2001ApJ...561..244H

Mapping observations of the J=6⟶5 transition of N₂O and the Π⁺_1/2, J=3/2⟶1/2 line of NO in the 2 mm band toward the core region of the Sagittarius B2 complex have been carried out using the Kitt Peak 12 m telescope. Emission from NO was found to be extended over a region 2'x5' in size that includes the Sgr B2 (N), Sgr B2 (M), and Sgr B2 (OH) positions, very similar to the distribution found for HNO. In contrast, N₂O emission was confined to a source approximately 1' in extent, slightly elongated in the north-south direction and centered on the Sgr B2 (N) core. A virtually identical distribution was found for the J_Kτ=14₀⟶14_–1 E transition of methanol, which lies 255 K above ground state and samples very hot gas. Excitation conditions are favorable for the J=6⟶5 line of N₂O over the entire NO region; hence, the confined nature of this species is a result of chemistry. The J=3⟶2 and J=9⟶8 lines of N₂O at 75 and 226 GHz, respectively, were also detected at Sgr B2 (N). Combined with the J=6⟶5 data, these transitions indicate a column density for this molecule of N_tot∼1.5x10¹⁵ cm^–2 at this position and an abundance of f(N₂O/H₂)∼1.5x10^–9. This fractional abundance is almost 2 orders of magnitude higher than predicted by low-temperature chemical models. The N₂O observations suggest that this molecule is preferentially formed in high-temperature gas; a likely mechanism is the neutral-neutral reaction NO+NH⟶N₂O+H, which has an appreciable rate only at T>125 K. The column density of NO found over the Sgr B2 cloud was N_tot~(0.8-1.5)x10¹⁶ cm^–2, corresponding to a fractional abundance of f(NO/H₂)~(0.8-1.5)x10^–8, which is about 1 order of magnitude less than model predictions. The similar distributions of NO and HNO suggest a chemical connection. It is likely that the major route to HNO is from NO via the ion-molecule process NO+HNO⁺⟶NO⁺+HNO, which occurs readily at low temperatures. The NO molecule thus appears to be the main precursor species in the N/O chemical network.