A multi-transition molecular line study of inward motions towards massive star-forming cores
Abstract
A multi-transition 3 mm molecular 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), 12CO(1-0) and optically thin lines C18O(1-0), 13CO(1-0) were observed towards each source. The normalized velocity differences (δV CS, δV HCO+) between the peak velocities of optically thick lines and optically thin line C18O(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.