A survey of the HCN J=1-0 hyperfine lines towards class 0 and I sources

Abstract

The HCN 1-0 hyperfine lines have been observed toward 24 young stellar objects (YSOs) of class 0 and I. We examined how their line shapes and velocity shifts vary along with their relative optical depths and compared them with those of CS 2-1, H2CO 2(12)-1(11), and HCO+ 4-3 & 3-2 transitions previously observed by Mardones et al. (1997) and Gregersen et al. (1997). It is found that all these molecular species do not always exhibit the same sense of line asymmetry and the correlation of velocity shift is better between HCN and CS than between HCN and H2CO. The most opaque transition of HCN F=2-1 has about the same velocity shift as that of CS despite of the larger beam size of this study, which suggests that HCN F=2-1 line may be more sensitive to the internal motion of YSOs than CS line. Systematic changes of the velocity shift are noted for many sources, as one goes from F=0-1 to 2-1. The monotonic decrease of velocity (blue shift) is apparently more frequent. A detailed model of radiative transfer allowing line overlap of HCN is employed to L483 which shows convincing signatures of infall on a scale of 0.1 pc. It appears that the observed line is not compatible with the standard Shu (1977) model, but is fitted with augmentations of density and infall velocity, by factors of 6 and 0.5, respectively, and with an inclusion of a diffuse, static, turbulent, and geometrically thick envelope.