Portal de Periódicos da CAPES

Brown Dwarfs, White Knights, and Demons

1998; IOP Publishing; Volume: 502; Issue: 1 Linguagem: Inglês

10.1086/311478

1538-4357

G. Gyuk, N. W. Evans, Evalyn Gates,

History and Developments in Astronomy

This Letter investigates the hypothesis that the lensing objects toward the Large Magellanic Cloud are brown dwarfs by analyzing the effects of velocity anisotropy on the inferred microlensing masses. To reduce the masses, the transverse velocity of the lenses with respect to the microlensing tube must be minimized. In the outer halo, radial anisotropy is best for doing this; closer to the solar circle, azimuthal anisotropy is best. By using a constraint on the total kinetic energy of the tracer population from the Jeans equations, the microlensing mass is minimized over orientations of the velocity dispersion tensor. This minimum mass is ≳0.1 M☉, which lies above the hydrogen-burning limit. This demonstrates explicitly that populations of brown dwarfs with smoothly decreasing densities and dynamically mixed velocity distributions cannot be responsible for the microlensing 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 distribution 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 and lumpy. In such a case, theorists cannot rule out brown dwarfs. Only exorcists can!