Figure 1: Artist’s impression of a brown dwarf. Heat from its warm interior “leaks” out through gaps in its cloud coverage. A cool brown dwarf would resemble Jupiter more than it would resemble a star.
Brown dwarfs are substellar objects that span the gap between the most massive planets and the least massive stars. Like stars, brown dwarfs can also come in pairs. Using a technique known as gravitational microlensing, Choi et al. (2013) reported the discovery of two pairs of very low-mass binary brown dwarfs identified as OGLE-2009-BLG-151 and OGLE-2011-BLG-0420. Gravitational microlensing is observed when the gravity of an intervening object (lens) magnifies the light from a background star (source). It happens as the intervening object crosses the line-of-sight between the observer and the background star.
OGLE-2009-BLG-151 was first detected by the Optical Gravitational Lensing Experiment (OGLE) group and then independently detected by the Microlensing Observations in Astrophysics (MOA) group in 2009, hence its alternate designation - “MOA-2009-BLG-232”. The other gravitational microlensing event, OGLE-2011-BLG-0420, was detected by the OGLE group in 2011. A number of ground-based telescopes also provided follow-up observations for both gravitational microlensing events.
Figure 2: Light-curves of the binary brown dwarf gravitational microlensing events OGLE-2009-BLG-151 and OGLE-2011-BLG-0420. Choi et al. (2013).
The light-curve of OGLE-2009-BLG-151 is characterised by two prominent spikes, consistent with a binary-lens model. Based on the light-curve, OGLE-2009-BLG-151 is inferred to be a tightly-bound pair of very low-mass brown dwarfs with masses 0.018 and 0.0075 times the Sun’s mass. The two brown dwarfs are projected to be 0.31 AU, or 46 million km apart from each other. That is equal to Mercury’s closest distance to the Sun. For comparison, the average Earth-Sun separation distance is 1 AU, or 149.6 million km.
As for OGLE-2011-BLG-0420, its light-curve appears smooth and symmetric. However, upon careful observations, the light-curve shows noticeable deviations indicative of a binary-lens model rather than a single-lens model. Similar to OGLE-2009-BLG-151, OGLE-2011-BLG-0420 consists of two very low-mass brown dwarfs in a tight binary system. The two brown dwarfs are 0.025 and 0.0094 times the Sun’s mass, and are spaced only 0.19 AU, or 28 million km apart. In fact, the two brown dwarfs of OGLE-2011-BLG-0420 are spaced as far apart as Jupiter’s outermost moons are from Jupiter.
The total system masses of OGLE-2009-BLG-151 and OGLE-2011-BLG-0420 are 0.025 and 0.034 times the Sun’s mass, respectively, placing them well below the hydrogen-burning limit of ~0.08 times the Sun’s mass. What makes the discovery of OGLE-2009-BLG-151 and OGLE-2011-BLG-0420 interesting is that the two systems have among the lowest total system masses known for brown dwarf binaries. Additionally, OGLE-2009-BLG-151 and OGLE-2011-BLG-0420 are also the tightest known brown dwarf binaries. The discovery of these two systems among the relatively small sample of gravitational microlensing events involving binary systems shows that tightly-bound pairs of very low-mass brown dwarfs are not uncommon.
Figure 3: Projected separation versus total system mass for a compilation of binaries. Grey circles indicate old field binaries, whereas blue squares indicate young (< 500 million year old) systems. The size of the symbols is proportional to the square root of the mass ratio (i.e. the ratio of the less massive component to the more massive component in the binary system). The red stars correspond to OGLE-2009-BLG-151 and OGLE-2011-BLG-0420. Choi et al. (2013).
Figure 4: Binding energy versus total system mass for the same binaries as shown in Figure 3. Choi et al. (2013).
Choi et al. (2013), “Microlensing Discovery of a Population of Very Tight, Very Low-mass Binary Brown Dwarfs”, arXiv:1302.4169 [astro-ph.SR]