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D. J. Hollenbach (NASA Ames Research Center), A. Parravano (Universidad de Los Andes), C. F. McKee (University of California, Berkeley)
We examine the formation of massive stars in the Galaxy, the resultant fluctuating UV radiation field, and the effect of this field on the star- forming interstellar medium. There are substantial fluctuations of the UV radiation field in space (scales of 100's of parsecs) and time (time- scales of order 100 million years) at the solar circle. The FUV (6 eV< h\nu < 13.6 eV) field and the pressure determines whether the thermal balance of the neutral gas results in cold clouds or warm (T~104 K) neutral medium. We show how to calculate the average fractions of the gas in the cold and warm phases when the interstellar gas is subject to this fluctuating FUV field. The knowledge of how these fractions depend on the gas properties and on the FUV sources is a basic step in building a model of the large scale behavior of the ISM and the mutual relation between the ISM and the star formation rate. Application is made to observations of spiral galaxies which correlate the star formation rate per unit area with the surface density of the interstellar gas. We acknowledge support from the NASA Astrophysical Theory program.