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Although ammonia is an abundant molecule commonly observed towards the dense interstellar medium, it has not yet been established whether its main formation route is from gas-phase ion-molecule reactions or grain-surface hydrogen additions on adsorbed nitrogen atoms. Deuterium fractionation can be used as a tool to constrain formation mechanisms. High abundances of deuterated molecules are routinely observed in the dense interstellar medium, with the ratio between deuterated molecules and the main isotopologue enhanced by several orders of magnitude with respect to the elemental D/H ratio. In the case of ammonia, the detection of its triply deuterated isotopologue hints at high abundances of the deuterated intermediate nitrogen radicals, ND, NHD and ND$_2$. So far however, only ND has been detected in the interstellar medium. In this paper, to constrain the formation of ammonia, we aim at determining the NHD/NH$_2$ and ND$_2$/NHD abundance ratios, and compare them with the predictions of both pure gas-phase and grain-surface chemical models. We searched for the fundamental rotational transitions of NHD and ND$_2$ towards the class 0 protostar IRAS16293-2422, towards which NH, NH$_2$ and ND had been previously detected. Both NHD and ND$_2$ are detected in absorption towards the source. The relative abundance ratios NH$_2$ : NHD : ND$_2$ are close to 8 : 4 : 1. These ratios can be reproduced by our gas-phase chemical model within a factor of two-three. Statistical ratios as expected from grain-surface chemistry are also consistent with our data. Further investigations of the ortho-to-para ratio in ND$_2$ , both theoretical and observational, could bring new constraints to better understand nitrogen hydride chemistry., Comment: 10 pages, 5 figures, accepted for publication in MNRAS

We present observations of the 3P1-3P0 fine structure transition of atomic carbon [CI], the J=3-2 transition of CO, as well as of the J=1-0 transitions of 13CO and C18O toward DR15, an HII region associated with two mid-infrared dark clouds (IRDCs). The 13CO and C18O J=1-0 emissions closely follow the dark patches seen in optical wavelength, showing two self-gravitating molecular cores with masses of 2000 Msun and 900 Msun, respectively, at the positions of the catalogued IRDCs. Our data show a rough spatial correlation between [CI] and 13CO J=1-0. Bright [CI] emission occurs in relatively cold gas behind the molecular cores, neither in highly excited gas traced by CO J=3-2 emission nor in HII region/molecular cloud interface. These results are inconsistent with those predicted by standard photodissociation region (PDR) models, suggesting an origin for interstellar atomic carbon unrelated to photodissociation processes., Comment: 11 pages Latex, 6 figures, Accepted for publication in The Astrophysical Journal

We present observations of the 3P1-3P0 fine structure transition of atomic carbon [CI], the J=3-2 transition of CO, as well as of the J=1-0 transitions of 13CO and C18O toward DR15, an HII region associated with two mid-infrared dark clouds (IRDCs). The 13CO and C18O J=1-0 emissions closely follow the dark patches seen in optical wavelength, showing two self-gravitating molecular cores with masses of 2000 Msun and 900 Msun, respectively, at the positions of the catalogued IRDCs. Our data show a rough spatial correlation between [CI] and 13CO J=1-0. Bright [CI] emission occurs in relatively cold gas behind the molecular cores, neither in highly excited gas traced by CO J=3-2 emission nor in HII region/molecular cloud interface. These results are inconsistent with those predicted by standard photodissociation region (PDR) models, suggesting an origin for interstellar atomic carbon unrelated to photodissociation processes., Comment: 11 pages Latex, 6 figures, Accepted for publication in The Astrophysical Journal

The pure rotational transitions of the protonated hydrogen cyanide ion, HCNH+, and its isotopic species, HCND+ and DCND+, were measured in the 107 - 482 GHz region with a source modulated microwave spectrometer. The ions were generated in the cell with a magnetically confined dc-glow discharge of HCN and/or DCN. The rotational constant B0 and the centrifugal distortion constant D0 for each ion were precisely determined by a least-squares fitting to the observed spectral lines. The observed rotational transition frequencies by laboratory spectroscopy and the predicted ones are accurate in about 30 to 40 kHz and are useful as rest frequencies for astronomical searches of HCNH+ and HCND+., Comment: 14 pages in TeX, 1 figures in JPEG

The pure rotational transitions of the protonated hydrogen cyanide ion, HCNH+, and its isotopic species, HCND+ and DCND+, were measured in the 107 - 482 GHz region with a source modulated microwave spectrometer. The ions were generated in the cell with a magnetically confined dc-glow discharge of HCN and/or DCN. The rotational constant B0 and the centrifugal distortion constant D0 for each ion were precisely determined by a least-squares fitting to the observed spectral lines. The observed rotational transition frequencies by laboratory spectroscopy and the predicted ones are accurate in about 30 to 40 kHz and are useful as rest frequencies for astronomical searches of HCNH+ and HCND+., Comment: 14 pages in TeX, 1 figures in JPEG