# Gas and Dust Properties in the Chamaeleon Molecular Cloud Complex based on the Optically Thick HI

## TL;DR

This study investigates gas and dust properties in the Chamaeleon molecular cloud complex using emission lines, dust optical depth, and infrared extinction, revealing optically thick HI and spatial variations in the CO-to-H2 conversion factor.

## Contribution

It introduces a nonlinear relation between dust optical depth and infrared extinction, and models optically thick HI around molecular clouds, providing new insights into gas properties and their spatial variations.

## Key findings

- Optically thick HI with $ au_{HI} \\sim$1.3 extends around clouds.
- A nonlinear $ au_{353}$-$A_J$ relation indicates dust evolution.
- Spatial variation in $X_{CO}$ suggests environmental dependence.

## Abstract

Gas and dust properties in the Chamaeleon molecular cloud complex have been investigated with emission lines from atomic hydrogen (HI) and 12CO molecule, dust optical depth at 353 GHz ($\tau_{353}$), and $J$-band infrared extinction ($A_{J}$). We have found a scatter correlation between the HI integrated intensity ($W_{\rm HI}$) and $\tau_{353}$ in the Chamaeleon region. The scattering has been examined in terms of possible large optical depth in HI emission ($\tau_{\rm HI}$) using a total column density ($N_{\rm H}$) model based on $\tau_{353}$. A nonlinear relation of $\tau_{353}$ with the $\sim$1.2 power of $A_{J}$ has been found in opaque regions ($A_{J}$ $\gtrsim$ 0.3 mag), which may indicate dust evolution effect. If we apply this nonlinear relation to the $N_{\rm H}$ model (i.e., $N_{\rm H} \propto \tau_{353}^{1/1.2}$) allowing arbitrary $\tau_{\rm HI}$, the model curve reproduces well the $W_{\rm HI}$-$\tau_{353}$ scatter correlation, suggesting optically thick HI ($\tau_{\rm HI} \sim$1.3) extended around the molecular clouds. Based on the correlations between the CO integrated intensity and the $N_{\rm H}$ model, we have then derived the CO-to-H$_{2}$ conversion factor ($X_{\rm CO}$) on $\sim$1.5$^{\circ}$ scales (corresponding to $\sim$4 persec) and found spatial variations of $X_{\rm CO}$ $\sim$(0.5-3)$\times$10$^{20}$ cm$^{-2}$ K$^{-1}$ km$^{-1}$ s across the cloud complex, possibly depending on the radiation field inside or surrounding the molecular clouds. These gas properties found in the Chamaeleon region are discussed through a comparison with other local molecular cloud complexes.