Non-Equilibrium Chemistry of Dynamically Evolving Prestellar Cores: II. Ionization and Magnetic Field

TL;DR

This study investigates how non-equilibrium chemistry affects magnetic field measurements, ionization, and ion properties in collapsing molecular cloud cores, revealing underestimations in magnetic field strength and lower ion molecular weights.

Contribution

It provides new insights into the chemical and magnetic properties of prestellar cores, highlighting the impact of non-equilibrium chemistry on observational measurements and ion dynamics.

Findings

Zeeman observations underestimate magnetic field strength in cores.

Ionization degree depends on density, cosmic-ray ionization rate, and temperature.

Mean molecular weight of ions approaches 3 at high densities, speeding up ambipolar diffusion.

Abstract

We study the effect that non-equilibrium chemistry in dynamical models of collapsing molecular cloud cores has on measurements of the magnetic field in these cores, the degree of ionization, and the mean molecular weight of ions. We find that OH and CN, usually used in Zeeman observations of the line-of-sight magnetic field, have an abundance that decreases toward the center of the core much faster than the density increases. As a result, Zeeman observations tend to sample the outer layers of the core and consistently underestimate the core magnetic field. The degree of ionization follows a complicated dependence on the number density at central densities up to 10^5 cm^{-3} for magnetic models and 10^6 cm^{-3} in non-magnetic models. At higher central densities the scaling approaches a power-law with a slope of -0.6 and a normalization which depends on the cosmic-ray ionization rate {\zeta} and the temperature T as ({\zeta}T)^1/2. The mean molecular weight of ions is systematically lower than the usually assumed value of 20 - 30, and, at high densities, approaches a value of 3 due to the asymptotic dominance of the H3+ ion. This significantly lower value implies that ambipolar diffusion operates faster.