2014ApJ...796...59F

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° to - 45°. The CO emission, dust opacity at 353 GHz (τ₃₅₃), 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 HI and τ₃₅₃ show a large scatter with a correlation coefficient of ∼0.6 for a T_drange from 16 K to 22 K. We find, however, that HI and τ₃₅₃show better correlation for smaller ranges of T_devery 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_dlower than 21.5 K. We have determined a relationship for the optically thin HI gas between atomic hydrogen column density and τ₃₅₃, HI under the assumption that the dust properties are uniform and we have applied this to estimate HI from τ₃₅₃ for the whole cloud. HI was then used to solve for T_sand 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/cm3. The HI envelope has a total mass of ∼1.2x10⁴ M_☉, 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 H₂in the dark gas, but we present indirect evidence that most of the gas mass is in the atomic state.