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MACHO velocities from satellite-based parallaxes
Gould, Andrew
AA(Ohio State University, Columbus, OH, US)
Astrophysical Journal, Part 2 - Letters (ISSN 0004-637X), vol. 421, no. 2, p. L75-L78 (ApJL Homepage)
Publication Date:
NASA/STI Keywords:
Astrometry, Dark Matter, Galactic Bulge, Gravitational Lenses, Magellanic Clouds, Applications Of Mathematics, Light Curve, Photometry, Satellite Observation, Solar Orbits, Space Infrared Telescope Facility, Velocity
Bibliographic Code:


Observation of microlensing events by two satellite telescopes in conjunction with Earth-based observations can measure three of the four physical parameters of the lenses. By making these measurements, one can (1) determine whether the lenses are in fact massive compact halo objects (MACHOs) in the Galactic halo or are objects in a thin or thick disk; (2) determine whether and how fast the halo is rotating; (3) identify individual lensing events that arise from Large Magellanic Cloud (LMC) MACHOs and so determine the density, flatness, and rotation speed of the LMC halo; (4) better contrain the distribution of low mass stars that give rise to lensing events toward the Galactic bulge. Earth-based observations alone yield only one combination of the physical parameters, the timescale, omega-1 = square root of (4GMDOL(1 - DOL/DOS)/(absolute value of nu)/(c)). Here M and DOL are the mass and distance of the lens, and v represents the two components of its transverse velocity. The distance to the source DOS is known. Observation by two satellites yields in addition the two components of the 'reduced' transverse velocity, tilde nu identically equal to (1 - DOL/DOS)-1nu. The telescopes should have apertures of d greater than or approximately 0.15 m and should be placed in solar orbit about 0.6 AU and 0.3 AU from the Earth. One or both should have eccentricity e approximately 0.3 and inclination i approximately 20 deg. A few hours of observations per month on the proposed Space Infrared Telescope Facility could substitute for one of the telescopes. Observation by a single satellite can measure tilde nu up to a fourfold ambiguity, including a twofold ambiguity in the reduced speed, tilde nu. However, it is possible to design a single-satellite experiment which could give good statistical information about tilde nu.

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