Kinematics in Partially Ionised Molecular Clouds: Implications for the Transition to Coherence

TL;DR

This study investigates how different ionisation profiles affect the kinematics, velocity structures, and spectral features of molecular clouds, revealing that ionisation levels influence the transition to coherence and core formation dynamics.

Contribution

It extends previous analyses by examining the impact of photoionisation and cosmic ray ionisation profiles on velocity structures and synthetic spectra in molecular cloud simulations.

Findings

High ionisation fractions lead to supersonic velocities up to twice the sound speed.

Cores exhibit small, sub-thermal motions relative to the background gas.

Velocity deviations are caused by line-of-sight intersections with different density regions.

Abstract

Bailey & Basu (2014) show analysis of density and mass-to-flux ratio maps for simulations with either an ionisation profile which takes into account photoionisation (step-like profile) or a cosmic ray only ionisation profile. We extend this study to analyse the effect of these ionisation profiles on velocity structures, kinematics, and synthetic spectra. Clump regions are found to occur at the convergence of two flows with a low velocity region and velocity direction transition occurring at the junction. Models with evident substructure show that core formation occurs on the periphery of these velocity valleys. Analysis of synthetic spectra reveals the presence of large non-thermal components within low-density gas, especially for models with the step-like ionisation profile. All cores show small, sub-thermal relative motions compared to background gas. Large deviations within this analysis are due to the line of sight intersecting low- and high-density regions across the velocity switch transition. Positive deviations correspond to a foreground core moving away from the observer while negative deviations correspond to a background core moving toward the observer. Comparison of velocities resulting from different ionisation profiles suggest that high ionisation fractions yield supersonic velocities, up to two times the sound speed, while regions with low ionisation fractions tend to be subsonic or mildly supersonic. This suggests that the transition to coherence within cores could be a transition between high and low ionisation fractions within the gas.