The Effect of Far-infrared Radiation on the Hyperfine Anomaly of the OH
  18 cm Transition
  We present observations of the four hyperfine structure components of the OH
18 cm transition (1612, 1665, 1667 and 1720 MHz) toward Heiles Cloud 2 (HCL2)
in Taurus and the dense cores L183 and L169. Toward the peculiar straight
structure in the northern part of HCL2, the 1720 MHz line shows absorption
against the cosmic microwave background at a velocity of $\sim$ 5.5 km
s$^{-1}$, whereas the 1612 MHz line shows brighter emission than expected in
local thermodynamic equilibrium (LTE). Such an intensity anomaly is also
observed toward TMC-1 (CP), L183, and L169, where the 1612 MHz line is stronger
and the 1720 MHz line is weaker than those expected under LTE. We conduct
statistical equilibrium calculations considering the effect of far-infrared
(FIR) radiation from surrounding clouds and find that the absorption feature of
the 1720 MHz line can be reproduced by the following conditions: gas kinetic
temperature lower than about 30 K, OH column density higher than 10$^{15}$
cm$^{-2}$, H$_2$ density lower than 10$^3$ cm$^{-3}$, and an ortho-to-para
ratio of H$_2$ lower than 1. Therefore, the absorption feature of the 1720 MHz
line is considered to trace relatively cold and dense gas that is surrounded by
a warm envelope consisting of CO-dark molecular gas.
