EXTENDING THE MACHO SEARCH TO SIMILAR-TO-10(6) M-CIRCLE-DOT

The search for a microlensing (changing light-curve) signature of massive compact halo objects (Machos) by the Macho Collaboration is currently believed to be sensitive in the range 10(-7)-10(2) M.. Microlensing events from higher mass objects last longer than the 4 yr duration of the planned experiments and therefore, according to current beliefs, cannot be distinguished from long-term variables. In fact the signature of Machos in the range 10(2)-10(3) M. can be distinguished from background events by the annual modulation in light magnification induced by the Earth's motion. For Machos in the range 10(3)-10(6) M., Hubble Space Telescope (HST), or even ground-based measurements can resolve the split lensed images, thus confirming the lens interpretation of an event. If the HST's optics were repaired, it could resolve images for Machos greater than or similar to 300 M.. The lower mass limit can be reduced to 4 x 10(-9) M. by conducting 1 month of rapid repeat observations of a single field. The standard view is that a Macho light curve yields only one physically relevant parameter, the time scale of the event. The time scale is a combination of the four parameters one would like to know: the mass, the distance, and the two components of transverse velocity of the Macho. I show that for masses 4-100 M., annual parallax oscillations in the light curve can be used to determine the transverse velocity. In the range 10(-3)-10(6) M. such measurements can be made using a small special-purpose satellite telescope. For masses 10(3)-10(6) M., one may determine all four Macho parameters by combining a number of techniques.