Brown dwarfs, white knights, and demons

This Letter investigates the hypothesis that the lensing objects toward the Large Magellanic Cloud are brown dwarfs by analyzing the effects of veloci ty anisotropy on the inferred microlensing masses. To reduce the masses, th e transverse velocity of the lenses with respect to the microlensing tube m ust be minimized. In the outer halo, radial anisotropy is best for doing th is; closer to the solar circle, azimuthal anisotropy is best. By using a co nstraint on the total kinetic energy of the tracer population from the Jean s equations, the microlensing mass is minimized over orientations of the ve locity dispersion tensor. This minimum mass is greater than or similar to 0 .1 M., which lies above the hydrogen-burning limit. This demonstrates expli citly that populations of brown dwarfs with smoothly decreasing densities a nd dynamically mixed velocity distributions cannot be responsible for the m icrolensing events. Brown dwarfs are no white knights! There is one caveat. If there are demons sitting on the microlensing tube, they can drop brown dwarfs so as to reproduce the microlensing data set exactly. Such a distrib ution is not smooth and does not give well-mixed velocities in phase space. It is a permissible solution only if the outer halo is dynamically young a nd lumpy. In such a case, theorists cannot rule out brown dwarfs. Only exor cists can!