On estimating angular momenta of infalling protostellar cores from observations

TL;DR

This study uses numerical simulations to assess how accurately molecular line observations can estimate the angular momenta of early protostellar cores, highlighting the importance of velocity field characteristics.

Contribution

It demonstrates that with proper corrections, observational estimates of core angular momentum are reasonably accurate, contrasting previous overestimation claims.

Findings

Estimates are within a factor of two to three of true values.

Projection effects can cause some underestimates.

Smoother velocity fields reduce overestimation issues.

Abstract

We use numerical simulations of molecular cloud formation in the colliding flow scenario to investigate the reliability of observational estimates of the angular momenta of early-state, low-mass protostellar cores. We show that, with suitable corrections for projection factors, molecular line observations of velocity gradients in NH$_3$ can be used to provide reasonable estimates of core angular momenta within a factor of two to three, with a few large underestimates due to unfavorable viewing angles. Our results differ from previous investigations which suggested that observations might overestimate true angular momenta by as much as an order of magnitude; the difference is probably due to the much smoother velocity field on small scales in our simulations, which result from allowing turbulence to decay and gravitational infall to dominate. The results emphasize the importance of understanding the nature of 'turbulent' velocities, with implications for the formation of protostellar disks during core collapse.