Lensing of Stars by Spherical Gas Clouds

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Abstract
Cold gas clouds of ~Jovian mass and ~AU size will act as lenses for optical light. If the Galaxy contains ~1011 M☉ in such clouds, background stars will be magnified at a detectable rate. The resulting light curves can resemble those due to gravitational lensing by a point mass, raising the possibility that some events attributed to gravitational lensing might instead be due to "gaseous lensing." During a lensing event, the lens would impose narrow infrared and far-red H2 absorption lines on the stellar spectrum. Existing programs to observe gravitational microlensing, supplemented by spectroscopy, can therefore be used to either detect such events or place limits on the number of such gas clouds present in the Galaxy.