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Title:
Are the ogle/macho microlenses in a heavy spheroid?
Authors:
Evans, N. W.
Affiliation:
AA(Theoretical Physics, Department of Physics, Oxford, UK)
Publication:
Astrophysical Journal, Part 2 - Letters (ISSN 0004-637X), vol. 445, no. 2, p. L105-L108 (ApJL Homepage)
Publication Date:
06/1995
Category:
Astrophysics
Origin:
STI
NASA/STI Keywords:
Brown Dwarf Stars, Dark Matter, Galactic Bulge, Galactic Evolution, Galactic Structure, Gravitational Lenses, Missing Mass (Astrophysics), Star Distribution, Stellar Evolution, Mass Distribution, Optical Thickness, Radial Distribution, Stellar Systems
DOI:
10.1086/187900
Bibliographic Code:
1995ApJ...445L.105E

Abstract

It is sometimes suggested that the Galaxy possesses a massive spheroid largely composed of low-mass stars and brown dwarfs. We investigate whether this is a possible origin of the high optical depth to microlensing toward the Galactic center reported by the OGLE/MACHO collaborations. A model of the inner Galaxy is built in which the bulge, spheroid, and disk provide almost all of the local circular speed. The microlensing observables are calculated including the effects of foreground and background contamination. We find an optical depth to microlensing of approximately 2 x 10-6 toward Baade's Window - within the experimental bounds. The model suggests that the OGLE should have found about nine events in its first two years of monitoring, which it did. But the distribution of timescales peaks at approximately 6 days and has a mean of approximately 20 days. These are shorter than implied by the data. The profusion of short events is caused by spheroid mass functions that rise steeply toward the faint end. If this discrepancy remains as the experiments continue, it may rule out a massive spheroid as a major constituent of the Galaxy.

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