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

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 ($W_{\mathrm{HI}}$) and ${\tau }_{353}$ in the Chamaeleon region. The scattering has been examined in terms of possible large optical depth in HI emission (${\tau }_{\mathrm{HI}}$) using a total column density ($N_{\mathrm{H}}$) 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 $N_{\mathrm{H}}$ model (i.e., $N_{\mathrm{H}}\propto {\tau }_{353}^{1/1.2}$) allowing arbitrary ${\tau }_{\mathrm{HI}}$, the model curve reproduces well the $W_{\mathrm{HI}}$-${\tau }_{353}$ scatter correlation, suggesting optically thick HI (${\tau }_{\mathrm{HI}}\sim$1.3) extended around the molecular clouds. Based on the correlations between the CO integrated intensity and the $N_{\mathrm{H}}$ model, we have then derived the CO-to-H${}_2$ conversion factor ($X_{\mathrm{CO}}$) on $\sim$1.5${}^{\circ }$ scales (corresponding to $\sim$4 persec) and found spatial variations of $X_{\mathrm{CO}}$ $\sim$(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.

30 pages, 21 figures, Accepted for ApJ

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