Nitrogen oxide in protostellar envelopes and shocks: the ASAI survey

TL;DR

This study uses the IRAM 30-m ASAI spectral survey to detect and analyze nitrogen oxide (NO) in protostellar environments and shocks, revealing different NO profiles and abundances that inform on physical conditions and shock chemistry.

Contribution

First detection of NO emission in both protostellar envelope and shock regions using the IRAM 30-m ASAI survey, with detailed analysis of physical conditions and shock effects on NO abundance.

Findings

NO detected in protostellar envelope and shock regions.

NO abundance is enhanced in shocks, reaching ~6 x 10^{-6}.

Physical conditions consistent with shock models and envelope chemistry.

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 $^2\Pi_{\rm 1/2}$ $J$ = 3/2 $\to$ 1/2 (at 151 GHz) and the $^2\Pi_{\rm 1/2}$ $J$ = 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 $T_{\rm ex}$ $\geq$ 4 K, $N(\rm NO)$ $\leq$ 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: $T_{\rm ex}$ $\simeq$ 4--5 K, and $N(\rm NO)$ = 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.