Nitrogen oxide in protostellar envelopes and shocks: the ASAI survey

Abstract

The high-sensitivity of the IRAM 30-m ASAI unbiased spectral survey in the mm-window allows us to detect NO emission towards both the Class I object SVS13-A and the protostellar outflow shock L1157-B1. We detect the hyperfine components of the ${}^2{\hat{I}}_{1/2}$ $J$ = 3/2 $\to$ 1/2 (at 151 GHz) and the ${}^2{\hat{I}}_{1/2}$ $J$ = 5/2 $\to$ 3/2 (250 GHz) spectral pattern. The two objects show different NO profiles: (i) SVS13-A emits through narrow (1.5 km s${}^{-1}$) lines at the systemic velocity, while (ii) L1157-B1 shows broad ($\sim$ 5 km s${}^{-1}$) blue-shifted emission. For SVS13-A the analysis leads to $T_{\mathrm{ex}}$ $\ge$ 4 K, $N(\mathrm{NO})$ $\le$ 3 $\times$ 10${}^{15}$ cm${}^{-2}$, and indicates the association of NO with the protostellar envelope. In L1157-B1, NO is tracing the extended outflow cavity: $T_{\mathrm{ex}}$ $\simeq$ 4--5 K, and $N(\mathrm{NO})$ = 5.5$\pm$1.5 $\times$ 10${}^{15}$ cm${}^{-2}$. Using C${}^{18}$O, ${}^{13}$C${}^{18}$O, C${}^{17}$O, and ${}^{13}$C${}^{17}$O ASAI observations we derive an NO fractional abundance less than $\sim$ 10${}^{-7}$ for the SVS13-A envelope, in agreement with previous measurements towards extended PDRs and prestellar objects. Conversely, a definite $X(NO)$ enhancement is measured towards L1157-B1, $\sim$ 6 $\times$ 10${}^{-6}$, showing that the NO production increases in shocks. The public code UCLCHEM was used to interpret the NO observations, confirming that the abundance observed in SVS13-A can be attained in an envelope with a gas density of 10${}^5$ cm${}^{-3}$ and a kinetic temperature of 40 K. The NO abundance in L1157-B1 is reproduced with pre-shock densities of 10${}^5$ cm${}^{-3}$ subjected to a $\sim$ 45 km s${}^{-1}$ shock.

MNRAS, in press