An Observational Study of Molecular Clouds at High Galactic Latitude : 高銀緯領域における分子雲の観測的研究

We have carried out the high Galactic molecular clouds observations with the NANTEN telescope in order to reveal the distribution and physical properties of molecular clouds at high Galactic latitude. The three types of an observation have been executed at high Galactic region.Firstly, a Survey for high Galactic latitude molecular clouds was carried out toward the 68 of far-infrared-excess clouds of Reach et al. (1998) by using 12CO(J=1-0) line. CO emission was detected from 32 infrared excess clouds, corresponding to the detection rate of 47%. The CO detection rates for the cold and warm infrared excess cloud whose dust temperature are lower and higher than 17K is 72% and 33%, respectively. This indicates that the cold clouds are well shielded from external UV radiation, resulting in a high CO abundance and a low temperature of the clouds. The infrared-excess clouds with no CO emission are most likely to be molecular hydrogen clouds because the temperature is similar to, or lower than, that of the surrounding HI gas. The molecular gas without CO emission seems to occupy more than 90% of the area of the infrared-excess clouds. Next, we carried out a CO survey of high Galactic latitude molecular clouds toward an HI filament including MBM 53, 54, and 55. We covered the whole area of the HI filament in 12CO (J=1-0) with a 4′grid spacing. The filament consists of many clumpy molecular clouds. 110 12CO clouds are identified and total mass is estimated to be ～1200M⊙ .13CO (J=1-0) observations were carried out toward the region of high 12CO intensities in order to measure the optical depth of molecular gas.There is no detection in C^18O (J=1-0) line in the observed region. This indicates that there are no clouds dense enough to lead star-formation in the near future.These observations spatially resolved the entire gas distribution of MBM 53,54, and 55 for the first time, and we have found a massive cloud, HLCG 92-35 whose mass is ～330M⊙, corresponding to 1/4 of the total mass. This CO cloud occupies the Galactic western half of a circular HI cloud toward (l, b) ～ (92°,-35°) , and the HI to CO mass ratio is estimated to be the largest in the observed region. Far-infrared-excess clouds toward HLCG 92-35 are the largest in the observed region. The ratio of the luminosity of the infrared excess to CO mass is also significantly larger than those of the other clouds, by a factor of ～5. These facts indicate that HLCG 92-35 is a CO-forming molecular cloud, which is younger than the MBM clouds in terms of molecular cloud formation. Some past explosive event has been suggested by Gir et al. (1994) toward the HI filament. Toward HLCG 92-35, the molecular gas distributed along the western edge of the HI cloud, which implies that the molecular gas may be formed by a compression of expanding HI shell. Finally, we carried out large scale CO observations toward a loop-like structure in far infrared whose angular extent is about 20x20 degree around (l, b) ~(109°, -45°) in Pegasus whose diameter corresponds to ～26 pc at a…