The initial magnetic criticality of prestellar cores

TL;DR

This study uses simulations and molecular line observations to suggest that prestellar cores are magnetically subcritical, challenging previous measurements indicating supercriticality, and highlights the importance of chemistry and line profiles in assessing magnetic support.

Contribution

The paper introduces an indirect method combining chemistry, radiative transfer, and simulations to determine the magnetic criticality of prestellar cores, providing new insights into their magnetic state.

Findings

Subcritical models match observed molecular line data.

Supercritical models cannot reproduce observed line strengths.

Magnetic support influences molecular line profiles and core evolution.

Abstract

Direct observational measurements of the magnetic field strength in prestellar cores typically find supercritical mass-to-flux ratios, suggesting that the magnetic field is insufficient to prevent gravitational collapse. These measurements suffer from significant uncertainties; an alternative approach is to utilise the sensitivity of prestellar chemistry to the evolutionary history, and indirectly constrain the degree of magnetic support. We combine non-ideal magnetohydrodynamic simulations of prestellar cores with time-dependent chemistry and radiative transfer modelling, producing synthetic observations of the model cores in several commonly-observed molecular lines. We find that molecules strongly affected by freeze-out, such as CS and HCN, typically have much lower line intensities in magnetically subcritical models compared to supercritical ones, due to the longer collapse timescales. Subcritical models also produce much narrower lines for all species investigated. Accounting for a range of core properties, ages and viewing angles, we find that supercritical models are unable to reproduce the distribution of CS and N$_2$H$^+$ line strengths and widths seen in an observational sample, whereas subcritical models are in good agreement with the available data. This suggests that despite presently having supercritical mass-to-flux ratios, prestellar cores form as magnetically subcritical objects.