Evaluating the N/O chemical network:: The distribution of N2O and NO in the Sagittarius B2 complex

Mapping observations of the J = 6 --> 5 transition of N2O and the Pi (+)(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(tau) = 14(o) --> 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 --> 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 similar to 1.5 x 10(15) cm(-2) at this position and an abundance of f(N2O/H-2) similar to 1.5 x 10(-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 similar to (0.8-1.5) x 10(16) cm(-2), corresponding to a fractional abundance of f(NO/H-2) similar to (0.8-1.5) x 10(-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.