New detections of interstellar HNO at 2 and 1.2 millimeters: More N-O bonds

The 2₀₂-1₀₁ and 3₀₃-2₀₂ transitions of HNO at 163 and 244 GHz have been detected toward several molecular clouds. In conjunction with previous measurements of the 1₀₁-0₀₀ line at 81 GHz, these observations secure the identification of interstellar HNO. The 2₀₂-1₀₁ transition of this species has been detected toward NGC 2024, Sgr B2 (NW), W51M, and DR 21 (OH), while the 3₀₃-2₀₂ line at 1.2 mm was observed only in NGC 2024. Typical column densities of HNO observed toward these sources are N_tot ∼ 10¹²-10¹⁴ cm^-2, corresponding to fractional abundances of f_HNO ∼ 10^-11 to 10^-10. These values imply NO/HNO ratios of ∼ 100-800. Both the HNO fractional abundances and NO/HNO ratios are in reasonable agreement with predictions of ion-molecule-type models of interstellar chemistry, provided early-time calculations are used. Such behavior contrasts with that of other nitrogen compounds, whose observed abundances are best duplicated by steady-state calculations. The chemistries of HNO and NO are thus likely to be related. These observations also suggest that interstellar N-O bonds may be more common than previously thought.