Nitrogen oxide in protostellar envelopes and shocks: the ASAI survey

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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 ²Π 1/2 J = 3/2 → 1/2 (at 151 GHz) and the ²Π 1/2 J = 5/2 → 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 (5 km s⁻¹) blue-shifted emission. For SVS13-A the analysis leads to T ex ≥ 4 K, N(NO) ≤ 3 × 10¹⁵ cm⁻², and indicates the association of NO with the protostellar envelope. In L1157-B1, NO is tracing the extended outflow cavity: T ex ≃ 4--5 K, and N(NO) = 5.5±1.5 × 10¹⁵ cm⁻². Using C¹⁸O, ¹³C¹⁸O, C¹⁷O, and ¹³C¹⁷O ASAI observations we derive an NO fractional abundance less than 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, 6 × 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 10⁵ cm⁻³ and a kinetic temperature of 40 K. The NO abundance in L1157-B1 is reproduced with pre-shock densities of 10⁵ cm⁻³ subjected to a 45 km s⁻¹ shock.