HCO+ and N2D+ dense cores in Perseus

We quantify the HCO^+^ asymmetry using a dimensionless asymmetry parameter {delta}_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 {delta}_v_ and collapse model results, we find that {delta}_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 (0.03km/s) to supersonic (0.40km/s), where the supersonic contraction speeds may trace global rather than local core contraction.