HI, CO, and Planck/IRAS dust properties in the high-latitude-cloud
  complex, MBM 53, 54, 55 and HLCG 92-35; Possible evidence for an optically
  thick HI envelope around the CO clouds
  We present an analysis of the HI and CO gas in conjunction with the
Planck/IRAS submillimeter/far-infrared dust properties toward the most
outstanding high latitude clouds MBM 53, 54, 55 and HLCG 92-35 at b = -30 deg
to -45 deg. The CO emission, dust opacity at 353 GHz (tau353), and dust
temperature (Td) show generally good spatial correspondence. On the other hand,
the correspondence between the HI emission and the dust properties is less
clear than in CO. The integrated HI intensity WHI and tau353 show a large
scatter with a correlation coefficient of ~0.6 for a Td range from 16 K to 22
K. We find, however, that WHI and tau353 show better correlation for smaller
ranges of Td every 0.5 K, generally with a correlation coefficient of 0.7-0.9.
We set up a hypothesis that the HI gas associated with the highest Td >= 21.5 K
is optically thin, whereas the HI emission is generally optically thick for Td
lower than 21.5 K. We have determined a relationship for the optically thin HI
gas between atomic hydrogen column density and tau353, NHI (cm-2) = (1.5 x
10^26) x tau353, under the assumption that the dust properties are uniform and
we have applied this to estimate NHI from tau353 for the whole cloud. NHI was
then used to solve for Ts and tauHI over the region. The result shows that the
HI is dominated by optically thick gas having a low spin temperature of 20-40 K
and a density of 40-160 cm-3. The HI envelope has a total mass of ~1.2 x 10^4
Msol, an order of magnitude larger than that of the CO clouds. The HI envelope
properties derived by this method do not rule out a mixture of HI and H2 in the
dark gas, but we present indirect evidence that most of the gas mass is in the
atomic state.
