1999ApJ...520..223P

The HCN 1-0 hyperfine lines have been observed toward 24 young stellar objects (YSOs) of class 0 and I. The hyperfine lines are well separated in most cases and show such rich structures as asymmetric double peaks and strong wings. We examined how their line shapes and velocity shifts vary along with their relative optical depths, and compared them with those of the CS 2-1, H₂CO 2₁₂-1₁₁, and HCO⁺ 4-3 and 3-2 transitions previously observed by Mardones et al. and Gregersen et al. We find 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 the larger beam size of this study, which suggests that the HCN F=2-1 line could 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 (blueshift) 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 model, but is fitted with augmentations of density and infall velocity, by factors of 6 and 0.5, respectively, and with the inclusion of a diffuse, static, turbulent, and geometrically thick envelope. The distribution of hyperfine line intensity ratios for these YSOs does not accord with the LTE condition and is essentially the same as ones previously noted in cold dark clouds or small translucent cores. Although this anomaly may be explained in terms of radiative transfer effects in the cores, which are either static or under systematic motion, some of them seem to invoke the existence of a scattering envelope. It is confirmed that HCN is detected more selectively in class 0 and I sources than in starless cores or class II objects, which implies that the YSO(s) form a dense (∼10⁶ cm^–3) envelope with a significant HCN abundance in a narrow time span of their evolution.