Philip Yock
, PASA, 17 (1), 35.
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Title/Abstract Page: Observations from Australasia using
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Dark Matter
The study of dark matter, and in particular brown dwarfs, was the first application of the gravitational microlensing technqiue to be proposed, and it motivated the early observations by the pioneering groups in the field, viz. MACHO, EROS and OGLE. The technique is particularly suited to the study of brown dwarfs or similar dark objects because it does not rely on optical detection. Paczynski (1986) showed that, if the Galactic halo is composed of brown dwarfs, then approximately one star in a million in the Magellanic Clouds would be lensed at any one time by a brown dwarf for a durationdays. Here M denotes the mass of the brown dwarf, which is
. Hence the expected duration for these lensing events is 40 days or less. The MACHO group has now monitored several million stars in the Magellanic Clouds for several years. Amongst their database of LMC stars to 1997, no events were found with duration less than 20 days. A similar result was obtained by the EROS group working from Chile. These results were combined to yield a 95% confidence upper limit on the halo-mass-fraction of low-mass brown dwarfs with masses in the range
(Alcock et al. 1998). This result eliminates low-mass brown dwarfs, planets that have been ejected from planetary systems, and any other compact non-luminous objects in the stated mass range as a significant component of halo dark matter for standard models of the halo.
. The EROS group did not detect any events caused by halo objects in this mass range in the direction of the SMC (Afonso et al. 1999a). The MACHO group has, however, recorded several events in this mass range in their database towards the LMC, corresponding to a halo fraction of of objects with mass
(Alcock et al. 1997a). The result, which is marginally consistent with the above result of the EROS group, was generally unexpected. It has proven difficult to interpret. The above results are shown collectively in Figure 1. This figure also includes data from Gilmore and Unavane (1998) and the MOA group (Abe et al. 1999) that were obtained by surface photometry of external galaxies. The surface photometry data apply to main-sequence stars, i.e. to red dwarfs. Similarly restrictive limits on the abundance of red dwarfs in the Galactic halo have been obtained from studies of the Hubble deep field (Graff & Freese 1996, Flynn, Gould & Bahcall 1996). It is clear that the objects being detected by the MACHO group cannot be red dwarfs. Two types of interpretations of the data have been proposed. The first assumes a halo comprised mainly of old white dwarfs (Alcock et al. 1997a) or of primordial black holes (Nakamura 1998). The white dwarf hypothesis requires the initial mass function to be strongly peaked at
, to enable the white dwarfs to cool sufficiently to escape detection in other surveys (Chabrier et al. 1996), and it leaves unexplained the metal enrichment that would be expected to accompany them (Gibson and Mould 1997). The primordial black hole hypothesis requires their mass function to be peaked at
, surprisingly close to the mass of a normal star. Both these interpretations may be tested in the future by independent observations. Direct observations of white dwarfs should be possible if they are a major component of the halo (Chabrier 1999), and gravitational mergers of primordial black holes may be detectable if primordial black holes are a major component (Nakamura 1998).
Next Section: Galactic Bar
Title/Abstract Page: Observations from Australasia using
Previous Section: Introduction
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