Optical properties and dust temperatures in clumpy diffuse medium

TL;DR

This paper investigates how inhomogeneities in the distribution of interstellar dust, especially in the form of clumps larger than 0.1 au, affect the optical properties and dust temperatures in the diffuse interstellar medium, revealing significant impacts on opacity and molecule synthesis.

Contribution

It demonstrates that dust clumping on scales larger than 0.1 au significantly alters optical properties and dust temperatures, challenging the uniform dust/gas mixture assumption in diffuse ISM models.

Findings

Opacity decreases with larger dust clumps.

Clumps >0.1 au are opaque to far UV radiation.

Dust temperature gradients exist within clumps.

Abstract

In this study, we explore the impact of inhomogeneities in the spatial distribution of interstellar dust on spatial scales of $\le1$ au caused by ion shadowing forces on the optical properties of diffuse interstellar medium (ISM) as well as on the dust temperature. We show that recently proposed possibility that interstellar dust grains in the diffuse ISM are grouped in spherical cloudlets (clumps) may significantly affect the observed optical properties of the diffuse ISM in comparison to that calculated under the commonly accepted assumption on the uniform dust/gas mixture if the size of clumps $\gtrsim0.1$ au. We found that opacity of an arbitrary region of diffuse ISM quickly decreases with growth of dusty clumps. We also studied the dependence of opacity and dust temperature inside the dusty clumps on their size. We show that the clumps larger than 0.1 au are opaque for far ultraviolet radiation. Dust temperature exhibit a gradient inside a clump, decreasing from the edge to the center by several degrees for a clump of a size of 0.1 au and larger. We argue that dust temperatures and high opacity within clumps larger than 0.1 au may facilitate somewhat more efficient synthesis of molecules on surfaces of interstellar grains in the diffuse ISM than it was anticipated previously. On the other hand, the presence of clumps with sizes below 0.1 au makes small or negligible influence on the optical properties of the diffuse ISM in comparison to the case with uniform dust/gas mixture.