Lensing of Stars by Spherical Gas Clouds

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Abstract
If an appreciable fraction of the mass of the Galaxy is contained in cold self-gravitating gas clouds of ~Jovian mass and ~AU size, these clouds will act as converging lenses for optical light, and will magnify background stars at a detectable rate. The resulting light curves closely resemble those due to gravitational lensing by a point mass, raising the possibility that some of the events attributed to gravitational microlensing might in fact be due to ``gaseous lensing''. During a lensing event, the light from the imaged star would be reddened due to Rayleigh scattering, and the lens would contribute narrow infrared and far-red H_2 absorption lines which could be detected in the stellar spectrum. Existing programs to observe gravitational microlensing, supplemented by spectroscopy in the near-infrared or far-red, can therefore be used to either detect such events or place limits on the number of such gas clouds present in the Galaxy.

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