Contraction Signatures toward Dense Cores in the Perseus Molecular Cloud

We report the results of an HCO⁺ (3-2) and N₂D⁺ (3-2) molecular line survey performed toward 91 dense cores in the Perseus molecular cloud using the James Clerk Maxwell Telescope, to identify the fraction of starless and protostellar cores with systematic radial motions. We quantify the HCO⁺ asymmetry using a dimensionless asymmetry parameter δ_v, and identify 20 cores with significant blue or red line asymmetries in optically thick emission indicative of collapsing or expanding motions, respectively. We separately fit the HCO⁺ profiles with an analytic collapse model and determine contraction (expansion) speeds toward 22 cores. Comparing the δ_v and collapse model results, we find that δ_v is a good tracer of core contraction if the optically thin emission is aligned with the model-derived systemic velocity. The contraction speeds range from subsonic (0.03 km s^-1) to supersonic (0.4 km s^-1), where the supersonic contraction speeds may trace global rather than local core contraction. Most cores have contraction speeds significantly less than their free-fall speeds. Only 7 of 28 starless cores have spectra well-fit by the collapse model, which more than doubles (15 of 28) for protostellar cores. Starless cores with masses greater than the Jeans mass (M/M_J > 1) are somewhat more likely to show contraction motions. We find no trend of optically thin non-thermal line width with M/M_J, suggesting that any undetected contraction motions are small and subsonic. Most starless cores in Perseus are either not in a state of collapse or expansion, or are in a very early stage of collapse.