The penetration of FUV radiation into molecular clouds

TL;DR

This paper develops an advanced method to model FUV radiation transfer in molecular clouds, accounting for dust scattering and gas absorption, revealing significant effects of grain properties on radiation penetration and chemistry.

Contribution

It extends the spherical harmonics method to include gas lines and non-uniform dust properties in FUV radiative transfer modeling.

Findings

FUV penetration depth increases with dust albedo and scattering anisotropy.

FUV radiation fields vary greatly with dust grain growth in dense clouds.

Assuming uniform dust properties oversimplifies the complex chemistry and radiation in clouds.

Abstract

The solution of the FUV radiative transfer equation can be complicated if the most relevant radiative processes such as dust scattering and gas line absorption are included, and have realistic (non-uniform) properties. We have extended the spherical harmonics method to solve for the FUV radiation field in illuminated clouds taking into account gas absorption and coherent, nonconservative and anisotropic scattering by dust grains. Our formalism allows us to consistently include: (i) varying dust populations and (ii) gas lines in the FUV radiative transfer. The FUV penetration depth rises for increasing dust albedo and anisotropy of the scattered radiation (e.g. when grains grow towards cloud interiors). Illustrative models of illuminated clouds where only the dust populations are varied confirm earlier predictions for the FUV penetration in diffuse clouds (A_V<1). For denser and more embedded sources (A_V>1) we show that the FUV radiation field inside the cloud can differ by orders of magnitude depending on the grain properties. We show that the photochemical and thermal gradients can be very different depending on grain growth. Therefore, the assumption of uniform dust properties and averaged extinction curves can be a crude approximation to determine the resulting scattering properties, prevailing chemistry and atomic/molecular abundances in ISM clouds or protoplanetary disks.