  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.
