Imaging Molecular Outflow in Massive Star-forming Regions with HNCO Lines

Protostellar outflows are considered a signpost of star formation. These outflows can cause shocks in the molecular gas and are typically traced by the line wings of certain molecules. HNCO (4-3) has been regarded as a shock tracer because of the high abundance in shocked regions. Here we present the first imaging results of HNCO (4-3) line wings toward nine sources in a sample of 23 massive star-forming regions using the Instituto de Radioastronomía Milimétrica 30 m Telescope. We adopt the velocity range of the full width of HC₃N (10-9) and H¹³CO⁺ (1-0) emissions as the central emission values, beyond which the emission from HNCO (4-3) is considered to be from line wings. The spatial distributions of the red and/or blue lobes of HNCO (4-3) emission nicely associate with those lobes of HCO⁺ (1-0) in most of the sources. High-intensity ratios of HNCO (4-3) to HCO⁺ (1-0) are obtained in the line wings. The derived column density ratios of HNCO to HCO⁺ are consistent with those previously observed toward massive star-forming regions. These results provide direct evidence that HNCO could trace outflow in massive star-forming regions. This work also implies that the formation of some HNCO molecules is related to shock, either on the grain surface or within the shocked gas.