Light centroid shifts in observed stars due to microlensing by intervening disk stars
Abstract
The light centroid shifts caused by microlensing due to intervening stars for given source objects is investigated, and their relevance to the upcoming astrometric space missions, such as SIM and GAIA, are discussed. It turns out that, despite the effect of the sun and the planets, the dominant contribution comes from the stars in the galactic disk. The probability that a disk star introduces a centroid shift larger than 0.7 micro-arcsec at a given time reaches 100% for a source towards the Baade's window, while this probability is about 2% for a centroid shifts larger than 10 micro-arcsec. However, this contribution of disk stars to the centroid shift is not likely to vary much during the observation time of a typical photometric microlensing event; so it is possible to unambiguously observe the variation of the centroid shift by the lens object which lead to the photometric signature. On the other hand, a survey using astrometric detection can be used to infer information about the disk stars, a population to which the photometric microlensing surveys are insensitive. For a bulge star towards the Baade's window, the probability for a variation larger than 5 micro-arcsec within one year is 10^{-3}, i.e. much larger than any probability for photometric microlensing. If one also requires a peak in the centroid shift (i.e. a peak in the component transverse to the motion between lens and source and a change of sign in the component parallel to it), this probability is 10^{-4}.Keywords
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