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We present an analysis of the HI and CO gas in conjunction with thePlanck/IRAS submillimeter/far-infrared dust properties toward the mostoutstanding high latitude clouds MBM 53, 54, 55 and HLCG 92-35 at b = -30 degto -45 deg. The CO emission, dust opacity at 353 GHz (tau353), and dusttemperature (Td) show generally good spatial correspondence. On the other hand,the correspondence between the HI emission and the dust properties is lessclear than in CO. The integrated HI intensity WHI and tau353 show a largescatter with a correlation coefficient of ~0.6 for a Td range from 16 K to 22K. We find, however, that WHI and tau353 show better correlation for smallerranges 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 Kis optically thin, whereas the HI emission is generally optically thick for Tdlower than 21.5 K. We have determined a relationship for the optically thin HIgas between atomic hydrogen column density and tau353, NHI (cm-2) = (1.5 x10^26) x tau353, under the assumption that the dust properties are uniform andwe have applied this to estimate NHI from tau353 for the whole cloud. NHI wasthen used to solve for Ts and tauHI over the region. The result shows that theHI is dominated by optically thick gas having a low spin temperature of 20-40 Kand a density of 40-160 cm-3. The HI envelope has a total mass of ~1.2 x 10^4Msol, an order of magnitude larger than that of the CO clouds. The HI envelopeproperties derived by this method do not rule out a mixture of HI and H2 in thedark gas, but we present indirect evidence that most of the gas mass is in theatomic state.

astrophysical journal/the astrophysical journal

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