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tokens: between W (CH) and A v than those between W (OH), W (CO) and A v . To better examine these relationships, we adopt the usual definition of “ X -factor”,where N H2 is the column density of H 2 and W is the integrated intensity of the molecular tracers CH, OH, CO. We plot the correlation of “ X -factor” of CH, OH and CO versus the visual extinction in Figure 8 . When the visual extinction A v is in the range between 0.8 and 2.1 mag, the X CH -factor is almost a constant (1.0 ± 0.06) × 10 22 cm −2 K −1 km −1 s. The dispersion of CH X -factor is visibly less than those of OH and CO at 0.8 ≤ A v ≤ 2.1 mag. CH appears to be a better tracer of molecular gas than CO and OH in the transition zone (0.8 ≤ A v ≤ 2.1 mag). Where extinction drops below 1 mag in TBR, the integrated intensities W of the three molecules lie above the overall fitting lines, indicating a stronger intensity of these spectra, which is likely the result of C-shock in the TBR-O (Section 5 ). Figure 8. Correlation plot of the integrated intensity W of CH, OH and CO vs. the visual extinction A v , and the correlation of the “ X -factor” of CH, OH and CO vs. the visual extinction A v . “ r ” means Pearson correlation coefficient. 5. CH ABUNDANCE AND C-SHOCK MODEL We have calculated the column density of CH in Section 3.2 . The column density of CH across the TBR is shown in Figure 7 . Mattila ( 1986 ) found a good correlation between the CH column density and visual extinction for diffuse, dark molecular cloud N (CH) = 3.4 × 10 13 ( A v − 03) cm −2 as the dashed line shows in Figure 7 . This correlation is reasonable considering the CH formation threshold, which requires substantial H 2 (Black Dalgarno 1973 ). The derived column density of CH below 1 mag extinction (blue shade in Figure 7 ) in TBR obviously lies above the typical value observed by Mattila ( 1986 ). Furthermore, considering the correction factor for excitation temperature and optical depth (as discussed in Section 3.2 ), the actual column density of CH in TBR-O is likely to be even larger by an additional factor of 3. The true CH column density in TBR-O can thus be one order of magnitude higher than the simple extension from the X -factor for higher extinctions. We compared the CH column density with the prediction of C-shock models. Flower Pineau des Forets ( 1998 ) made a prediction that CH column density would increase from 10 13 to 10 14 cm −2 when extinction increases from 0.1 to 1 mag at a C-shock front, which matches exactly the measured values here. Xu et al. ( 2016 ) also found an overabundance of OH below 1 mag extinction (by a factor of 80). The overabundance of CH and OH suggests that there may be an additional channel of CH and OH production, possibly due to the shock (e.g., Draine Katz 1986a ) produced by the colliding streams (Xu et al. 2016 , Figure 4). When shock waves propagate through the molecular ISM, the gas is compressed, heated, and accelerated. CH can be produced during CH + synthesis