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Astron. Astrophys. 319, 1-6 (1997)

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Transition between atomic and molecular hydrogen in the Galaxy: vertical variation of the molecular fraction

K. Imamura and Y. Sofue

Institute of Astronomy, University of Tokyo, Mitaka, Tokyo 181, Japan (e-mail: sofue@mtk.ioa.s.u-tokyo.ac.jp)

Received 28 December 1995 / Accepted 22 June 1996


We derive radial and vertical distributions of H i and H2 gas densities in our Galaxy by using the terminal velocity method. We calculate the molecular fraction ([FORMULA]) defined as the ratio of the molecular hydrogen to total hydrogen gas density at galactic longitude [FORMULA] and galactic latitude [FORMULA]. The thickness of the molecular dominant region [FORMULA] is approximately constant ([FORMULA] pc) at galactocentric distance [FORMULA] 4.7-7.2 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 [FORMULA] variation can be reproduced by a model which takes into account the phase transition between H i and H2 gases in the interstellar matter.

Key words: ISM: general; atoms; molecules – galaxy: structure

Send offprint requests to: Y. Sofue

© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998