from very dense and hot clumps of gas rather than the more ordinary 
regions of molecular clouds. If it is abundant, water may also be the major 
coolant of dense star-forming regions, since the water molecule has a strong 
asymmetric rotor spectrum with transitions throughout the submillimeter and 
far-infrared. 
0.4 
g 
0.2 
..... :< 
0 
- 50 0 50 100 
Velocity (km/s) 
Figure 3. Detection of the 110 - 101 547 GHz ground-state 
t ransition of ortho-H2 180 in Orion-KL. 
To probe the abundance of water in a more general way, one must observe 
lower lying transitions which are more likely to be populated under the typical 
conditions found in interstellar clouds. However, absorption by atmospheric H2 0 
precludes this approach, unless one takes advantage of the fact that the lines of 
H2180 are slightly shifted in frequency due to the somewhat larger moments of 
inertia. However, the isotopic ratio is 160jl8 0:::::: 500, so the H2 180 lines are weak. 
Nonetheless, we have recently detected the 547 GHz ground-state transition and 
the 745 GHz transition (Blower :=:::: 100 K) in Orion-KL using our SIS receiver on 
38 J. Zmuidzinas et al. 
the KAO. We have also detected the 547 GHz line in absorption toward Sgr B2. 
Using the Caltech Submillimeter Observatory (CSO), we have followed up the 
airborne observations with detections of the 391 GHz transition {Eiower::::: 300 K) 
in Orion-KL, Sgr B2, and several other sources as well. 
The 547 GHz H2 180 absorption line in Sgr B2 is shown in Figure 2. This 
detection can be analyzed in the same manner as the HCl absorption line, since 
the critical densities and A-coefficients of these two ground-state transitions are 
similar. The integrated optical depth determined from the line to continuum 
ratio implies a minimum H2 180 column density of 1.1 x 1014 cm-2 . As in the 
case of HCl, we suspect that this is the actual column density in the low 
excitation molecular envelope that we are sampling. Thus, we arrive at a 
fractional abundance of H2 180 / H2 ::::: 7 x w- 10 , again taking the relevant H2 
column density to be 1.5 x 1023 cm-2 for the absorbing envelope. Since the 
isotopic ratio near the center of the galaxy is thought to be 160jl8 0 ::::: 250, we 
obtain H20/ H2::::: 1.7 X w-7 . This value is low, but within the range predicted 
by current chemical models. 
ORION-KL 
H2
160 745 GHz 
0.5 
g 
0 
- 0 . 5 '--'--'--'----''"--'L........JL........JL........JL........JL......JL........JL........JL........J--lL........JL........JL......JL......J---I__J 
-50 0 50 100 
Velocity (km/s) 
Figure 4. Detection of the 2u - 202 745 GHz transition of 
para-H2 180 in Orion-KL. The energy of the lower level in this 
transition is about 100 K. 
H2180 may have previously been detected in Orion-KL through the 203 
GHz and 321 GHz transitions using ground-based telescopes. However, these 
observations are badly confused by strong nearby 802 lines which hampers their 
interpretation. As Figures 3 and 4 show, we have unambigously detected the 
547 and 745 GHz lines from the KAO. In addition, we have obtained a solid 
1.2 Submillimeter Spectroscopy of Interstellar Hydrides 39 
detection of the 391 GHz line from the CSO. All of these H2180 observations of 
Orion-KL are summarized in the table below. 
H 2180 LINES DETECTED IN OMC-1 
Telescope v Line Elower Bbeam r· A 
(GHz) (K) (arcsec) (K) 
NRAO a 203 3t3 _, 22o 194 30 0.9 
cso b 322 5ts _, 422 452 20 0.6 
cso c 391 414 _, 321 303 17 2.5 
KAO c 547 lto _, lot 0 150 0.55 
KAO c 745 2u -+ 2o2 101 llO 0.65 
a Jacq et al. 1988; blended with 802 12o,l2 -+ 111,11 · 
b Menten et al. 1990; blended with 802 3~.30 _, 33s,29· 
c Zmuidzinas et al. 1994. 
We have attempted a simple rotation diagram analysis of these lines, which 
yields H20 abundances in the w- 7 range, but the large variation in beamwidths 
makes the results fairly sensitive to the assumed source size. In addition, the 547 
GHz line appears to be weaker than what one would extrapolate from the higher-
lying transitions, and furthermore, the line profile of the 547 GHz transition 
does not seem to be consistent with either the 745 or 391 GHz profiles. Thus, 
we speculate that there may be foreground lower excitation gas which partially 
absorbs the 54 7 GHz radiation and distorts the line shape. We hope to learn 
more about the spatial distribution of H2180 by mapping the 391 GHz t ransition 
at the CSO, and perhaps by mapping the 203 GHz transition with the OVRO 
interferometer. This could give us the source size and therefore reduce the 
ambiguity in the rotation diagram analysis. In addition, these maps might tell 
us whether the "hot core" or "low-velocity outflow" component is primarily 
responsible for the H2 180 emission, and therefore whether evaporation from 
radiatively heated dust grains or shock chemistry is leading to a substantial 
enhancement of the H2180 abundance. We have some hint of the answer from 
the linewidths of the H2 180 transitions that we observe, which seem to be more 
consistent with the ·'low-velocity outflow" source than with the "hot core", but 
the inconsistency in the observed line profiles points to several emission (or 
absorption) components. 
ACKNOWLEDGEMENTS 
'vVe are deeply grateful to the staff of the Kuiper Airborne Observatory for their 
invaluable assistance with this new project, and commend their enthusiasm and 
dedication to airborne astronomy. We also thartk T. Phillips, D. Neufeld, W. 
Langer, and G. Melnick for illuminating discussions. We are also indebted to 
A. Betz and R. Boreiko for the loan of their AOS. This work was supported by 
a grant from the NASA Airborne Astronomy program (NAG2-744). SIS mixer 
development at Caltech is supported by NASA grant NAGW-107, NASA/ JPL, 
and a NSF PYI grant to J. Zmuidzinas. 
40 J. Zmuidzinas et al. 
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Submillimeter spectroscopy of interstellar hydrides

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