Line Profiles of Cores within Clusters: II Signatures of Dynamical Collapse during High Mass Star Formation

TL;DR

This study models line profiles from hydrodynamic simulations of collapsing massive star forming regions to identify signatures of collapse and assess observational diagnostics.

Contribution

It provides detailed predictions of molecular line profiles during collapse, highlighting the effects of velocity gradients and beam size on observable features.

Findings

Optically thick lines are marginally blue-shifted and often lack self-absorption dips.

Lower order HCO+ transitions better indicate collapse than higher order lines.

High-resolution observations can test these model predictions and reveal core dynamics.

Abstract

Observations of atomic or molecular lines can provide important information about the physical state of star forming regions. In order to investigate the line profiles from dynamical collapsing massive star forming regions (MSFRs), we model the emission from hydrodynamic simulations of a collapsing cloud in the absence of outflows. By performing radiative transfer calculations, we compute the optically thick HCO+ and optically thin N2H+ line profiles from two collapsing regions at different epochs. Due to large-scale collapse, the MSFRs have large velocity gradients, reaching up to 20 km/s/pc across the central core. The optically thin lines typically contain multiple velocity components resulting from the superposition of numerous density peaks along the line-of-sight. The optically thick lines are only marginally shifted to the blue side of the optically thin line profiles, and frequently do not have a central depression in their profiles due to self-absorption. As the regions evolve the lines become brighter and the optically thick lines become broader. The lower order HCO+ (1-0) transitions are better indicators of collapse than the higher order (4-3) transitions. We also investigate how the beam sizes affect profile shapes. Smaller beams lead to brighter and narrower lines that are more skewed to the blue in HCO+ relative to the true core velocity, but show multiple components in N2H+. High resolution observations (e.g. with ALMA) can test these predictions and provide insights into the nature of MSFRs.