NO molecule in massive star forming regions
Context. Among diatomic molecules composed of the abundant elements C, N and O, NO has been detected far less than the well studied CN and CO, making it a crucial yet under-observed component in nitrogen-containing chemical networks. NO was thought to serve as a potential tracer of shocks, as evidenced with orders abundance enhancements reported in literature. Aims. Large-sample observations for NO molecule in widespread interstellar environments are needed to confirm if the enhancement of NO is due to shock chemistry or not. Methods. Single-point survey for NO lines around 150 GHz was carried out by Arizona Radio Observatory 12-meter telescope towards a sample of 36 massive star forming regions containing SiO emission, which include three evolutionary stages: 4 IRDCs, 6 protostars and 26 H II regions. Results. The NO emission was detected in 28 sources with a detection rate of 78%. Beam-averaged NO column densities and abundances relative to H2 were derived from integrated intensities of two main hyperfine lines. Correlations between NO and SiO in integrated intensity and relative abundance are similar to the corresponding correlations of c-C3H2, indicating that NO enrichment may not significantly involve pronounced shock activities, which coincides with the trend in line widths: NO is close to c-C3H2, both smaller than H2CO and far smaller than SiO. Conclusions. Observational evidence does not strongly support significant NO enhancement by shock chemistry in the observed sources, indicating that the formation of NO does not necessarily require shocks.