Fe b 19 99 A Survey of the HCN J = 1 – 0 Hyperfine Lines towards Class 0 and I Sources

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 CS 2–1, H 2 CO 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 H 2 CO. 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. 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. Though this anomaly may be explained in terms of radiative transfer effect in the cores which are either – 3 – static or under systematic motion, some of them seem to invoke an existence of 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 core embedding YSO(s) form a dense (∼ 10 6 cm −3) envelope with a significant HCN abundance in a narrow time span of their …