Nitrogen oxide in protostellar envelopes and shocks: the ASAI survey

Abstract

The high sensitivity of the IRAM 30-m Astrochemical Surveys At IRAM (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 <SUP>2</SUP>Π<SUB>1/2</SUB>J = 3/2 → 1/2 (at 151 GHz) and the <SUP>2</SUP>Π<SUB>1/2</SUB>J = 5/2 → 3/2 (at 250 GHz) spectral pattern. The two objects show different NO profiles: (i) SVS13-A emits through narrow (1.5 km s<SUP>-1</SUP>) lines at the systemic velocity, while (ii) L1157-B1 shows broad (∼5 km s<SUP>-1</SUP>) blueshifted emission. For SVS13-A, the analysis leads to T<SUB>ex</SUB> ≥ 4 K, N(NO) ≤ 3 × 10<SUP>15</SUP> cm<SUP>-2</SUP>, and indicates the association of NO with the protostellar envelope. In L1157-B1, NO is tracing the extended outflow cavity: T<SUB>ex</SUB> ≃ 4-5 K, and N(NO) = 5.5 ± 1.5 × 10<SUP>15</SUP> cm<SUP>-2</SUP>. Using C<SUP>18</SUP>O, <SUP>13</SUP>C<SUP>18</SUP>O, C<SUP>17</SUP>O, and <SUP>13</SUP>C<SUP>17</SUP>O ASAI observations, we derive an NO fractional abundance less than ∼10<SUP>-7</SUP> for the SVS13-A envelope, in agreement with previous measurements towards extended photodissociation regions (PDRs) and prestellar objects. Conversely, a definite X(NO) enhancement is measured towards L1157-B1, ∼6 × 10<SUP>-6</SUP>, 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<SUP>5</SUP> cm<SUP>-3</SUP> and a kinetic temperature of 40 K. The NO abundance in L1157-B1 is reproduced with pre-shock densities of 10<SUP>5</SUP> cm<SUP>-3</SUP> subjected to a ∼45 km s<SUP>-1</SUP> shock.