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 degrees to -45 degrees. The CO emission, dust opacity at 353 GHz (tau(353)), and dust temperature (T-d) 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 W-Hi and tau(353) show a large scatter with a correlation coefficient of similar to 0.6 for a T-d range from 16 K to 22 K. We find, however, that WHi and tau(353) show better correlation for smaller ranges of T-d 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 T-d >= 21.5 K is optically thin, whereas the Hi emission is generally optically thick for T-d lower than 21.5 K. We have determined a relationship for the optically thin Hi gas between atomic hydrogen column density and tau(353), N-Hi (cm(-2)) = (1.5 x 10(26))center dot tau(353), under the assumption that the dust properties are uniform and we have applied this to estimate N-Hi from tau(353) for the whole cloud. N-Hi was then used to solve for T-s and tau(Hi) 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 similar to 1.2 x 10(4) M-circle dot, 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 H-2 in the dark gas, but we present indirect evidence that most of the gas mass is in the atomic state.