Correlation study of carbon ions and hydroxyl molecules toward galactic nebulae

New observations of the C 92a recombination line and of the 2 (3i2), j =3/2 rotational transition of OH are reported in the direction of the five strongest carbon-line sources: W3A, W10, W12, W22, and W5 1A. For every carbon line, there is an OH absorbing feature with a very similar radial velocity; a linear-regression analysis yields a correlation coefficient of 0.999. The widths of the velocity correlated features are also related. The functional form of the linewidth relation suggests a separation of the carbon ions (at the periphery) and the OH molecules (in the core) within the predominantly neutral region. Depletion of charged-particle abundance in the region of the OH molecules allows the short-range collisional excitation rate W(OH-neutrals) to dominate the longer range rate W(OH-ions). For total densities n(H + 2H2) > /cm3, the excitation of the A-doublet state of the OH molecule remains largely unresponsive to radiative processes. For the shielded, central cores of the predominantly neutral regions, we conclude that the kinetic temperature of the OH molecules must be 5 K, considerably less than the canonical value of 30-100 K usually employed.