Infall motions in massive star-forming regions: results from years 1 and 2 of the MALT90 survey

Abstract

Massive star-forming regions with observed infall motions are good sites for\nstudying the birth of massive stars. In this paper, 405 compact sources have\nbeen extracted from the APEX Telescope Large Area Survey of the Galaxy\n(ATLASGAL) compact sources that also have been observed in the Millimetre\nAstronomy Legacy Team 90 GHz (MALT90) survey during Years 1 and 2. These\nobservations are complemented with Spitzer GLIMPSE/MIPSGAL mid-IR survey data\nto help classify the elected star-forming clumps into three evolutionary\nstages: pre-stellar, proto-stellar and UCHII regions. The results suggest that\n0.05 g cm$^{-2}$ is a reliable empirical lower bound for the clump surface\ndensities required for massive-star formation to occur. The optically thick\nHCO$^{+}$(1-0) and HNC(1-0) lines, as well as the optically thin\nN$_{2}$H$^{+}$(1-0) line were used to search for infall motions toward these\nsources. By analyzing the asymmetries of the optically thick HCO$^{+}$(1-0) and\nHNC(1-0) lines and the mapping observations of HCO$^{+}$(1-0), a total of 131\nreliable infall candidates have been identified. The HCO$^{+}$(1-0) line shows\nthe highest occurrence of obvious asymmetric features, suggesting that it may\nbe a better infall motion tracer than other lines such as HNC(1-0). The\ndetection rates of infall candidates toward pre-stellar, proto-stellar and\nUCHII clumps are 0.3452, 0.3861 and 0.2152, respectively. The relatively high\ndetection rate of infall candidates toward UCHII clumps indicates that many\nUCHII regions are still accreting matter. The peak column densities and masses\nof the infall candidates, in general, display a increasing trend with\nprogressing evolutionary stages. However, the rough estimates of the mass\ninfall rate show no obvious variation with evolutionary stage.\n