Constraints on Cold H2Clouds from Gravitational Microlensing Searches

It has been proposed that the Galaxy might contain a population of coldclouds in numbers sufficient to account for a substantial fraction of the totalmass of the Galaxy. These clouds would have masses of the order of 10^{-3}Solar mass and sizes of the order of 10 AU. We consider here the lensingeffects of such clouds on the light from background stars. A semianalyticalformalism for calculation of the magnification event rate produced by suchgaseous lensing is developed, taking into account the spatial distribution ofthe dark matter in the Galaxy, the velocity distribution of the lensing cloudsand source stars, and motion of the observer. Event rates are calculated forthe case of gaseous lensing of stars in the Large Magellanic Cloud and resultsare directly compared with the results of the search for gravitationalmicrolensing events undertaken by the MACHO collaboration. The MACHO experimentstrongly constrains the properties of the proposed molecular clouds, but doesnot completely rule them out. Future monitoring programs will either detect ormore strongly constrain this proposed population.Comment: 36 pages, 9 figures, 1 table, typos corrected, minor change