1. Satoshi Yamamoto and Nobuyuki Kuboi Department of Physics The University of Tokyo Submillimeter-wave CI Line Survey in Molecular Clouds

2. Why Atomic Carbon? The fourth abundant element after H, He, and O Two fine structure lines at submm band 3 P 1 - 3 P 0 492 GHz, 3 P 2 - 3 P 1 809 GHz Low ionization potential HII region PDR MC C + C CO Tracer of an interface between PDR and MC → Formation and Evolution of Molecular Clouds

3. [CI] Observations with Mt. Fuji Submillimeter-wave Telescope [CI] 3 P 1 - 3 P 0 (492 GHz) Large-scale Mapping (> 50 deg 2 ) GMC: Orion A, Orion B, M17, W51, DR15, DR21, W3, W44 etc. Dark Cloud: Taurus, NGC1333, rho Oph, L134, Bright Rimmed Clouds, etc. Translucent Cloud: G216-2.5, MBM12, Draco [CI] 3 P 2 - 3 P 1 (809 GHz) Mapping toward representative sources Orion A, Orion B, DR21, NGC1333 etc.

4. Taurus Dark Cloud Distance ～ 140 pc Active formation site of low-mass stars (Extensive studies with molecular lines) No OB stars associated No SNR associated ↓ Spontaneous formation and evolution of molecular clouds

6. CI Integrated Intensity CI rich cloud Translucent Cloud Dense Cores CI/ 13 CO(1-0) Integrated Intensity Ratio

7. Late Early Hirahara et al. 1992 TMC-1

8. C I Rich Cloud in HCL2 High C/CO ratio > 0.8 Av > 6 H 2 density ～ 5 x 10 3 cm -3 No enhancement in IRAS color Early stage of chemical evolution? Interface between translucent part and dense part Consistent with chemical evolution scheme in the TMC-1 ridge → Formation site of molecular cloud cores

9. CI Peak Av=6, C/CO=0.6 CI Integrated Intensity CI/C 18 O(1-0) Integrated Intensity Ratio CI/ 13 CO(1-0) Integrated Intensity Ratio

10. CI Peak of L1495 Early Stage of Chemical Evolution?

11. CI/ 13 CO(1-0) Integrated Intensity Ratio CI/C 18 O(1-0) Integrated Intensity Ratio No translucent part Is associated. Relatively low C/CO ratio (0.2)

12. B18 is more evolved than HCL2 and L1495 ? A lack of translucent part in B18

13. Summary CI/ 13 CO(1-0) intensity ratio is a good indicator of a translucent part of a molecular cloud. HCL2 and L1495 regions contain an interface between a translucent part and a dense part. >>> Formation site of molecular cloud cores? B18 does not contain a clear interface. >>> More evolved than HCL2 and L1495 CI line >>>Unique probe of cloud core formation and evolution

15. Diffuse part is not Associated.