tokens: et al. 2014 ). When the collision velocity is assumed to be the velocity difference between the stem and bowl region (∼1.5 km s −1 ), the kinetic energy released by the collision is estimated to be erg cm −3 , where the H 2 density of 10 3 cm −3 is arbitrarily assumed. This can be balanced by the cooling rate by Goldsmith Langer ( 1978 ), and the equilibrium temperature is roughly estimated to be 100 K. Here, we assume the cooling time of 10 4 yr according to Inoue Inutsuka ( 2012 ). This temperature is comparable with that observed in the interacting region. Thus, this rough estimate implies that the slight temperature enhancement is possible in the interacting region. Warm gas with a temperature of 74 K, heated by the filament–filament collision, is cooled down mainly by rotational line emission of CO for an H 2 density of 10 3 cm −3 (Inoue Inutsuka 2012 ). As mentioned above, the cooling time () is estimated to be around 10 4 yr according to Koyama Inutsuka ( 2000 ) with a cooling rate of 10 −26 erg cm −3 s −1 (Inoue Inutsuka 2012 ). Assuming that this warm gas expands with a constant velocity of 1.5 km s −1 , which corresponds to the typical velocity difference between the blueshifted and redshifted components of the Pipe nebula of 3.0 km s −1 and 4–5 km s −1 , respectively, the gas can extend in space by about 0.015 pc within the cooling time. This length corresponds to a typical spatial scale of a shocked warm region, and hence we hereafter denote it as (=0.015 pc). On the other hand, the extent of the cloud along the line of sight () can roughly be estimated from the H 2 density ( n (H 2 )), the OH column density ( N (OH)), and the OH fractional abundance ( X (OH)) relative to H 2 with the following equation:We employ an H 2 density of 10 3 cm −3 and an OH column density of 5.3 × 10 13 cm −2 , the latter of which is determined for the (0′, 32′) position (Table 5 ). Assuming an OH abundance of 10 −7 , which is a typical value in diffuse clouds (Wiesemeyer et al. 2012 ), is estimated to be ∼0.2 pc. If the OH fractional abundance is one order of magnitude higher ( X (OH) = 10 −6 ), which could be possible in a shocked gas according to Draine Katz ( 1986 ), could be 0.02 pc. The latter value ( ∼ 0.02 pc) for the shocked gas case ( X (OH) = 10 −6 ) is comparable to the of 0.015 pc. This result suggests the presence of shock-induced compression at the interface of the two filaments, and supports the filament–filament collisions picture in the Pipe nebula. 7. Summary We have observed the four hfs components of the OH 18 cm transitions toward the Pipe nebula along the two strip lines and determined the gas kinetic temperatures accurately. Along strip-1, the gas kinetic temperature is found to have no relation to the apparent distance to the nearby star, θ -Oph. This result suggests that the heating effect from θ -Oph does not significantly contribute to the heating of the Pipe nebula, although a possibility that θ -Oph illuminates the
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