Dense Molecular Clumps with Large Blue Asymmetries: Evidence for Collapse

TL;DR

This study validates large blue asymmetries in dense clumps selected from the MALT90 survey as signatures of rapid gravitational collapse. Using deeper Mopra observations of 27 candidate clumps, the authors quantify an exceptionally high blue excess in HCO$^{+}$ ($\bar{A}_{HCO^+} = 0.69$) and progressively smaller asymmetries in less optically thick tracers, consistent with an exterior cooler layer and interior warmer region. Hyperfine ratios reveal LTE consistency for N$_2$H$^{+}$ but non-LTE anomalies for HCN, which are explained by self-absorption in a two-cloud collapsing model that also reproduces the line profiles. The two-cloud modeling yields infall speeds around $V_{inf} \sim 2.4$ km s$^{-1}$, indicating some of the most rapid collapse observed in dense clumps, and supporting a scenario of extreme collapse with strong excitation gradients. Overall, the results corroborate collapse as the dominant process in these clumps and highlight the role of optical depth and foreground self-absorption in shaping the observed spectra.

Abstract

An analysis of the Millimetre Astronomy Legacy Team 90 GHz (MALT90) 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 \hcop\, line. %with respect to the optically thin \nthp\, line. New, more sensitive molecular line observations of this sample, conducted with the Mopra 22-m telescope, confirm the blue asymmetries in the \hcop\, line profiles, with large, positive values of the asymmetry parameter $A$ ($\bar{A}_{HCO^+} = 0.69\pm0.01$), and positive, but smaller asymmetries in the \hcn\, and \hnc\, lines: ($\bar{A}_{HCN} = 0.35\pm0.01$ and $\bar{A}_{HNC} = 0.28\pm0.01$), as expected for a less optically thick tracer in collapsing clumps. The small, positive mean asymmetry parameters for \cch\, and \htcop, $\bar{A}_{C_2H} = 0.15\pm0.02$ and $\bar{A}_{H^{13}CO^+} = 0.18\pm0.03$, likely indicate slightly optically thick emission for at least some clumps. The hyperfine ratios for \nthp\, are in their optically thin, LTE, values, but for \hcn\ they are not; the $F=1 \to 1$ hyperfine line shows abnormally weak intensities. A simple two-component model shows that self-absorption of the background $F = 1 \to 1$ hyperfine line by the main $F = 2 \to 1$ hyperfine line of a cold, foreground, redshifted cloud can reproduce the observed \hcn\, hyperfine intensities and match the \hcn\, and \hcop\, 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. A simple two-cloud model suggests that this sample represents dense clumps with extreme collapse velocities, $V_{inf} \sim 2.4$ \kms.

Figures (30)

• Figure 1: Spitzer mid- (GLIMPSE) and far-infrared (MIPSGAL) images of the selected sources, with 3.6 $\mu$m in blue, 8.0 $\mu$m in green, and 24 $\mu$m in red. Red crosses indicate the source position. The green circle indicates the size of the Mopra beam and the observed position. Yellow contours indicate ATLASGAL 870 $\mu$m flux with levels at 0.1 Jy/beam.
• Figure 2: Spitzer mid- (GLIMPSE) and far-infrared (MIPSGAL) images of the selected sources, with 3.6 $\mu$m in blue, 8.0 $\mu$m in green, and 24 $\mu$m in red. Red crosses indicate the source position. The green circle indicates the size of the Mopra beam and the observed position. Yellow contours indicate ATLASGAL 870 $\mu$m flux with levels at 0.1 Jy/beam.
• Figure 3: Spitzer mid- (GLIMPSE) and far-infrared (MIPSGAL) images of the selected sources, with 3.6 $\mu$m in blue, 8.0 $\mu$m in green, and 24 $\mu$m in red. Red crosses indicate the source position. The green circle indicates the size of the Mopra beam and the observed position. Yellow contours indicate ATLASGAL 870 $\mu$m flux with levels at 0.1 Jy/beam.
• Figure 4: Mopra 3 mm spectra of the HCO$^{+}$ (1--0), N$_{2}$H$^{+}$ (1--0), HCN (1--0), and HNC (1--0) lines. The vertical lines show the fitted velocity of the N$_{2}$H$^{+}$ (1--0) line in orange, if available, otherwise the velocity of the peak HCO$^{+}$ (1--0) intensity in cyan.
• Figure 5: Mopra 3 mm spectra of the HCO$^{+}$ (1--0), N$_{2}$H$^{+}$ (1--0), HCN (1--0), and HNC (1--0) lines. Same as Fig, \ref{['fig:fig0_bright']}, but for different sources.