THE OH ROTATIONAL POPULATION AND PHOTODISSOCIATION OF H2O IN DG Tauri

We analyze the OH rotational emission in the Spitzer Space Telescope mid- infrared spectrum of the T Tauri star DG Tau. OH is observed in emission from upper level energies of 1900 K to 28,000 K. The rotational diagram cannot be fit with any single combination of temperature and column density and has slopes that correspond to excitation temperatures ranging from 200 K to 6000 K. The relative A-doublet population within each rotational level is not equal, showing that the OH population is not in thermal equilibrium. The symmetric A-doublet state is preferred in all rotational states, with an average of 0.5 for the population ratio of the anti-symmetric to symmetric state. We show that the population distribution of the high rotational lines and the A-doublet ratio are consistent with the formation of OH following the photo- dissociation of H2O by FUV photons in the second absorption band of water (similar to 1150-1400 angstrom), which includes Ly alpha. Other processes, OH formation from either photo-dissociation of water in the first absorption band (1450-1900 angstrom) or the reaction O(D-1) + H-2, or collisional excitation, cannot explain the observed emission in the high rotational states but could potentially contribute to the population of lower rotational levels. These results demonstrate that the photo-dissociation of water is active in DG Tau and support the idea that the hot rotational OH emission commonly observed in Classical T Tauri stars is due to the dissociation of H2O by FUV radiation.