When a foreground object crosses the line of sight to a background object, the gravitational field of the foreground object can bend light from the background object, resulting in an observable magnification of the background object. This astrophysical phenomenon is known as gravitational microlensing. The foreground object is referred to as the “lens” and the background object is referred to as the “source”. Gravitational microlensing does not depend on the brightness of the “lens”, and so enables the detection of faint or even dark foreground objects that happen to pass in front of luminous background sources.
Figure 1: Artist’s impression of a very low-mass binary system comprised of what could be a brown dwarf circling a low-mass star.
Figure 2: Light-curve of OGLE-2013-BLG-0102. Jung et al. (2014).
As part of the Optical Gravitational Lensing Experiment (OGLE), Jung et al. (2014) present the analysis of a gravitational microlensing event. The “lens” object, identified as OGLE-2013-BLG-0102, turns out to be a very low-mass binary system with a mass-ratio of 0.13 between the two components. The primary (i.e. the more massive component) and secondary (i.e. the less massive component) have estimated masses of 0.097 ± 0.011 and 0.013±0.002 solar-masses, respectively. This places the primary at the star/brown-dwarf boundary and the secondary at the brown-dwarf/planet boundary. It is generally accepted that the division between low-mass stars and brown dwarfs is ~0.075 solar-masses, while the division between brown dwarfs and giant planets is ~0.012 solar-masses.
The projected separation between the primary and secondary is 0.80 ± 0.04 AU, indicating OGLE-2013-BLG-0102 is a close-separation very low-mass binary system. Furthermore, OGLE-2013-BLG-0102 is estimated to lie at a distance of ~10,000 light years. At such a distance, it would not have been detectable by other means, demonstrating that gravitational microlensing is a useful technique for detecting close-separation very low-mass binary systems. OGLE-2013-BLG-0102 is located in a sparsely populated region of parameter space (Figure 3). Other close-separation very low-mass binary systems that were detected via gravitational microlensing include OGLE-2009-BLG-151L, OGLE-2011-BLG-420L and OGLE-2012-BLG-0358L.
Figure 3: Total mass versus separation (left panel) and primary versus secondary masses (right panel) for a compilation of low-mass binaries. Microlensing binaries are denoted in ‘star’ marks while those discovered by other methods are marked by ‘dots’. The ‘red star’ is OGLE-2013-BLG-0102 and the three ‘blue stars’ are the binaries OGLE-2009-BLG-151L, OGLE-2011-BLG-420L and OGLE-2012-BLG-0358L. The vertical and horizontal dashed lines represent the star/brown-dwarf and brown-dwarf/planet boundaries, respectively. Jung et al. (2014).
Jung et al. (2014), “OGLE-2013-BLG-0102La,b: Microlensing binary with components at star/brown-dwarf and brown-dwarf/planet boundaries”, arXiv:1407.7926 [astro-ph.SR]