2019ApJ...878..131H

Gas and dust properties in the Chamaeleon molecular cloud complex have been investigated with emission lines from the atomic hydrogen (H I) and ¹²CO molecules, dust optical depth at 353 GHz (τ₃₅₃), and J-band infrared extinction (A_J). We have found a scatter correlation between the H I integrated intensity (W_H I_) and τ₃₅₃ in the Chamaeleon region. The scattering has been examined in terms of a possible large optical depth in H I emission (τ_H I_) using a total column density (N_H) model based on τ₃₅₃. A nonlinear relation of τ₃₅₃ with the ∼1.2 power of A_J has been found in opaque regions (A_J >= 0.3 mag), which may indicate a dust evolution effect. If we apply this nonlinear relation to the N_H model (i.e., N_H ∝ τ₃₅₃^1/1.2) allowing arbitrary τ_H I_, the model curve reproduces well the W_H I_–τ353_ scatter correlation, suggesting optically thick H I (τ_H I_ ∼ 1.3) extended around the molecular clouds. Based on the correlations between the CO integrated intensity and the N_H model, we have derived the CO-to-H₂ conversion factor (X_CO) on ∼1.°5 scales (corresponding to ∼4 pc) and found spatial variations of X_CO ∼ (0.5-3) x 10²⁰ 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.