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
e-mail: carina.persson@chalmers.se
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

Abstract

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-NH₂N_Ka,KcJ = 1_1,1 3/2–0_0,0 1/2 (953 GHz) and para-NH₂ 2_1,2 5/2–1_0,1 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.