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Session 42 - Structure and Evolution of The Universe.
Display session, Tuesday, June 09
Current microlensing data in the direction of the LMC indicate that in the context of a spherical isothermal model with a Maxwellian velocity distribution, some significant fraction of the Galactic halo is composed of MACHOs with masses roughly in the range 0.1 to 1.0 M\sun. Such masses are consistent with several astrophysical candidates for MACHOs -- white dwarfs, neutron stars, and black holes -- each of which presents serious challenges for stellar formation and evolution theories. However, the MACHO component of the halo, if it is not the major component, as in Cold Dark Matter scenarios, may have a very different distribution from the typically assumed spherical isothermal model. The MACHO distribution may be in a significantly flattened halo and/or, due to dissipation, more centrally condensed. In addition, such a distribution is likely to be anisotropic and may have a significant rotational component. The velocity dispersion and mass density distribution which describe the halo model are crucial input parameters in extracting an estimate of the lens mass from the data. We investigate several non-standard halo models and their implications for the lens mass derived from event timescales. We examine two classes of models with a bulk rotational component of the halo velocity distribution: a highly flattened 1/r^2 halo, and a spheroid-like population with whose density falls off as 1/r^3.5. The highly flattened 1/r^2 models can decrease the implied average MACHO mass only marginally and the spheroid models not at all. Generally, rotational models cannot bring the MACHO mass implied by the current microlensing data down to the sub-stellar range. We also analyze the effects of velocity anisotropy on the inferred microlensing masses and find that populations of brown dwarfs with smoothly decreasing densities and dynamically mixed velocity distributions cannot be responsible for the microlensing events.
Program listing for Tuesday