A multi-transition molecular line study of inward motions towards massive star-forming cores

Amulti-transition 3mmmolecular line single-pointing and mapping survey was carried out towards 29 massive star-forming cores in order to search for the signature of inward motions. Up to seven different transitions, optically thick lines HCO(1-0), CS(2-1), HNC(1-0), HCN(1-0), CO(1-0) and optically thin lines CO(1-0), CO(1-0) were observed towards each source. The normalized velocity differences (δVCS, δVHCO+) between the peak velocities of optically thick lines and optically thin line CO(1-0) for each source were derived. Prominent inward motions are probably present in either HCO(1-0) or CS(2-1) or HNC(1-0) observations in most sources. Our observations show that there is a significant difference in the incidence of blue shifted line asymmetric line profiles between CS(2-1) and HCO(1-0). The HCO(1-0) shows the highest occurrence of obvious asymmetric feature, perhaps owing to different optical depth between CS(2-1) and HCO(1-0). HCO(1-0) appears to be the best inward motion tracer. The mapping observations of multiple line transitions enable us to identify six strong infall candidates G123.07-6.31,W75(OH), S235N, CEP-A,W3(OH), NGC7538. The infall signature is extended up to a linear scale > 0.2 pc.