HI, CO, and Dust in the Perseus Cloud
  Comparison analyses between the gas emission data (HI 21cm line and CO 2.6 mm
line) and the Planck/IRAS dust emission data (optical depth at 353 GHz tau353
and dust temperature Td) 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 HI 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 tau353 increases as a function of the 1.3-th power of column number
density of the total hydrogen (NH), and this implies dust evolution in high
density regions. This calibrated tau353-NH relationship shows that the amount
of the HI gas can be underestimated to be ~60% if the optically-thin HI method
is used. Based on this relationship, we calculated optical depth of the 21 cm
line (tauHI), and found that <tauHI> ~ 0.92 around the molecular cloud. The
effect of tauHI 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 (XCO), and we found its average value is <XCO> ~ 1.0x10^20 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.
