Annu. Rev. Astron. Astrophys. 1994. 32: 531-590 Copyright © 1994 by Annual Reviews. All rights reserved

7.7. Microlensing of Stars by Halo Objects in our own Galaxy

Attempts to detect microlensing by objects in our own halo by looking for intensity variations in stars in the Magellanic Clouds and the Galactic Bulge have now been underway for several years and may already have met with success. In this case, the timescale for the variation is P = 0.2(M / M)^1/2 y, so one can seek lenses over the mass range 10^-8-10² M, but the probability of an individual star being lensed is only ~ 10^-6, so one has to look at many stars for a long time (Paczynski 1986). The likely event rate is ~ N P^-1 ~ (M / M)^-1/2y^-1, where N ~ 10⁶ is the number of stars. Thus, small masses give frequent short-duration events (e.g. 0.01 M events would last a week and occur a few times a year) and are best sought with CCDs, white large masses give rare long-duration events (e.g. 10 M events would last a year and occur every few years) and are best sought with photographic plates. The key feature of these microlensing events is that the light-curves are time-symmetric and achromatic; this may allow them to be distinguished from intrinsic stellar variations (Griest 1991).

Three groups are involved; each now claims to have detected lensing events. The American group (MACHO) has used a dedicated telescope at Mount Stromlo to study 10⁷ stars in red and blue light in the LMC, the SMC, and the Galactic Bulge. After analyzing 4 fields near the center of the LMC (2 × 10⁶ stars with 250 observations per star), they have obtained one event (Alcock et al 1993): The duration is 34 days (corresponding to a mass of 0.1 M) and the amplification is A = 6.8. The French group (EROS) has been studying stars in the LMC and their approach is two-pronged: They are seeking 1-100 day events (corresponding to 10^-4-1 M lenses) with digitized red and blue Schmidt plates obtained with the ESO telescope in Chile and 1 hour to 3 day events (corresponding to 10^-7-10^-3 M with CCDs taken at the Observatoire de Haute Provence. The CCD searches have given no results, which presumably implies a limit _C(10^-7-10^-3 M) < 0.1, but analysis of 3 × 10⁶ stars on the Schmidt plates yields two events (Auborg et al 1993): One is associated with a main-sequence star and has A = 2.5 and P = 54 d (corresponding to a mass of 0.2 M); the other is associated with a star between the main-sequence and the giant branch and has A = 3.3 and P = 60 d (corresponding to a mass of 0.3 M). They have also confirmed the MACHO event in red light. The Polish collaborative (OGLE) are using the Las Companas telescope in Chile to look at 7 × 10⁵ stars in the Galactic bulge (Udalski et al 1993). They have claimed one event with A = 2.4 and P = 42 d (corresponding to a mass of 0.3 M) which they attribute to a disk M-dwarf, but they only have data in one color. The rough values of M and _c for these events are indicated in Figure 5, but there is considerable uncertainty in both these values.