2017ApJ...838..132O

Comparison analyses between the gas emission data (H I 21 cm line and CO 2.6 mm line) and the Planck/IRAS dust emission data (optical depth at 353 GHz τ₃₅₃ and dust temperature T_d) allow us to estimate the amount and distribution of the hydrogen gas more accurately, and our previous studies revealed the existence of a large amount of optically thick H I gas in the solar neighborhood. Referring to this, we discuss the neutral hydrogen gas around the Perseus cloud in the present paper. By using the J-band extinction data, we found that τ₃₅₃ increases as a function of the 1.3th power of column number density of the total hydrogen (N_H), and this implies dust evolution in high density regions. This calibrated τ₃₅₃–N_H relationship shows that the amount of the H I gas can be underestimated to be ∼60 % if the optically thin H I method is used. Based on this relationship, we calculated the optical depth of the 21 cm line (τ_HI) and found that <τ_HI>∼0.92 around the molecular cloud. The effect of τ_HI is still significant, even if we take into account the dust evolution. We also estimated a spatial distribution of the CO-to-H₂ conversion factor (X_CO), and we found its average value is <X_CO>∼1.0×10²⁰ cm^–2 K^–1 km^–1 s. Although these results are inconsistent with some previous studies, these discrepancies can be well explained by the difference of the data and analyses methods.