TRANSITION BETWEEN ATOMIC AND MOLECULAR-HYDROGEN IN THE GALAXY - VERTICAL VARIATION OF THE MOLECULAR FRACTION

We derive radial and vertical distributions of H I and H-2 gas densiti es in our Galaxy by using the terminal velocity method. We calculate t he molecular fraction (f(mol)) defined as the ratio of the molecular h ydrogen to total hydrogen gas density at galactic longitude l = 33 deg rees similar to 64 degrees and galactic latitude b = -2 degrees simila r to +2 degrees. The thickness of the molecular dominant region (f(mol ) greater than or equal to 0.8) is approximately constant (109 +/- 12 pc) at galactocentric distance R similar or equal to 4.7-7.2 kpc. The molecular fraction decreases suddenly at a critical height from the ga lactic plane, below which the gas disk is almost totally molecular, wh ile 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 H I and H-2 gases in the interstellar ma tter.