# Herschel observations of EXtra-Ordinary Sources (HEXOS): Detection of hydrogen fluoride in absorption towards Orion KL

T. G. Phillips, E. A. Bergin, D. C. Lis, D. A. Neufeld, T. A. Bell, S. Wang, N.

R. Crockett, M. Emprechtinger, G. A. Blake, E. Caux, et al.

► To cite this version:

T. G. Phillips, E. A. Bergin, D. C. Lis, D. A. Neufeld, T. A. Bell, et al. Herschel observations of EXtra-Ordinary Sources (HEXOS): Detection of hydrogen fluoride in absorption towards Orion KL. Astronomy and Astrophysics - A&A, 2010, 518, 10.1051/0004-6361/201014570. insu-03625646

HAL Id: insu-03625646

https://insu.hal.science/insu-03625646

Submitted on 31 Mar 2022

**HAL** is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
LETTER TO THE EDITOR

# Herschel observations of EXtra-Ordinary Sources (HEXOS): Detection of hydrogen fluoride in absorption towards Orion KL*

T. G. Phillips¹, E. A. Bergin², D. C. Lis¹, D. A. Neufeld³, T. A. Bell¹, S. Wang², N. R. Crockett², M. Emprechtinger¹, G. A. Blake¹, E. Caux⁴,⁵, C. Ceccarelli⁶, J. Cernicharo⁷, C. Comito⁸, F. Daniel⁷,⁹, M.-L. Dubernet¹⁰,¹¹, P. Encrenaz⁹, M. Gerin⁹, T. F. Giesen¹², J. R. Goicoechea⁷, P. F. Goldsmith¹³, E. Herbst¹⁴, C. Joblin⁴,⁵, D. Johnstone¹⁵,

W. D. Langer¹³, W. D. Latter¹⁶, S. D. Lord¹⁶, S. Maret⁶, P. G. Martin¹⁷, G. J. Melnick¹⁸, K. M. Menten⁸, P. Morris¹⁶, H. S. P. Müller¹², J. A. Murphy¹⁹, V. Ossenkopf¹²,²⁰, J. C. Pearson¹³, M. Pérault⁹, R. Plume²¹, S.-L. Qin¹²,

P. Schilke⁸,¹², S. Schlemmer¹², J. Stutzki¹², N. Trappe¹⁹, F. F. S. van der Tak²¹, C. Vastel⁴,⁵, H. W. Yorke¹³, S. Yu¹³, J. Zmuidzinas¹, A. Boogert¹⁶, R. Güsten⁸, P. Hartogh²², N. Honingh¹², A. Karpov¹, J. Kooi¹,

J.-M. Krieg⁹, and R. Schieder¹²

*(Affiliations are available in the online edition)*

Received 30 March 2010 / Accepted 20 April 2010

ABSTRACT

We report a detection of the fundamental rotational transition of hydrogen fluoride in absorption towards Orion KL using Herschel/HIFI. After the removal of contaminating features associated with common molecules ("weeds"), the HF spectrum shows a P-Cygni profile, with weak redshifted emission and strong blue-shifted absorption, associated with the low-velocity molecular outflow. We derive an estimate of 2.9 × 10¹³ cm⁻² for the HF column density responsible for the broad absorption component. Using our best estimate of the H₂ column density within the low-velocity molecular outflow, we obtain a lower limit of ~1.6 × 10⁻¹⁰ for the HF abundance relative to hydrogen nuclei, corresponding to ~0.6% of the solar abundance of fluorine. This value is close to that inferred from previous ISO observations of HF *J* = 2–1 absorption towards Sgr B2, but is in sharp contrast to the lower limit of 6 × 10⁻⁹ derived by Neufeld et al. for cold, foreground clouds on the line of sight towards G10.6-0.4.

**Key words.** ISM: abundances – ISM: molecules – submillimeter: ISM

1. Introduction

Hydrogen fluoride (HF) is expected to be the main reservoir of fluorine in the interstellar medium because it is easily produced by the exothermic reaction of F with H₂ (Neufeld et al. 2005; Neufeld & Wolfire 2009) and its very strong chemical bond makes this molecule relatively insensitive to UV photodissociation. Interstellar HF was first detected by Neufeld et al. (1997) with ISO. The *J* = 2–1 rotational transition was observed in absorption towards Sgr B2, at a low spectral resolution using the ISO long-wavelength spectrometer (LWS). The HIFI instrument (de Graauw et al. 2010) aboard the Herschel Space Observatory (Pilbratt et al. 2010) has allowed observations of the ground state rotational transition of HF at 1.232 THz to be performed for the first time, at high spectral resolution. This transition is expected to be generally observed in absorption because of the very large A coefficient (e.g. Neufeld et al. 2010). Only extremely dense regions could possibly generate enough collisional excitation to yield an HF feature with a positive frequency-integrated flux.

A full HIFI spectral scan of band 5a, with frequency coverage from 1.109 to 1.239 THz, was carried out as part of the guaranteed time key program Herschel observations of EXtra-Ordinary Sources: The Orion and Sagittarius B2

star-forming regions (HEXOS). With a strong continuum, it might be expected that Orion would exhibit numerous absorption lines; however it is well known to exhibit no absorption lines at mm wavelengths. For instance even CO *J* = 1–0, which is seen with self-reversals towards many star-forming regions, has a smooth emission line profile with no self-absorption (e.g. Tauber et al. 1991). The lack of absorption has been attributed to competing excitation gradients (external heating from θ¹C and internal heating from the embedded massive protostars), the location of the H II region on the front of the cloud, and the presence of numerous unresolved dense (*n*H₂ > 10⁵ cm⁻³) clumps along the line of sight (Tauber et al. 1991). At shorter wavelengths some evidence for absorption is found. Betz & Boreiko (1989) find that the blue side of the fundamental rotational transition of OH is completely absorbed, with only a redshifted emission component. The far-infrared survey of Lerate et al. (2006) with ISO-LWS (188 to 44 µm), has shown that the shapes of water and OH lines gradually change from pure emission at the longest wavelengths to mostly P-Cygni profiles at the shortest wavelengths. At even shorter mid-infrared wavelengths, strong water absorption has been detected by Wright et al. (2000) using ISO-SWS. With the ~10 km s⁻¹ resolution of the SWS Fabry Perot, the absorption is shown to be blue-shifted with respect to the source systemic velocity, extending down to about −50 km s⁻¹. In recent observations with the CRIRES spectrograph on the Very Large Telescope, Beuther et al. (2010)

<footnote>* Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.</footnote>
have obtained spectra of the R-branch of the v = 1–0 band of both ¹³CO and ¹²CO toward the BN object and “source n” within Orion KL. These spectra, which are consistent with earlier spectra obtained at lower resolution and signal-to-noise ratio by Scoville et al. (1983), also show strong absorption at velocities between about –25 and 12 km s⁻¹. As we will discuss below, the particular excitation of these lines (ground rotational state HF and ground vibrational state CO) makes them strong candidates to be seen in absorption, provided favorable geometry and strong background continuum exist. Ground state rotational lines of water isotopologues have similar excitation requirements and HIFI observations of para-H₂¹⁸O and para-H₂¹⁷O, observed separately in band 4b, are also discussed here.

## 2. Observations

HIFI observations presented here were carried out on March 6, 2010 using the dual beam switch (DBS) mode pointed towards the Orion Hot Core at $\alpha_{J2000} = 5^{h}35^{m}14.3^{s}$ and $\delta_{J2000} = -5^{\circ}22'36.7''$. The DBS reference beams lie approximately 3' east and west (i.e. perpendicular to the orientation of the Orion Molecular Ridge; e.g. Ungerechts et al. 1997). We used the Wide Band Spectrometer providing a spectral resolution of 1.1 MHz (0.26 km s⁻¹) over a 4 GHz IF bandwidth. Although both H and V polarization data were obtained, we only present here data from the H polarization, reduced using HIPE (Ott 2010) with pipeline version 2.4.

The band 4b and 5a spectral scans consist of double sideband spectra with a redundancy of 6, which gives observations of a lower or upper sideband frequency with 6 different frequency settings of the local oscillator. This allows for the deconvolution and isolation of a single sideband spectrum (Comito & Schilke 2002). We applied the standard HIFI deconvolution using the *doDeconvolution* task within HIPE. All data presented here are deconvolved single sideband spectra, including the continuum. Regions of the spectrum free of lines were isolated and give a typical rms of $T_A^* = 0.17$ K at the original spectral resolution. The HIFI beam size at 1.23 THz is 17'', with an assumed main beam efficiency of 0.67.

A ¹³CO *J* = 2–1 spectrum towards Orion BN/KL was also obtained using the facility receiver and spectrometers of the Caltech Submillimeter Observatory (CSO) atop Mauna Kea in Hawaii. The CSO beam size at 220 GHz is ~33'' and the main beam efficiency is ~0.68.

## 3. Results

### 3.1. First detection of submm absorption towards Orion

Figure 1 shows the detection of HF *J* = 1–0 and para-H₂¹⁸O 1₁₁–0₀₀ in emission and absorption towards Orion BN/KL (blue and red histograms, respectively). Both lines show high-velocity emission line wings on the red side of the systemic velocity of 9 km s⁻¹, a sharp drop near the systemic velocity, and broad absorption extending towards lower (blue) velocities. In contrast, ¹³CO *J* = 2–1 emission (black histogram) shows broad line wings superposed on a narrow feature at 9 km s⁻¹, but no evidence for absorption.

There are a number of issues which must be addressed. First these data were obtained using DBS which places the reference beams 3' away from the central hot core. This is large enough to avoid any reference position contamination from the hot core and shock, but might encompass emission from the extended molecular ridge. Both HF and para-H₂¹⁸O have high dipole moments and fast (>10⁻² s⁻¹) spontaneous de-excitation

<figure><img src="image_2.png" /></figure>

**Fig. 1.** HIFI detection of interstellar HF *J* = 1–0 (rest frequency 1232.47627 GHz; Nolt et al. 1987) and para-H₂¹⁸O 1₁₁–0₀₀ (at 1101.698256 GHz) towards Orion BN/KL (blue and red, respectively). The region of low-velocity HF absorption is highlighted in grey. Both observations are from HIFI band 5a and are referenced to the temperature scale on the left. CSO ¹³CO *J* = 2–1 spectrum towards the same position is shown in black, using the temperature scale on the right. The HF absorption is blended with the emission of CH₃OH (~–10 km s⁻¹) and SO₂ (~–20 and –65 km s⁻¹), shown in green. Subtraction of these contaminating lines results in the dark blue HF spectrum.

rates leading to critical densities in excess of 10⁸ cm⁻³ (Reese et al. 2005; Grosjean et al. 2003). Beyond the hot core, the density is well below this value (Bergin et al. 1996) and, for HF, extended emission is unlikely. This may not be the case for para-H₂¹⁸O, as the ground state emission of *ortho*-H₂O is strong and extended (Snell et al. 2000; Olofsson et al. 2003). Because the DBS mode alternates between two reference positions, we have used the Level 1 data to compute a difference spectrum between the two reference positions; we see no evidence for emission or absorption in such a difference spectrum for either line. It is very unlikely that the same level of emission or absorption would be present in the two reference beams, separated by 6' on the sky. In addition, the extended emission component in Orion is centered at 9 km s⁻¹ and has a narrow line width of 2–5 km s⁻¹. For HF we do see absorption at the systemic velocity, but also a broad blue-shifted absorption. We thus conclude that the observed absorption is real, and not an artifact of the observing mode employed.

### 3.2. Contamination by interfering lines

In the case of HF there is an additional complication in that the low-velocity blue absorption is blended with emission from CH₃OH *J* = 21₁–2₀₀ E and SO₂ *J* = 307,23–296,24 (see grey area in Fig. 1). A more extensive look at the spectrum near HF *J* = 1–0 is provided in Fig. 2. In this 7 GHz wide region of the spectrum we see multiple prominent lines of both CH₃OH and SO₂. For CH₃OH, one of the detected transitions is *J* = 21₁–2₀₀ A⁺, which has a similar ground state energy and 120% stronger line strength than the *J* = 21₁–2₀₀ E transition. Thus we confirm the CH₃OH contamination. We have modeled the extensive emission from SO₂ (and CH₃OH) seen in Band 5a assuming LTE (with a correction for optical depth). This analysis also confirms that SO₂ *J* = 307,23–296,24 will emit at appreciable levels and interfere with the HF absorption.

Line contamination by abundant “weed” molecules, such as methanol or SO₂, will be a common problem affecting HIFI
<figure><img src="image_3.png" /></figure>

**Fig. 2.** A more extensive look at the molecular spectrum surrounding HF *J* = 1–0. Transitions of CH₃OH and SO₂ were isolated using the frequencies taken from Müller & Brünken (2005) and Xu et al. (2008). Three lines are blended with (or provide background to) the HF absorption.

observations of molecular hot core sources and modeling tools
are being developed to deal with this problem. The two SO₂ lines
in the spectrum have been removed using such a model<fnref n="1" />. In the
case of CH₃OH, the LTE model is not accurate enough to
deal with the problem and we removed the interfering line by
fitting a single-component gaussian and subtracting the emis-
sion. The emission fit and the final HF absorption spectrum
are shown in Fig. 3. This spectrum is then used for the anal-
ysis in Sect. 4. The HF absorption full-width at zero intensity
(FWZI) is ~50 km s⁻<fnref n="1" />, which is less than that of para-H₂¹⁸O
(FWZI ~ 80 km s⁻<fnref n="1" />). For completeness, in the computation of
the line-to-continuum ratio, we also explored the possibility that
the CH₃OH and SO₂ lines contribute to the background emis-
sion for the absorbing HF gas. This will be reflected in our error
analysis.

The analysis is simpler for H₂¹⁸O, which shows no evidence
for any contamination within the absorption velocity range.
For this line we have assumed a continuum value based on the
level measured at frequencies adjacent to the water line.

**4. Discussion**

After the removal of features attributed to CH3OH and SO2,
the HF J = 1-0 spectrum shows a P-Cygni profile with a broad,
blueshifted absorption at LSR velocities between about -45 and
9 km s-1 and a redshifted emission component at LSR velocities
in the range 12 to 50 km s-1. The analysis of the HF emission,
along with other spectral lines detected in Orion, will be dis-
cussed in a future paper. The HF spectrum is strikingly similar to
that of another transition with an extremely high critical density:
the CO fundamental vibrational band (Beuther et al. 2010) which
shows the same combination of absorption at LSR velocities be-
tween about -25 and 12 km s-1 and weak emission extending
to VLSR ~ 30 km s-1. We suggest that exactly the same physical
processes give rise to the CO v = 1-0 and HF J = 1-0 spectra:
outflowing material surrounding the hot core in Orion absorbs
and re-emits continuum radiation from the central source. The
absorption - occurring in front of the source - naturally gives
rise to a blueshifted absorption feature, while re-emission from

<figure><img src="image_4.png" /></figure>

**Fig. 3.** HIFI HF *J* = 1–0 spectrum (blue-green histogram) with fit to CH₃OH and SO₂ emission shown in yellow. Subtraction of these contaminating lines provides a more complete look at the HF absorption (blue histogram).

the back side of the outflow provides the weak redshifted emis-
sion feature. For transitions with a very high critical density, the
absorption of a “resonance line photon” (such as HF *J* = 1–0)
will inevitably be followed by re-emission. Were the continuum
source surrounded by a spherical shell that is small compared to
the telescope beam, the areas of the emission and absorption fea-
tures would be equal. The simplest interpretation of the observed
HF *J* = 1–0 spectrum – which shows an absorption feature that
is stronger than the emission feature – is that the outflowing ma-
terial is not entirely encompassed by the beam so that part of the
emission flux is unobserved; this explanation could be tested by
means of mapping observations.

Most of the material appears to have an outflow velocity
≤20 km s⁻<fnref n="1" /> and is likely associated with the “Low Velocity
Flow” (e.g. Genzel & Stutzki 1989). The para-H₂¹⁸O and para-
H₂¹⁷O 1₁₁–0₀₀ (not shown) lines also show absorption at nega-
tive LSR velocities, although the absorption extends further, to
LSR velocities as negative as ~ − 80 km s⁻<fnref n="1" />. This behavior may
reflect the presence of enhanced water abundances in the High
Velocity Flow (Franklin et al. 2008). The para-H₂¹⁸O (and para-
H₂¹⁷O 1₁₁–0₀₀) line profiles are also different from HF in ex-
hibiting an emission feature that is stronger than the absorption
feature. This behavior must imply that collisional excitation pro-
vides an additional excitation mechanism, and may suggest that
the rate coefficients for excitation of the para-water transition
are significantly larger than those for excitation of the HF transi-
tion. To date, the collisional excitation of HF has been computed
only in the case where He is the collision partner (Reese et al.
2005); the rate coefficients thereby derived are indeed an order
of magnitude smaller than those computed for the excitation of
para-water by H₂ (Grosjean et al. 2003), but the rate coefficients
for excitation of HF by H₂ might be expected to be larger than
those for excitation by He.

We have determined the column density of absorbing
molecules responsible for the broad blueshifted absorption fea-
tures. If the absorbing material is assumed to cover the contin-
uum source, we estimate the velocity-integrated optical depth
HF *J* = 1–0 as 11.8 km s⁻<fnref n="1" />, which implies an HF column den-
sity of 2.9 × 10¹³ cm⁻² if all molecules are in the ground-state.
Uncertainties introduced by the need to correct for the SO₂ and
CH₃OH emission features result in possible errors that we esti-
mate as about ±25%. Derived under the same set of assumptions,
the column densities of absorbing para-H₂¹⁸O and para-H₂¹⁷O
are 1.3 × 10¹³ cm⁻² and 7 × 10¹² cm⁻², respectively. Strictly,

<footnote n="1">¹ We made use of the myXCLASS program (http://www.astro.uni-koeln.de/projects/schilke/XCLASS), which accesses the CDMS (Müller et al. 2001, 2005; http://www.cdms.de) and JPL (Pickett et al. 1998; http://spec.jpl.nasa.gov) molecular data bases.</footnote>
these values are all lower limits, because the source could be partially-covered by clumps of arbitrarily large optical depth and the strong continuum could lead to some excitation. In addition, the water lines have strong emission which might lie behind the absorbing material. However, the fact that all three spectral lines show absorption profiles of similar shapes but different depths, suggests that the optical depths are not extremely large.

In order to estimate the molecular abundances implied by these column densities, we require an estimate of the total column density of H₂. Beuther et al. (2010) used the observed strength of the ¹³CO *v* = 1–0 absorption features along the sight-line to the BN object, together with standard assumptions about the ¹³CO/¹²CO and CO/H₂ abundance ratios, to derive an H₂ column density of 4.9 × 10²² cm⁻² for the outflowing absorbing gas. An alternative estimate has been obtained by Persson et al. (2007) from observations of emission from C¹⁷O pure rotational lines; this yields a value 3 × 10²³ cm⁻² for the total H₂ column density within the Low Velocity Flow, half of which (1.5 × 10²³ cm⁻²) would be associated with the blue outflow lobe. We have estimated the H₂ column density independently from the strength of the broad ¹³CO 2–1 emission component. Assuming LVG, 100 K kinetic temperature, an H₂ density of 1 × 10⁵ cm⁻³ (Persson et al. 2007), and a ¹³CO fractional abundance of 2 × 10⁻⁶ (Dickman 1987), we derive N(H₂) = 9 × 10²² cm⁻² in the blue outflow lobe. This value, intermediate between the Persson et al. and Beuther et al. estimates, is used in the subsequent calculations (a factor of 2 estimated uncertainty).

With these assumptions, our observations of the blueshifted absorption feature imply an HF abundance of 1.6 × 10⁻¹⁰ relative to hydrogen nuclei, corresponding to ~0.6% of the solar abundance of fluorine (Asplund et al. 2009). This value is in sharp contrast with the lower limit N(HF)/N<sub>H</sub> ≥ 6 × 10⁻⁹ obtained by Neufeld et al. (2010) for foreground material along the sight-line to G10.6-0.4, but close to that inferred (N(HF)/N<sub>H</sub> = 1.5 × 10⁻¹⁰) from previous observations of HF *J* = 2–1 absorption toward Sgr B2 (Neufeld et al. 1997). Since the density in the Low Velocity Flow, *n*(H<sub>2</sub>) ~ 10<sup>5</sup> cm<sup>-3</sup> (Persson et al. 2007) is two orders of magnitude larger than the typical density of the foreground material probed in observations of G10.6-0.4, and since HF is expected to be the dominant reservoir of fluorine in the gas-phase, this abundance difference reflects a density-dependent depletion of fluorine nuclei onto grain mantles. A depletion factor ~10<sup>2</sup> does not seem unreasonable, in light of the polar nature of the HF molecule; indeed, the freezing and boiling points of HF are relatively large (190 K and 293 K respectively at 1 atm. pressure), so – as in the case of interstellar water – the freeze-out of HF can be expected to be quite efficient. On the other hand, shocks of velocity greater than ~15 km s<sup>-1</sup> are predicted to release grain mantles into the gas phase as the result of sputtering (Draine 1995). If high depletions are indeed attained in gas traveling at outflow velocities up to ~35 km s<sup>-1</sup>, then our observations suggest that the acceleration to a given terminal velocity can occur without the material suffering shocks of equal velocity.

## 5. Conclusions

Our observations of hydrogen fluoride toward Orion-KL have revealed an unusual absorption feature in the spectrum of this source. To our knowledge, this is the first report of a submillimeter spectral line with a net negative flux in this archetypical

emission line source. The unusual behavior of the HF *J* = 1–0 transition is a consequence of its extremely large critical density, and is mirrored by mid-infrared observations of the CO *v* = 1–0 band. Thanks to the high spectral resolution achievable with HIFI, the HF *J* = 1–0 line promises to provide a unique probe of the kinematics of – and depletion within – absorbing material along the sight-line to bright continuum sources, and one that is uncomplicated by the collisionally-excited line emission that is usually present in the spectra of other transitions. Redshifted HF *J* = 1–0 absorption may also prove to be an excellent tracer of the interstellar medium in the high-redshift Universe; the range of redshifts accessible from ground-based submillimeter telescopes is indicated by Neufeld et al. (2005, see their Fig. 11).

*Acknowledgements.* HIFI has been designed and built by a consortium of institutes and university departments from across Europe, Canada and the United States under the leadership of SRON Netherlands Institute for Space Research, Groningen, The Netherlands and with major contributions from Germany, France and the US. Consortium members are: Canada: CSA, U.Waterloo; France: CESR, LAB, LERMA, IRAM; Germany: KOSMA, MPIfR, MPS, Ireland, NUI Maynooth; Italy: ASI, IFSI-INAF, Osservatorio Astrofisico di Arcetri- INAF; Netherlands: SRON, TUD; Poland: CAMK, CBK; Spain: Observatorio Astronómico Nacional (IGN), Centro de Astrobiología (CSIC-INTA). Sweden: Chalmers University of Technology – MC2, RSS & GARD; Onsala Space Observatory; Swedish National Space Board, Stockholm University – Stockholm Observatory; Switzerland: ETH Zurich, FHNW; USA: Caltech, JPL, NHSC. Support for this work was provided by NASA through an award issued by JPL/Caltech. CSO is supported by the NSF, award AST-0540882.

## References

Asplund, M., Grevesse, N., Sauval, A. J., & Scott, P. 2009, ARA&A, 47, 481

Bergin, E. A., Snell, R. L., & Goldsmith, P. F. 1996, ApJ, 460, 343

Betz, A. L., & Boreiko, R. T. 1989, ApJ, 346, L101

Beuther, H., Linz, H., Bik, A., Goto, M., & Henning, T. 2010, A&A, 512, A29

Comito, C., & Schilke, P. 2002, A&A, 395, 357

de Graauw, Th., et al. 2010, A&A, 518, L6

Dickman, R. L. 1978, ApJS, 37, 407

Draine, B. T. 1995, Ap&SS, 233, 111

Franklin, J., Snell, R. L., Kaufman, M. J., et al. 2008, ApJ, 674, 1015

Genzel, R., & Stutzki, J. 1989, ARA&A, 27, 41

Grosjean, A., Dubernet, M.-L., & Ceccarelli, C. 2003, A&A, 408, 1197

Lerate, M. R., Barlow, M. J., Swinyard, B. M., et al. 2006, MNRAS, 370, 597

Müller, H. S. P., & Brünken, S. 2005, J. Mol. Spectrosc., 232, 213

Müller, H. S. P., Thorwirth, S., Roth, D. A., & Winnewisser, G. 2001, A&A, 370, L49

Müller, H. S. P., Schlöder, F., Stutzki, J., & Winnewisser, G. 2005, J. Mol. Struct, 742, 215

Neufeld, D. A., & Wolfire, M. G. 2009, ApJ, 706, 1594

Neufeld, D. A., Zmuidzinas, J., Schilke, P., & Phillips, T. G. 1997, ApJ, 488, L141

Neufeld, D. A., Wolfire, M. G., & Schilke, P. 2005, ApJ, 628, 260

Neufeld, D. A., et al. 2010, A&A, 518, L108

Nolt, I. G., Radostitz, J. V., Dilonardo, G., et al. 1987, J. Mol. Spectrosc., 125, 274

Olofsson, A. O. H., Olofsson, G., Hjalmarson, Å., et al. 2003, A&A, 402, L47

Ott, S. 2010, in Astronomical Data Analysis Software and Systems XIX, ed. Y. Mizumoto, K.-I. Morita, & M. Ohishi, ASP Conf. Ser., in press

Persson, C. M., Olofsson, A. O. H., Koning, N., et al. 2007, A&A, 476, 807

Pickett, H. M., Poynter, R. L., Cohen, E. A., et al. 1998, J. Quant. Spectrosc. Radiat. Transfer, 60, 883

Pilbratt, G. L., et al. 2010, A&A, 518, L1

Reese, C., Stoecklin, T., Voronin, A., & Rayez, J. C. 2005, A&A, 430, 1139

Scoville, N., Kleinmann, S. G., Hall, D. N. B., & Ridgway, S. T. 1983, ApJ, 275, 201

Snell, R. L., Howe, J. E., Ashby, M. L. N., et al. 2000, ApJ, 539, L93

Tauber, J. A., Goldsmith, P. F., & Dickman, R. L. 1991, ApJ, 375, 635

Ungerechts, H., Bergin, E. A., Goldsmith, P. F., et al. 1997, ApJ, 482, 245

Wright, C. M., van Dishoeck, E. F., Black, J. H., et al. 2000, A&A, 358, 689

Xu, L.-H., Fisher, J., Lees, R. M., et al. 2008, J. Mol. Spectrosc., 251, 305
1 California Institute of Technology, Cahill Center for Astronomy and Astrophysics 301-17, Pasadena, CA 91125, USA
e-mail: tgp@submm.caltech.edu

2 Department of Astronomy, University of Michigan, 500 Church Street, Ann Arbor, MI 48109, USA

3 Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218, USA

4 Centre d'étude Spatiale des Rayonnements, Université de Toulouse [UPS], 31062 Toulouse Cedex 9, France

5 CNRS/INSU, UMR 5187, 9 avenue du Colonel Roche, 31028 Toulouse Cedex 4, France

6 Laboratoire d'Astrophysique de l'Observatoire de Grenoble, BP 53, 38041 Grenoble, Cedex 9, France

7 Centro de Astrobiología (CSIC/INTA), Laboratorio de Astrofísica Molecular, Ctra. de Torrejón a Ajalvir, km 4 28850, Torrejón de Ardoz, Madrid, Spain

8 Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany

9 LERMA, CNRS UMR8112, Observatoire de Paris and École Normale Supérieure, 24 Rue Lhomond, 75231 Paris Cedex 05, France

10 LPMAA, UMR7092, Université Pierre et Marie Curie, Paris, France

11 LUTH, UMR8102, Observatoire de Paris, Meudon, France

12 I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany

13 Jet Propulsion Laboratory, Caltech, Pasadena, CA 91109, USA

14 Departments of Physics, Astronomy and Chemistry, Ohio State University, Columbus, OH 43210, USA

15 National Research Council Canada, Herzberg Institute of Astrophysics, 5071 West Saanich Road, Victoria, BC V9E 2E7, Canada

16 Infrared Processing and Analysis Center, California Institute of Technology, MS 100-22, Pasadena, CA 91125, USA

17 Canadian Institute for Theoretical Astrophysics, University of Toronto, 60 St George St, Toronto, ON M5S 3H8, Canada

18 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge MA 02138, USA

19 National University of Ireland Maynooth, Ireland

20 SRON Netherlands Institute for Space Research, PO Box 800, 9700 AV, Groningen, The Netherlands

21 Department of Physics and Astronomy, University of Calgary, 2500 University Drive NW, Calgary, AB T2N 1N4, Canada

22 MPI für Sonnensystemforschung, 37191 Katlenburg-Lindau, Germany