Tuesday, July 14, 2015

Detection of a Brown Dwarf Orbiting a Sun-Like Star

Using a method known as gravitational microlensing, where the gravitational field of a foreground object distorts and magnifies the light from a background star, Ranc et al. (2015) present the discovery of MOA-2007-BLG-197Lb, a brown dwarf in orbit around a Sun-like star. Brown dwarfs are substellar objects that occupy the mass range between gas giant planets and the least massive stars. In fact, MOA-2007-BLG-197Lb is the first brown dwarf to be detected around a Sun-like star through gravitational microlensing. Previous detections of brown dwarf companions to stars through gravitational microlensing have all been detections of brown dwarfs around diminutive red dwarf stars. MOA-2007-BLG-197Lb is a brown dwarf with 41 ± 2 times the mass of Jupiter and it orbits a Sun-like star with 0.82 ± 0.04 times the Sun’s mass at an observed projected separation of 4.3 ± 0.1 AU (i.e. one AU is the distance of Earth from the Sun).

Both MOA-2007-BLG-197Lb and its host star are located at a distance of roughly 14,000 light years from Earth. MOA-2007-BLG-197Lb is a valuable find because there appears to be a lack of brown dwarfs around Sun-like stars. One reason could be because brown dwarfs orbiting Sun-like stars lose angular momentum due to tidal interactions with their host stars, causing the brown dwarfs to spiral in and fall into their host stars. Nonetheless, gravitational microlensing is a great technique for detecting free-floating brown dwarfs, brown dwarf binaries and brown dwarf companions to stars. Future detections of brown dwarfs around Sun-like stars will offer better insight on the population distribution of brown dwarfs.

Light curve of the gravitational microlensing event that led to the detection of MOA-2007-BLG-197Lb and the best-fit model (solid line). The insert on the right shows a zoom-in of the “spike” in the light curve generated by the presence of MOA-2007-BLG-197Lb. Ranc et al. (2015).

Ranc et al. (2015), “MOA-2007-BLG-197: Exploring the brown dwarf desert”, arXiv:1505.06037 [astro-ph.EP]