Molecular line signatures of cloud-cloud collisions

TL;DR

This paper uses simulations and radiative transfer models to identify molecular line signatures that distinguish genuine cloud-cloud collisions from chance superpositions, aiding star formation studies.

Contribution

It introduces a method to observationally confirm cloud collisions by analyzing molecular line ratios, specifically NH$_3$ and HCN, in PV space.

Findings

Genuinely colliding clouds show peak NH$_3$ and HCN ratios of 0.5 and 0.2 relative to CO.

Chance superpositions have co-located NH$_3$, HCN, and CO peaks.

Molecular line ratios can differentiate true collisions from superpositions.

Abstract

Collisions between interstellar gas clouds are potentially an important mechanism for triggering star formation. This is because they are able to rapidly generate large masses of dense gas. Observationally, cloud collisions are often identified in position-velocity (PV) space through bridging features between intensity peaks, usually of CO emission. Using a combination of hydrodynamical simulations, time-dependent chemistry, and radiative transfer, we produce synthetic molecular line observations of overlapping clouds that are genuinely colliding, and overlapping clouds that are just chance superpositions. Molecules tracing denser material than CO, such as NH$_3$ and HCN, reach peak intensity ratios of $0.5$ and $0.2$ with respect to CO in the `bridging feature' region of PV space for genuinely colliding clouds. For overlapping clouds that are just chance superpositions, the peak NH$_3$ and HCN intensities are co-located with the CO intensity peaks. This represents a way of confirming cloud collisions observationally, and distinguishing them from chance alignments of unrelated material.