tokens: s −1 ). Figure 2 shows the integrated intensity maps of C 18 O ( J = 1–0) (cyan contours) and 13 CO ( J = 1–0) (color) observed with the NRO 45 m telescope for the entire region of HCL2 (Sunada Kitamura 1999 ). A prominent straight structure is seen in the northern part of HCL2 in the C 18 O map, which also has been pointed out by Narayanan et al. ( 2008 ) and Goldsmith et al. ( 2010 ). Interestingly, the velocity of the blueshifted component detected in the OH 18 cm transition ( km s −1 ) corresponds to that of C 18 O, exhibiting the straight structure, whereas the velocity of the redshifted component seen in OH ( 6–10 km s −1 ) corresponds to that of the southwestern extended gas in the 13 CO map. These two structures are interfacing with each other, suggesting a compressive motion as the origin of the straight structure (Figure 20 ). Such a compressive motion is consistent with the decreasing velocity from southwest to northeast across the straight structure observed in 13 CO and C 18 O. In addition, the existence of an [H i ] cloud in the northeast of TMC-1FN is suggested by the archival data of the Galactic Arecibo L-band Feed Array H i (GALFA-H i ) Survey maps (Peek et al. 2011 ). Furthermore, the 6–10 km s −1 component traces a diffuse warm gas, whereas the 5.5 km s −1 component traces a cold and dense core illuminated by the FIR radiation from dust grains, according to our statistical equilibrium calculation. It is likely that the 5.5 km s −1 component traces cold cores surrounded by the warm envelope gas in HCL2. Figure 20. Schematic illustration of the cross section of TMC-1FN along the line of sight. The cold (∼20 K) and dense core (5 km s −1 ) is formed by compressive motion of the extended warm gas (6–10 km s −1 ). We observe the source from the right-hand side. The 6.5 and 8.2 km s −1 components show the 1612 MHz line absorption, in spite of the FIR pumping effect seen in the 5.5 km s −1 component. This can be explained by the proposed picture of TMC-1FN (Figure 20 ). A warm envelope gas traced by the 6.5 and 8.2 km s −1 components is likely to be illuminated by fainter FIR radiation than the gas traced by the 5.5 km s −1 component. The OH column density for the 6.5 and 8.2 km s −1 components is also lower. According to the results in Appendix C , the FIR intensity at 160 μ m is estimated to be ∼400 and 100 MJy sr −1 at positions 0.5 and 1.0 pc distant from the center of the TMC-1FN filament, respectively. The OH column densities are determined to be 3.6 × 10 14 cm −2 and 1.4 × 10 14 cm −2 for the 6.5 and 8.2 km s −1 components, respectively (Table 3 ). As described in Section 4.1.7 , our calculation shows that the FIR pumping effect is negligible for an OH column density lower than 10 15 cm −2 and I 160 lower than 100 MJy sr −1 . On the other hand, the 1612 MHz line indeed appears in absorption for I 160 lower than 4 × 10 3 MJy sr −1 , if the OH column density is 3 × 10 14 cm −2 (Figure 15 (d)). Hence, even though there might be
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