ORFEUS II echelle observations of molecular hydrogen in the Galactic halo

The far-ultraviolet spectra of HD 219188, HD 94473, and HD 18100, three ear ly-type stars in the Galactic halo, were obtained with the echelle spectrom eter on the ORFEUS II telescope in 1996 November. We derive H-2 column dens ities for the rotational levels up to J " = 6 and estimate excitation tempe ratures and H-2 dissociation rates for the clouds toward HD 219188 and HD 9 4473. The results indicate that strong radiation fields might exist near th ese clouds, but these radiation fields do not seem to act as major heating sources. No significant H-2 absorption is seen toward HD 18100. The correla tion between our H-2 column densities and published extinction maps confirm s that the molecular clouds are associated with dust clouds that lie betwee n the observed stars and the Sun. We estimate the conversion factor between N(H-2) and W((CO)-C-12) to be X-CO = 4 +/- 2 x 10(19) cm(-2) (K km s(-1))( -1) for the clouds along the line of sight to HD 94473, consistent with pre vious estimates to within a factor of a few.