Has MACHO seen microlensing of a binary star?

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Session 33 -- Relativistic Astrophysics
Display presentation, Tuesday, 31, 1994, 9:20-6:30

[33.02] Has MACHO seen microlensing of a binary star?

T. Axelrod, C. Alcock, R. Allsman, D. Bennett, K. Cook (LLNL), K. Freeman, B. Peterson, P. Quinn, A. Rodgers (MSSSO), K. Griest, S. Marshall, S. Perlmutter, M. Pratt, C. Stubbs, W. Sutherland (CfPA)

The Macho project has now accumulated lightcurves for about 10 million stars in the LMC which span approximately two years and include dwarfs and giants with magnitudes down to about V = 20. In this paper, we consider the lightcurves for a sample of the brighter main sequence stars: most of them show emission lines in their spectra and sporadic asymmetric and chromatic fluctuations which are clearly intrinsic and not consistent with microlensing. Among these brighter main sequence stars, we might occasionally expect to see microlensing of a binary star system by a dark object, and we present here a possible example of such an event.

This particular lightcurve is well fit by a microlensing event in which a binary star system in the LMC is lensed by a dark object in the galactic halo with mass of order 0.1 solar masses. If this interpretation is correct, the projected separation of the binary is about 27 AU, and the two components have a luminosity ratio of about 2 with very similar colors. The V and R magnitudes of the object are both about 16.1. Spectra of the object have been taken at CTIO and the AAT, and are consistent with a binary composed of two main sequence B stars, an interpretation which is also consistent with the luminosity ratio and colors determined from the microlensing fit. No spectroscopic evidence is seen for a binary, nor would such be expected, since the orbital velocity is only about 7 km/sec. No emission lines are seen in the spectrum.

The luminosity and color of the object (or system) places it in the same regime as known Be variables. The distinguishing features of this variable are its lack of Balmer emission, achromaticity, and the good fit to a model of lensing of a binary with plausible parameters. It is clear that phenomena such as this which occur in regions of the C-L array where intrinsic variability is known can not be considered as contributing reliably to the statistics of halo lensing events. Nevertheless, binaries exist and lensing amplification of these unresolved systems in the LMC will occur.

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