The CH+ Abundance in Turbulent, Diffuse Molecular Clouds

TL;DR

This paper investigates how turbulent dissipation in diffuse molecular clouds can explain the high observed abundance of CH+ by modeling large-scale turbulence and comparing results with observations.

Contribution

It links large-scale turbulence properties to small-scale dissipation zones, providing a method to estimate CH+ abundance in diffuse clouds.

Findings

Computed CH+ column densities match observations.

Turbulent dissipation can account for high CH+ levels.

Model reproduces emission line observations of excited H2.

Abstract

The intermittent dissipation of interstellar turbulence is an important energy source in the diffuse ISM. Though on average smaller than the heating rates due to cosmic rays and the photoelectric effect on dust grains, the turbulent cascade can channel large amounts of energy into a relatively small fraction of the gas that consequently undergoes significant heating and chemical enrichment. In particular, this mechanism has been proposed as a solution to the long-standing problem of the high abundance of CH+ along diffuse molecular sight lines, which steady-state, low temperature models under-produce by over an order of magnitude. While much work has been done on the structure and chemistry of these small-scale dissipation zones, comparatively little attention has been paid to relating these zones to the properties of the large-scale turbulence. In this paper, we attempt to bridge this gap by estimating the temperature and CH+ column density along diffuse molecular sight-lines by post-processing 3-dimensional MHD turbulence simulations. Assuming reasonable values for the cloud density (30 / cm^3), size (20 pc), and velocity dispersion (2.3 km / s), we find that our computed abundances compare well with CH+ column density observations, as well as with observations of emission lines from rotationally excited H2 molecules.