Transition between Atomic and Molecular Hydrogen in the Galaxy: Vertical Variation of the Molecular Fraction

We derive radial and vertical distributions of HI and H2 gas densities in our Galaxy by using the terminal velocity method. We calculate the molecular fraction (f_mol) defined as the ratio of the molecular hydrogen to total hydrogen gas density at galactic longitude l= 33 to 64 deg and galactic latitude b=-2 to +2 deg. The thickness of the molecular dominant region f_mol > 0.8 is approximately constant (109 +/- 12 pc) at galactocentric distance R = 4.7 to 7.2 kpc. The molecular fraction decreases suddenly at a critical height from the galactic plane, below which the gas disk is almost totally molecular, while it is almost atomic beyond this height. We show that the vertical f_mol variation can be reproduced by a model which takes into account the phase transition between HI and H2 gases in the interstellar matter. Key words: Atomic Hydrogen --- the Galaxy --- Interstellar Matter --- Molecular Hydrogen