New Detections of Interstellar HNO at 2 and 1.2 Millimeters: More N-O Bonds

The 2₀₂ to 1₀₁ and 3₀₃ to 2₀₂ transitions of HNO at 163 and 244 GHz have been detected toward several molecular clouds. In conjunction with previous measurements of the 1₀₁ to 0₀₀ line at 81 GHz, these observations secure the identification of interstellar HNO. The 2₀₂ to 1₀₁ transition of this species has been detected toward NGC 2024, Sgr B2 (NW), W51M, and Dr 21 (OH), while the 3₀₃ to 2₀₂ line at 1.2 mm was observed only in NGC 2024. Typical column densities of HNO observed toward these sources are N_tot approximately 10¹² to 10¹⁴/sq cm, corresponding to fractional abundances of f_HNO approximately 10^-11 to 10^-10. These values imply NO/HNO ratios of approximately 100 to 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.