Variation of the Core Lifetime and Fragmentation Scale in Molecular Clouds as an Indication of Ambipolar Diffusion

TL;DR

This paper investigates how ambipolar diffusion influences core formation and fragmentation in molecular clouds, using models that fit observed core lifetimes and numbers, revealing a transcritical mass-to-flux ratio and a specific magnetic field-density relation.

Contribution

The study introduces a model linking ambipolar diffusion to core lifetimes and fragmentation scales, fitting observational data and revealing a consistent magnetic field-density relation in prestellar cores.

Findings

Prestellar cores likely have a transcritical mass-to-flux ratio.

The magnetic field scales with density as B ∝ n^{0.43}.

Model matches observed core lifetimes and formation rates.

Abstract

Ambipolar diffusion likely plays a pivotal role in the formation and evolution of dense cores in weakly-ionized molecular clouds. Linear analyses show that the evolutionary times and fragmentation scales are significantly greater than the hydrodynamic (Jeans) values even for clouds with mildly supercritical mass-to-flux ratio. We utilize values of fragmentation scales and growth times that correspond to typical ionization fractions within a molecular cloud, and apply to the context of the observed estimated lifetime of prestellar cores as well as the observed number of such embedded cores forming in a parent clump. By varying a single parameter, the mass-to-flux ratio, over the range of observationally measured densities, we fit the range of estimated prestellar core lifetimes ($\sim 0.1$ to a few Myr) identified with Herschel as well as the number of embedded cores formed in a parent clump measured in Perseus with the Submillimeter Array (SMA). Our model suggests that the prestellar cores are formed with a transcritical mass-to-flux ratio and higher densities correspond to somewhat higher mass-to-flux ratio but the normalized mass-to-flux ratio $\mu$ remains in the range $1 \lesssim \mu \lesssim 2$. Our best-fit model exhibits $B \propto n^{0.43}$ for prestellar cores, due to partial flux-freezing as a consequence of ambipolar diffusion.