2021ApJS..252....1T

We have conducted a OH 18 cm survey toward 141 molecular clouds in various environments, including 33 optical dark clouds, 98 Planck Galactic cold clumps (PGCCs), and 10 Spitzer dark clouds with the Arecibo telescope. The deviations from local thermal equilibrium are common for intensity ratios of both OH main lines and satellite lines. Line intensity of OH 1667 MHz is found to correlate linearly with visual extinction A_V when A_V is less than 3 mag. It was converted into OH column density by adopting the excitation temperature derived from Monte Carlo simulations with 1σ uncertainty. The relationship between OH abundance X(OH) relative to H₂ and A_V is found to follow an empirical formula, X(OH)/10^–7=1.3_–0.4^+0.4+6.3_–0.5^+0.5×exp(-A_V/2.9_–0.6^+0.6). Linear correlation is found between OH and ¹³CO intensity. Besides, nonthermal velocity dispersions of OH and ¹³CO are closely correlated. These results imply tight chemical evolution and spatial occupation between OH and ¹³CO. No obvious correlation is found between column density and nonthermal velocity dispersion of OH and H I narrow self-absorption (HINSA), indicating different chemical evolution and spatial volume occupation between OH and HINSA. Using the age information of HINSA analysis, OH abundance X(OH) is found to increase linearly with cloud age, which is consistent with previous simulations. Fourteen OH components without corresponding CO emission were detected, implying the effectiveness of OH in tracing the "CO-dark" molecular gas.