Probing inward motions in starless cores using the HCN(J=1-0) hyperfine transitions:: A pointing survey toward central regions

We have carried out a survey toward the central regions of 85 starless cores in HCN(J = 1-0) to study inward motions in the cores. Sixty-four cores were detected with HCN lines. The infall asymmetry in the HCN spectra is found to be more prevalent, and more prominent than in any other previously used infall tracers such as CS(J = 2 - 1), DCO+ (J = 2 - 1), and N2H+(J = 1 - 0). We have found a close relation between the intensities of the HCN and N2H+ lines. This implies that the HCN is not very depleted in the central regions of the cores. In some cores, the HCN spectra show different signs of asymmetry than other molecular lines. A few cores show various signs of asymmetry in individual HCN hyperfine lines. The distribution of the velocity shift delta V of the HCN profiles with respect to the systemic velocity of the optically thin tracer is found to be more shifted toward the bluer side than those of other infall tracers, indicating that the HCN traces inward motions more frequently. The delta V distribution of each HCN hyperfine line for all sources is similar. Moreover, the delta V values obtained from different HCN hyperfine lines for each source are nearly similar. These may mean that most starless cores are in similar kinematic states across the layers of the cores. We identify 17 infall candidates using all available indicators such as the velocity shift delta V and the blue-to-red peak intensity ratio of double-peaked profiles for HCN(J = 1 - 0), CS(J = 2 - 1), CS(J = 3 - 2), DCO+(J = 2 - 1), and N2H+(J = 1 - 0). Four of them, L63, L492, L694-2, and L1197, are found to show a higher blue-to-red ratio in the HCN hyperfine line along the lower opacity, suggesting that infall speed becomes higher toward the center.