2026ApJ...998..167J

An analysis of the Millimetre Astronomy Legacy Team 90 GHz survey has produced a sample of 27 candidate dense molecular clumps with large collapse motions, as revealed by large “blue” asymmetrical line profiles of the optically thick HCO⁺ (1-0) line. New, more sensitive molecular line observations of this sample, conducted with the Mopra 22 m telescope, confirm the blue asymmetries in the HCO⁺ (1-0) line profiles, with large, positive values of the asymmetry parameter A (${\overline{A}}_{{\mathrm{HCO}}^{+}}=0.69\pm 0.01$), and positive, but smaller asymmetries in the HCN (1-0) and HNC (1-0) lines (${\overline{A}}_{\mathrm{HCN}}=0.35\pm 0.01$ and ${\overline{A}}_{\mathrm{HNC}}=0.28\pm 0.01$), as expected for a less optically thick tracer in collapsing clumps. The small, positive mean asymmetry parameters for C₂H (N = 1-0; J = 3/2-1/2; F = 2-1) and H¹³CO⁺ (1-0), ${\overline{A}}_{{{\rm{C}}}_{2}{\rm{H}}}=0.15\pm 0.02$ and ${\overline{A}}_{{{\rm{H}}}^{13}{\mathrm{CO}}^{+}}=0.18\pm 0.03$, likely indicate slightly optically thick emission for at least some clumps. The hyperfine ratios for N₂H⁺ (1-0) are in their optically thin, LTE, values, but for HCN (1-0) they are not; the F = 1 → 1 hyperfine line shows abnormally weak intensities. A simple two-component model shows that self-absorption of the background F = 1 → 1 hyperfine line by the main F = 2 → 1 hyperfine line of a cold, foreground, redshifted cloud can reproduce the observed HCN (1-0) hyperfine intensities and match the HCN (1-0) and HCO⁺ (1-0) line profiles. All of these results are consistent with self-absorption of the optically thick lines on the red side of the profile, as expected for collapsing clumps. The simple two-cloud model suggests that this sample represents dense clumps with extreme collapse velocities, V_inf ∼ 2.4 km s^–1.