Ambipolar Diffusion-Mediated Thermal Fronts in the Neutral ISM

TL;DR

This paper investigates the structure of thermal fronts in the neutral interstellar medium, emphasizing the role of ambipolar diffusion in magnetic field transport and its effects on thermal and magnetic profiles.

Contribution

It introduces a detailed calculation of steady-state thermal fronts considering magnetic fields and ambipolar diffusion, highlighting their impact on magnetic field uniformity and thermal structure.

Findings

Ambipolar diffusion flattens magnetic field profiles across fronts.

Magnetic fields do not significantly alter thermal profiles of the fronts.

Magnetic field strength remains nearly uniform across the fronts.

Abstract

In a thermally bistable medium, cold, dense gas is separated from warm, rareified gas by thin phase transition layers, or fronts, in which heating, radiative cooling, thermal conduction, and convection of material are balanced. We calculate the steady-state structure of such fronts in the presence of magnetic fields, including the processes of ion-neutral drift and ion-neutral frictional heating. We find that ambipolar diffusion efficiently transports the magnetic field across the fronts, leading to a flat magnetic field strength profile. The thermal profiles of such fronts are not significantly different from those of unmagnetized fronts. The near uniformity of the magnetic field strength across a front is consistent with the flat field strength-gas density relation that is observed in diffuse interstellar gas.