Nitric oxide in star-forming regions: Further evidence for interstellar N-O bonds

Nitric oxide has been newly detected towards several star-forming clouds, including Orion-KL, Sgr B2(N), W33A, W51M, and DR21(OH) via its J = 3/2 → 1/2 transitions near 150 GHz, using the FCRAO 14 m telescope. Both λ-doubling components of NO were observed towards all sources. Column densities derived for nitric oxide in these clouds are N ∼ 10¹⁵-10¹⁶ cm^-2, corresponding to fractional abundances of f∼ 0.5-1.0 × 10^-8, relative to H₂. Towards Orion-KL, the NO line profile suggests that the species arises primarily from hot, dense gas. Nitric oxide may arise from warm material toward the other clouds as well. Nitric oxide in star-forming regions could be synthesized by high-temperature reactions, although the observed abundances do not disagree with values predicted from low-temperature, ion-molecule chemistry by more than one order of magnitude. The abundance of NO, unlike other simple interstellar nitrogen compounds, does appear to be reproduced by chemical models, at least to a good approximation. Regardless of the nature of formation of NO, it appears to be a common constituent of warm, dense molecular clouds. N-O bonds may therefore be more prevalent than previously thought.