This thesis presents the results of the large-scale observations of the physical proper-ties of the molecular clouds in multiple molecular lines, and then it is found that the combination of the optically thick line and different transitions of the optically thin lines are important to derive the cloud properties. Large-scale observations of the J =2–1 transition of CO lines are however anything more than just started. Hence, there are lots of challenges to be addressed. Here we point out some directions for the future studies.
The first is a study of the other molecular clouds. In this thesis, we carried out the study of the giant molecular clouds associated with HII regions and revealed the spatial distribution of the temperature and density. Similar studies toward the dark clouds (e.g., Taurus, Polaris Flare) and the other GMCs (e.g., Cygnus, California) would provide vital information to understand the relation between the cloud properties and the surrounding environment. These fundamental datasets of the GMC properties are necessary for the comparisons with the surveys in other wavelengths of the Galactic GMCs (e.g., taken with Spitzer and Herschel) and the surveys of the GMCs in the external galaxies taken with ALMA.
The second is a study of the C18O(J =2–1). In this thesis, we used12CO and13CO of J =1–0 and J =2–1 for the analyses, and the cloud properties are derived toward the region where density is about 103 cm−3. By using the combination of the C18O and
13CO, it is expected that the properties of the higher density region about 104 cm−3 are derived. These analyses will enable us to investigate the hierarchical structure of the density which exists in the molecular clouds.
The third is an application to the lines of the other molecules. We confirmed the usefulness of the analyses for the combination of the CO lines. The similar analyses for the combination of the higher density tracers such as CS and HCO+ by observ-ing with the better angular resolution (e.g., usobserv-ing Nobeyama 45-m, ALMA), would
104 4.2. FUTURE PROSPECTS
provide us wealth of information of the density structure in the inner dense region.
For example, the combination of the C32S(J =2–1), C34S(J =1–0), and C34S(J =2–1) lines is possibly worthy for the analyses. These observations would provide us the significant perceptions for the whole picture of the evolution of molecular clouds.
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