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A&A
Volume 521, October 2010
Herschel/HIFI: first science highlights
Article Number L45
Number of page(s) 6
Section Letters
DOI https://doi.org/10.1051/0004-6361/201015105
Published online 01 October 2010
A&A 521, L45 (2010)

Herschel/HIFI: first science highlights

LETTER TO THE EDITOR

Nitrogen hydrides in interstellar gas

Herschel[*]/HIFI observations towards G10.6-0.4 (W31C)[*]

C. M. Persson1 - J. H. Black1 - J. Cernicharo2 - J. R. Goicoechea2 - G. E. Hassel3 - E. Herbst4 - M. Gerin5 - M. De Luca5 - T. A. Bell6 - A. Coutens7,8 - E. Falgarone5 - P. F. Goldsmith9 - H. Gupta9 - M. Kazmierczak10 - D. C. Lis6 - B. Mookerjea11 - D. A. Neufeld12 - J. Pearson9 - T. G. Phillips6 - P. Sonnentrucker12 - J. Stutzki13 - C. Vastel7,8 - S. Yu9 - F. Boulanger14 - E. Dartois14 - P. Encrenaz5 - T. R. Geballe15 - T. Giesen13 - B. Godard5 - C. Gry16 - P. Hennebelle5 - P. Hily-Blant17 - C. Joblin7 - R. Ko\los18 - J. Kre\lowski10 - J. Martín-Pintado2 - K. Menten20 - R. Monje6 - M. Perault5 - R. Plume21 - M. Salez5 - S. Schlemmer13 - M. Schmidt19 - D. Teyssier22 - I. Péron5,23 - P. Cais24 - P. Gaufre24 - A. Cros7,8 - L. Ravera7,8 - P. Morris25 - S. Lord25 - P. Planesas26

1 - Onsala Space Observatory, Chalmers University of Technology, 43992 Onsala, Sweden
2 - Centro de Astrobiologìa, CSIC-INTA, 28850 Madrid, Spain
3 - Depts. of Physics, Ohio State Univ., USA
4 - Depts. of Physics, Astronomy & Chemistry, Ohio State Univ., USA
5 - LERMA, CNRS, Observatoire de Paris and ENS, France
6 - California Institute of Technology, Cahill Center for Astronomy and Astrophysics 301-17, Pasadena, CA 91125, USA
7 - Centre d'Étude Spatiale des Rayonnements (CESR), Université de Toulouse [UPS], 31062 Toulouse Cedex 9, France
8 - CNRS/INSU, UMR 5187, 9 Avenue du Colonel Roche, 31028 Toulouse Cedex 4, France
9 - JPL, California Institute of Technology, Pasadena, USA
10 - Nicolaus Copernicus University, Torun, Poland
11 - Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai 400005, India
12 - The Johns Hopkins University, Baltimore, MD 21218, USA
13 - I. Physikalisches Institut, University of Cologne, Germany
14 - Institut d'Astrophysique Spatiale (IAS), Orsay, France
15 - Gemini telescope, Hilo, Hawaii, USA
16 - LAM, OAMP, Université Aix-Marseille & CNRS, Marseille, France
17 - Laboratoire d'Astrophysique de Grenoble, France
18 - Institute of Physical Chemistry, PAS, Warsaw, Poland
19 - Nicolaus Copernicus Astronomical Center, Torun, Poland
20 - MPI für Radioastronomie, Bonn, Germany
21 - Dept. of Physics & Astronomy, University of Calgary, Canada
22 - European Space Astronomy Centre, ESA, Madrid, Spain
23 - Institut de Radioastronomie Millimétrique, IRAM, 300 rue de la Piscine, 38406 St. Martin d'Hères, France
24 - Institute Université de Bordeaux, Laboratoire d'Astrophysique de Bordeaux, 33000 Bordeaux, France; and CNRS/INSU, UMR 5804, BP 89, 33271 Floirac Cedex, France
25 - Infrared Processing and Analysis Center, California Institute of Technology, MS 100-22, Pasadena, CA 91125, USA
26 - Observatorio Astronácional (IGN) and Atacama Large Millimeter/Submillimeter Array, Joint ALMA Office, Santiago, Chile

Received 31 May 2010 / Accepted 6 July 2010

Abstract
The HIFI instrument on board the Herschel Space Observatory has been used to observe interstellar nitrogen hydrides along the sight-line towards G10.6-0.4 in order to improve our understanding of the interstellar chemistry of nitrogen. We report observations of absorption in NH N =  $1\leftarrow0$, J = $2\leftarrow1$ and ortho-NH2  $1_{1,1} \gets 0_{0,0}$. We also observed ortho-NH3 $1_0 \gets 0_0$, and $2_0 \gets 1_0$, para-NH3 $2_1 \gets 1_1$, and searched unsuccessfully for NH+. All detections show emission and absorption associated directly with the hot-core source itself as well as absorption by foreground material over a wide range of velocities. All spectra show similar, non-saturated, absorption features, which we attribute to diffuse molecular gas. Total column densities over the velocity range 11-54 km s-1 are estimated. The similar profiles suggest fairly uniform abundances relative to hydrogen, approximately 6 $\times$ 10-9, 3 $\times$ 10-9, and 3 $\times$ 10-9 for NH, NH2, and NH3, respectively. These abundances are discussed with reference to models of gas-phase and surface chemistry.

Key words: astrochemistry - ISM: abundances - ISM: molecules - molecular processes - submillimetre: ISM - line: formation

1 Introduction

Molecular hydrides are important in the chemistry of the interstellar medium since they often appear in the first steps in chains of reactions that lead to other more complex species. The production pathways of nitrogen-bearing molecules are still rather uncertain since key species, such as NH+, NH, and NH2 have not been widely observed. Even the first identified polyatomic interstellar molecule, ammonia (NH3), has been widely observed mainly in its para symmetry form, which leaves its formation mechanism poorly constrained in the diffuse molecular gas. Both gas-phase chemistry and grain surface reactions have been proposed as formation mechanisms, but clearly more observations are needed for a better understanding.

Both NH and NH2 are well known in comets (e.g. Meier et al. 1998; Feldman et al. 1993; Swings et al. 1941), and have been observed in stellar photospheres (e.g. Farmer & Norton 1989; Schmitt 1969) via their electronic, vibration-rotation, and high rotational transitions. Interstellar NH was first detected in the interstellar medium by Meyer & Roth (1991) by optical absorption spectroscopy. Subsequent observations by Crawford & Williams (1997) and Weselak et al. (2009) have yielded six lines of sight where column densities of both NH and H2 are directly measured. The average value of the column density ratio in these diffuse and translucent sightlines is $N({\rm NH})/N({\rm H}_2)=3$ $\times$ 10-9. Interstellar NH2 was first observed by van Dishoeck et al. (1993) in absorption towards Sgr B2 in three fine-structure components of the para-NH2 11,0-10,1 transition with partially resolved hyperfine structure at frequencies 461 to 469 GHz. Further absorption lines of both ortho and para forms of NH2 and NH were observed through use of the long-wavelength spectrometer aboard ISO (Goicoechea et al. 2004; Polehampton et al. 2007; Cernicharo et al. 2000). The ISO observations were unable to resolve the hyperfine structure of either molecule. Ammonia, NH3, was the first polyatomic molecule to be identified in interstellar space (Cheung et al. 1968) by means of its microwave inversion transitions. Although NH3 has been widely observed in dark clouds and star-forming regions, there have been very few measurements of the inversion lines in the diffuse interstellar gas (Liszt et al. 2006; Nash 1990).

With the unique capabilities of Herschel Space Observatory (Pilbratt et al. 2010) transitions between 157 and 625 $\mu$m (0.48-1.9 THz) are available with the Heterodyne Instrument for the Far-Infrared (HIFI; de Graauw et al. 2010) with very high sensitivity. This allows searches for spectrally resolved, ground-state rotational transitions of NH+, NH, NH2, and NH3 with the same instrument. Abundances of these species are key diagnostics for the nitrogen chemistry.

The most sensitive and model-independent method for measuring column densities of interstellar molecules is high-resolution absorption line spectroscopy, which is being exploited by the Herschel PRISMAS key programme (PRobing InterStellar Molecules with Absorption line Studies). This paper presents the first observations and analysis of nitrogen hydrides along the sight-line towards the massive star-forming region G10.6-0.4 (W31C) as a part of the PRISMAS program. Observations of interstellar absorption towards seven additional submillimetre-wave continuum sources will be presented and analysed in later publications.

The ultra-compact HII region G10.6-0.4 in the star-forming W31 complex is an extremely luminous submillimetre and infrared continuum source. The source is located within the so-called 30 km s-1 arm at a kinematic distance of 4.8 kpc (Fish et al. 2003). The gas associated directly with G10.6-0.4 is detected in the velocity range $V_{\rm LSR}\approx -15$ to +5 km s-1, while the foreground gas along the line of sight is detected at $V_{\rm LSR}\approx 5$ to 55 km s-1. The focus of this paper is on the diffuse interstellar gas traced by the absorption lines.

2 Observations

The observations, which took place in March 2010, are summarised in Table 1. The 20-10 and 21-11 NH3 transitions were observed in the same band. We used the dual beam switch mode and the wide band spectrometer (WBS) with a bandwidth of 4 GHz and an effective spectral resolution of 1.1 MHz. The corresponding velocity resolution is about 0.3 km s-1 at the higher frequencies and 0.6 km s-1 at 572 GHz. In addition, simultaneous observations were performed using the high- resolution spectrometer (HRS) with an effective spectral resolution of 0.25 MHz ( $\Delta \upsilon$ $\sim $ 0.1 km s-1) and a bandwidth of 230 MHz. Three observations were carried out with different frequency settings of the local oscillator (LO) corresponding to a change of approximately 15 km s-1 to determine the sideband origin of the lines. Two orthogonal polarisations were used during all observations.

Table 1:   Observed transitions.

\begin{figure} \par\includegraphics[width=8.5cm,clip]{15105Fig1.eps} \end{figure} Figure 1:

Double sideband spectra of NH, o-NH2, o and p-NH3, and NH+ over the LSR velocity range -50 to 85  km s-1.

Open with DEXTER

The half-power beam width of the telescope is 37, 22, and 17 $^{\prime\prime}$ at 572, 953, and 1215 GHz, respectively. Pointing was centered at RA = 18 $\hbox{$.\!\!^{\rm h}$ }$10 $\hbox{$.\!\!^{\rm m}$ }$28 $\hbox{$.\!\!^{\rm s}$ }$70, Dec = -19 $\hbox{$.\!\!^\circ$ }$55 $\hbox{$.\mkern-4mu^\prime$ }$50 $\hbox{$^{\prime\prime}$ }$ (J2000). The reference beams were located 3$^\prime$ on either side of the source and the calibration is described by Roelfsema (2010).

We have used the standard Herschel pipeline to Level 2 which provides fully calibrated spectra. The data were first analysed using Herschel interactive processing environment (HIPE[*]; Ott 2010) version 2.4, and in parallel we also used the software package xs[*]. The data quality is excellent with very low intensity ripples, two polarisations that are in agreement to better than 10%, and very good agreement between the three LO-tunings without contamination from the image sideband. We thus average the three LO-tunings and both polarisations in both spectra, except for NH3 at 572 GHz and NH+ which display ripples in the V- and H-polarisation, respectively, and were therefore not included. The resulting rms noise is 11, 120, 50, 51, and 74 mK for NH3 at 572 and 1215 GHz, for NH, NH2, and NH+, respectively.

3 Results

Figure 1 shows the double sideband WBS spectra of all observed transitions. The frequency scale is converted as customary to Doppler velocities relative to the local standard of rest ( $V_{\rm LSR}$). All spectra, except NH+ which is not detected, show emission at negative velocities and a deep absorption at -1 km s-1 associated with G10.6-0.4 as measured by observations of OH masers. At higher velocities we see a number of absorption features which are also seen in atomic hydrogen (Fish et al. 2003), HCO+, HNC, HCN and CN (Godard et al. 2010). The only transition which does not show absorption components from the foreground material is ortho-NH3 20-10.

The double-sideband calibrated antenna temperatures shown in Fig. 1 have to be divided by two in order to get the correct continuum levels,  $T_{\rm A}{(\rm cont)}$, which are 0.6, 2.6. 2.6, 3.0 and 3.4 K for NH3 at 572 GHz, NH2, NH, NH+, and NH3 at 1215 GHz, respectively. The sideband gain ratio has been shown to be close to unity in previous PRISMAS observations (Gerin et al. 2010; Neufeld et al. 2010b), although departures at the 10% level have been seen at some frequencies (Neufeld et al. 2010a). We here adopt a sideband ratio of unity.

Figure 2 shows a comparison of NH, NH2, and para-NH3 where the intensities are normalised to the continuum in a single sideband as $T_{\rm A}$/$T_{\rm A}$(cont)-1 assuming a sideband gain ratio of unity. The NH and NH2 spectra are strikingly similar, despite their complicated hyperfine structures. The para-NH3 spectra also show the same absorption pattern, although not as strongly. Figure 3 shows a similar comparison of ortho- and para-NH3. Here, too, we see the same absorption pattern in both species, and strong emission and self-absorption in the ortho-line from the source itself. The strongest velocity components in the foreground material lie at $V_{\rm LSR}$ = 16, 22.5, 27.5, 30 and 40.5 km s-1.

The line opacities are calculated as $\tau$ = $-\ln$( $T_{\rm line}/T_{\rm A}{(\rm cont)}$ - 1). To be conservative and not include absorption from the source itself, we use the velocity interval 11-54 km s-1. The total integrated opacities are $\int \tau {\rm d}V$ = 20, 16, 9, and 3.4 km s-1 for NH, NH2, NH3 at 572 and 1215 GHz, respectively.

\begin{figure} \par\includegraphics[width=8cm,clip]{15105Fig2.eps} \end{figure} Figure 2:

Comparison of NH, o-NH2 and p-NH3 at 1215.2 GHz where the intensities have been normalised to single sideband continuum.

Open with DEXTER

\begin{figure} \par\includegraphics[width=8cm,clip]{15105Fig3.eps} \end{figure} Figure 3:

Comparison of o- and p-NH3 at 572.5 and 1215.2 GHz where the intensities have been normalised to single sideband continuum.

Open with DEXTER

Even though unsaturated absorption lines provide straightforward determinations of opacity and column densities, the numerous hyperfine-structure components and velocity features complicate the analysis of both NH and NH2. The spectroscopic properties of NH and NH2 have been studied extensively and references, rest frequencies and molecular properties for NH and NH2 are listed in the Cologne Database for Molecular Spectroscopy[*] (CDMS, Müller et al. 2005), and NH3 in Jet Propulsion Laboratory[*] (JPL, Pickett et al. 1998).

In order to estimate the total column density from the integrated opacity, it is necessary to correct for the population of molecules in unobserved excited levels and to quantify any possible effect of stimulated emission. We have done this through non-equilibrium excitation calculations of integrated opacity with an enhanced version of the RADEX code[*] (van der Tak et al. 2007). Where collision rates are unknown, we have made guesses scaled in proportion to radiative line strengths. We assume diffuse molecular cloud conditions with $T_{\rm k}$ = 30 K and n(H2) = 500 cm-3. The background radiation is a model of the average Galactic background radiation in the solar neighbourhood plus the cosmic microwave background radiation. The excitation of the hydride molecules in diffuse gas is dominated by the background continuum radiation and not by collisions. The resulting excitation temperatures of the observed submillimetre transitions are typically 4-5 K, which are small enough compared to $h\nu/k$ that no correction for emission is required. All the optical depths of the hyperfine components that contribute to the line are summed. We have treated ortho- and para-forms of molecules together in the expectation that their relative abundances are fixed by the chemical formation process. Plausible rates of formation and destruction are included explicitly in the excitation calculation. Thus we derive total column densities as summarised in Table 2. The analysis is complicated for NH3 by the existence of metastable excited states, especially in the para species. The entry for NH3 in Table 2 is the sum of the measured column density in JK =00, the measured column density in the lower inversion sublevel of 11, and calculated values in the upper inversion sublevel of 11 and in both sublevels of 22. All other states are expected to have negligible populations. The uncertainties of measurement and in the excitation analysis do not fully constrain the ortho/para ratio in NH3, but it appears that the effective formation temperature must be greater than 20 K, otherwise the strength of the 1215 GHz line cannot be explained.

Table 2:   Total column densities, N, and abundances with respect to hydrogen, X.

We note that the three neutral nitrogen hydrides have comparable abundances: NH is approximately twice as abundant as NH2 and NH3. To estimate the abundance with respect to hydrogen we use $N_{\rm H}$ = 2.7 $\times$ 1022 cm-2 as total column density of hydrogen in all forms (Neufeld et al. 2010b). This value is consistent with the estimated atomic hydrogen column density in this line-of-sight, N(H) = 1.2 $\times$ 1022 cm-2 (Godard et al. 2010) inferred from absorption profiles observed with the VLA (Fish et al. 2003), and implies that the absorbing material is mostly molecular even if there most likely are large variations between the individual velocity components. Our estimate of the abundance of NH relative to total hydrogen is somewhat higher than the NH/H2 ratio of 3 $\times$ 10-9 for six lines of sight where both are observed directly. The resulting abundances are found in Table 2. The non-detection of NH+ is used to put an upper limit on its column density and abundance (3$\sigma$). A previous limit on NH+ towards $\zeta $ Oph from ultraviolet observations, $N({\rm NH}^+)/N_{\rm H}<1$ $\times$ 10-6, is orders of magnitude less sensitive than our limit (de Almeida & Singh 1982; Snow 1979).

4 Discussion

The production of NH and NH2 by purely gas-phase processes is problematic since their formation is inhibited by a lack of a sufficient source of N+. The atomic ion can be formed by cosmic ray ionization of N or by reaction of He+ with N2 or CN, which were formed by neutral-neutral reactions. The standard gas-phase ion-molecule chemistry of nitrogen hydrides is then initiated by the reaction N++H2  $\rightarrow$ NH+ + H, because H3+ does not react rapidly with N. Typical steady state dark cloud models (n = 1 $\times$ 103-5 $\times$ 104 cm-3, T = 10-40 K, $A_{\rm V} \ga 10$), predict an NH2 abundance of (1-10) $\times$ 10-8, an NH abundance 10 times lower, and an NH2/NH3 ratio of 0.3-1.5 for a wide range of assumptions (e.g. Millar et al. 1991; Langer & Graedel 1989), but these are not directly applicable to diffuse molecular gas.

Previous observations of NH in diffuse clouds were taken to imply that NH is produced on grain surfaces (Mann & Williams 1984; Wagenblast et al. 1993; O'Neill et al. 2002). Such models, however, often predict up to 1000 times more NH3 than NH2 (Hasegawa & Herbst 1993). An additional source of NH and NH2 is fragmentation of ammonia by photodissociation in diffuse clouds. Models of ultraviolet illuminated photon-dominated regions predict NH2/NH3 < 1 and NH3/NH < 1 in regions with extinction $A_{\rm v}$ $\la$  $5~{\rm mag}$ (e.g. Sternberg & Dalgarno 1995). The abundance patterns that we see in diffuse molecular gas are clearly different from those in Sgr B2, where NH3/NH2/NH $\sim $ 100/10/1 and the fractional abundance of NH is a few times $\times$10-9 (Goicoechea et al. 2004; Cernicharo et al. 2000).

We have used two different approaches to chemical models in order to compare the nitrogen-hydride abundances with theory. First, we constructed a pseudo-time-dependent model with constant physical conditions including both gas-phase and surface chemistry (Hasegawa et al. 1992) with $A_{\rm V}$ = 2 and 3, $n_{\rm H}$ = 500 and 5000 cm-3, $T_{\rm gas}$ = 30 and 50 K, and $T_{\rm dust}$ = 10 K. The ultraviolet field is 1 G0, and the cosmic ray ionization rate is $\zeta $ = 1.3 $\times$ 10-17 s-1. The resulting abundances are found in Fig. A.1. Gas-phase chemistry alone does not match the observed abundances, failing by factors of 10 to 100 for NH and NH2. Inclusion of processes on dust surfaces increases the abundances of the three neutral hydrides to 10-8-10-9, but fails to match the high NH/NH3 ratio. If the dominant source of NH3 is association on surfaces and if the ortho/para ratio is fixed at a surface temperature below 20 K, then the 1215 GHz para line should be much weaker relative to the ortho 572 GHz line than is observed. The observed line ratio is perhaps somewhat better explained by a high formation temperature, which points to exoergic gas-phase formation of NH3.

A second approach is to construct models of weak PDRs through use of the Meudon PDR code (Goicoechea & Le Bourlot 2007; Le Petit et al. 2006), which is a steady-state, depth-dependent model with pure gas-phase chemistry. We use parameters appropriate for diffuse clouds, UV = 0.5-5 G0, $n_{\rm H}$ = 100-1000 cm-3, $\zeta $ = 10-17-10-15 s-1, $A_{\rm V}$ < 5 and illuminate one side only. The results of the best fitting model are found in Fig. A.2. Although the resulting relative abundances of nitrogen-hydrides approach the observed ratios inside the cloud, the abundances relative to hydrogen are still too low.

5 Conclusions

Our first detections of spectrally resolved rotational transitions of nitrogen hydrides in the interstellar medium show remarkable similarities of line profiles and abundances towards one background source. The formation mechanisms are, however, difficult to explain. Neither standard gas-phase nor grain-surface chemistry can fully explain our observations, which may suggest that important reactions have been overlooked in the chemistry of interstellar nitrogen. Further analysis of the line profiles will yield abundance ratios in separate velocity components and will allow the nitrogen species to be compared directly with tracers of molecular hydrogen like CH and HF. Our sample will also be enlarged with seven additional sight-lines in the PRISMAS project. The chemical models will be refined and the possible role of turbulent dissipation regions will be assessed.

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 Astronmico Nacional (IGN), Centro de Astrobiologa (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. C.P. and J.H.B. acknowledge generous support from the Swedish National Space Board. J.C. and J.R.G. thanks spanish MICINN for funding support under projects AYA2009-07304 and CSD2009-00038 M.S. acknowledges support from grant N 203 393334 from Polish MNiSW.

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Online Material

Appendix A: Figures

\begin{figure} \par\includegraphics[angle=90,width=13cm,clip]{15105Fig4.eps} \end{figure} Figure A.1:

A pseudo-time-dependent model with constant physical conditions taking into account both gas-phase and grain surface chemistry. For all lines: $T_{\rm dust}$ = 10 K and the solid/dashed lines show $T_{\rm gas}$ = 30 and 50 K, respectively. The black thin lines show the observed abundances, and the dashed black line the NH+ upper limit. Black line: gas phase & grain surface chemistry using $A_{\rm V}$ = 3, $n_{\rm H}$ = 5000  cm-3. Red line: gas phase & grain surface chemistry using $A_{\rm V}$ = 2, $n_{\rm H}$ = 500  cm-3. Grey line: only gas phase chemistry using $A_{\rm V}$ = 3, $n_{\rm H}$ = 5000  cm-3. Blue line: only gas phase chemistry using $A_{\rm V}$ = 2, $n_{\rm H}$ = 500  cm-3.

Open with DEXTER

\begin{figure} \includegraphics[angle=-90,width=12.5cm,clip]{15105Fig5.eps} \end{figure} Figure A.2:

The cloud depth-dependent Meudon PDR model for pure gas-phase chemistry using $n_{\rm H}$ = 1000  cm-3, $\zeta $ = 10-17-10-15 s-1, and a UV-field of 10 times the interstellar radiation field in Draine units ($\sim $5 G0). The left panel shows the relative abundances of NH/NH2 and NH2/NH3, and the right panel show the absolute abundances.

Open with DEXTER

Footnotes

...Herschel[*]
Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.
... (W31C)[*]
Figures A.1 and A.2 (page 6) are only available in electronic form at http://www.aanda.org
...(HIPE[*]
HIPE is a joint development by the Herschel Science Ground Segment Consortium, consisting of ESA, the NASA Herschel Science Center, and the HIFI, PACS and SPIRE consortia.
... package xs[*]
Developed by Per Bergman at Onsala Space Observatory, Sweden; http://www.chalmers.se/rss/oso-en/observations/
... Spectroscopy[*]
http://www.cdms.de
... Laboratory[*]
http://spec.jpl.nasa.gov
... code[*]
The published version will soon include these enhancements, see http://www.sron.rug.nl/ vdtak/radex/index.shtml

All Tables

Table 1:   Observed transitions.

Table 2:   Total column densities, N, and abundances with respect to hydrogen, X.

All Figures

  \begin{figure} \par\includegraphics[width=8.5cm,clip]{15105Fig1.eps} \end{figure} Figure 1:

Double sideband spectra of NH, o-NH2, o and p-NH3, and NH+ over the LSR velocity range -50 to 85  km s-1.

Open with DEXTER
In the text

  \begin{figure} \par\includegraphics[width=8cm,clip]{15105Fig2.eps} \end{figure} Figure 2:

Comparison of NH, o-NH2 and p-NH3 at 1215.2 GHz where the intensities have been normalised to single sideband continuum.

Open with DEXTER
In the text

  \begin{figure} \par\includegraphics[width=8cm,clip]{15105Fig3.eps} \end{figure} Figure 3:

Comparison of o- and p-NH3 at 572.5 and 1215.2 GHz where the intensities have been normalised to single sideband continuum.

Open with DEXTER
In the text

  \begin{figure} \par\includegraphics[angle=90,width=13cm,clip]{15105Fig4.eps} \end{figure} Figure A.1:

A pseudo-time-dependent model with constant physical conditions taking into account both gas-phase and grain surface chemistry. For all lines: $T_{\rm dust}$ = 10 K and the solid/dashed lines show $T_{\rm gas}$ = 30 and 50 K, respectively. The black thin lines show the observed abundances, and the dashed black line the NH+ upper limit. Black line: gas phase & grain surface chemistry using $A_{\rm V}$ = 3, $n_{\rm H}$ = 5000  cm-3. Red line: gas phase & grain surface chemistry using $A_{\rm V}$ = 2, $n_{\rm H}$ = 500  cm-3. Grey line: only gas phase chemistry using $A_{\rm V}$ = 3, $n_{\rm H}$ = 5000  cm-3. Blue line: only gas phase chemistry using $A_{\rm V}$ = 2, $n_{\rm H}$ = 500  cm-3.

Open with DEXTER
In the text

  \begin{figure} \includegraphics[angle=-90,width=12.5cm,clip]{15105Fig5.eps} \end{figure} Figure A.2:

The cloud depth-dependent Meudon PDR model for pure gas-phase chemistry using $n_{\rm H}$ = 1000  cm-3, $\zeta $ = 10-17-10-15 s-1, and a UV-field of 10 times the interstellar radiation field in Draine units ($\sim $5 G0). The left panel shows the relative abundances of NH/NH2 and NH2/NH3, and the right panel show the absolute abundances.

Open with DEXTER
In the text

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