Dense Molecular Clumps with Large Blue Asymmetries: Evidence for Collapse

Abstract 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 ( A ¯ HCO + = 0.69 ± 0.01 ), and positive, but smaller asymmetries in the HCN (1–0) and HNC (1–0) lines ( A ¯ HCN = 0.35 ± 0.01 and A ¯ HNC = 0.28 ± 0.01 ), as expected for a less optically thick tracer in collapsing clumps. The small, positive mean asymmetry parameters for C 2 H ( N = 1–0; J = 3/2–1/2; F = 2–1) and H 13 CO + (1–0), A ¯ C 2 H = 0.15 ± 0.02 and A ¯ H 13 CO + = 0.18 ± 0.03 , likely indicate slightly optically thick emission for at least some clumps. The hyperfine ratios for N 2 H + (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 .