H I, CO, and Dust in the Perseus Cloud

Abstract

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} {τ }₃₅₃ 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 {τ }₃₅₃ 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 {τ }₃₅₃{--}{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 }_{{H}{{I}}}) and found that < {τ }_{{H}{{I}}}> ∼ 0.92 around the molecular cloud. The effect of {τ }_{{H}{{I}}} is still significant, even if we take into account the dust evolution. We also estimated a spatial distribution of the {CO}-to-{{{H}}}₂ conversion factor ({X}_CO}), and we found its average value is < {X}_CO}> ∼ 1.0× {10}²⁰ {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.