OH 18 cm Transition as a Thermometer for Molecular Clouds
  We have observed the four hyperfine components of the 18 cm OH transition
toward the translucent cloud eastward of Heiles Cloud 2 (HCL2E), the cold dark
cloud L134N, and the photodissociation region of the $\rho$-Ophiuchi molecular
cloud with the Effelsberg 100 m telescope. We have found intensity anomalies
amongst the hyperfine components in all three regions. In particular, an
absorption feature of the 1612 MHz satellite line against the cosmic microwave
background has been detected toward HCL2E and two positions of the
$\rho$-Ophiuchi molecular cloud. On the basis of statistical equilibrium
calculations, we find that the hyperfine anomalies originate from the non-LTE
population of the hyperfine levels, and can be used to determine the kinetic
temperature of the gas over a wide range of H$_2$ density (10$^2$ - 10$^7$
cm$^{-3}$). Toward the center of HCL2E, the gas kinetic temperature is
determined to be 53$\pm$1 K, and it increases toward the cloud peripheries
($\sim$ 60 K). The ortho-to-para ratio of H$_2$ is determined to be 3.5 $\pm$
0.9 from the averaged spectrum for the 8 positions. In L134N, a similar
increase of the temperature is also seen toward the periphery. In the
$\rho$-Ophiuchi molecular cloud, the gas kinetic temperature decreases as a
function of the distance from the exciting star HD147889. These results
demonstrate a new aspect of the OH 18 cm line as a good thermometer of
molecular cloud envelopes. The OH 18 cm line can be used to trace a new class
of warm molecular gas surrounding a molecular cloud, which is not well traced
by emission of CO and its isotopologues.
