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

Abstract

A multitransition 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) and 12 CO(1-0), and optically thin lines C 1S O(1-0) and 13 CO(1-0) were observed towards each source. The normalized velocity differences (δ V CS , δV HCO+ ) between the peak velocities of optically thick lines and optically thin line C 18 O(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 blueshifted asymmetric line profiles between CS(2-1) and HCO + (1-0). The HCO + (1-0) shows the highest occurrence of obvious asymmetric features, 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: G 123.07-6.31, W75(OH), S235N, CEP-A, W3(OH) and NGC 7538. The infall signature is extended up to a linear scale >0.2 pc.