Volume 586, February 2016
|Number of page(s)||20|
|Section||Interstellar and circumstellar matter|
|Published online||05 February 2016|
Ortho-to-para ratio of NH2
Herschel-HIFI observations of ortho- and para-NH2 rotational transitions towards W31C, W49N, W51, and G34.3+0.1⋆
1 Chalmers University of Technology, Department of Earth and Space Sciences, Onsala Space Observatory, 439 92 Onsala, Sweden
2 Department of Chemistry, University of Virginia, McCormick Road, Charlottesville, VA 22904, USA
3 Department of Physics & Astronomy, Siena College, Loudonville, NY 12211, USA
4 Université Grenoble Alpes and CNRS, IPAG, 38000 Grenoble, France
5 LERMA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, UPMC Univ. Paris 06, École normale supérieure, 75005 Paris, France
6 LERMA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, UPMC Univ. Paris 06, 75014 Paris, France
7 Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
Received: 18 June 2015
Accepted: 30 November 2015
We have used the Herschel-HIFI instrument to observe the two nuclear spin symmetries of amidogen (NH2) towards the high-mass star-forming regions W31C (G10.6−0.4), W49N (G43.2−0.1), W51 (G49.5−0.4), and G34.3+0.1. The aim is to investigate the ratio of nuclear spin types, the ortho-to-para ratio (OPR) of NH2 in the translucent interstellar gas, where it is traced by the line-of-sight absorption, and in the envelopes that surround the hot cores. The HIFI instrument allows spectrally resolved observations of NH2 that show a complicated pattern of hyperfine structure components in all its rotational transitions. The excited NH2 transitions were used to construct radiative transfer models of the hot cores and surrounding envelopes to investigate the excitation and possible emission of the ground-state rotational transitions of ortho-NH2NKa,KcJ = 11,1 3/2–00,0 1/2 (953 GHz) and para-NH2 21,2 5/2–10,1 3/2 (1444 GHz) used in the OPR calculations. Our best estimate of the average OPR in the envelopes lie above the high-temperature limit of three for W49N, specifically 3.5 with formal errors of ±0.1, but for W31C, W51, and G34.3+0.1 we find lower values of 2.5 ± 0.1, 2.7 ± 0.1, and 2.3 ± 0.1, respectively. Values this low are strictly forbidden in thermodynamical equilibrium since the OPR is expected to increase above three at low temperatures. In the translucent interstellar gas towards W31C, where the excitation effects are low, we find similar values between 2.2 ± 0.2 and 2.9 ± 0.2. In contrast, we find an OPR of 3.4 ± 0.1 in the dense and cold filament connected to W51 and also two lower limits of ≳4.2 and ≳5.0 in two other translucent gas components towards W31C and W49N. At low temperatures (T ≲ 50 K) the OPR of H2 is <10-1, far lower than the terrestrial laboratory normal value of three. In this para-enriched H2 gas, our astrochemical models can reproduce the variations of the observed OPR, both below and above the thermodynamical equilibrium value, by considering nuclear-spin gas-phase chemistry. The models suggest that values below three arise in regions with temperatures ≳20−25 K, depending on time, and values above three at lower temperatures.
Key words: ISM: abundances / ISM: molecules / astrochemistry / line: formation / molecular processes / submillimeter: ISM
© ESO, 2016
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