Friday, December 25, 2015

KIC 5621294 and its Circumbinary Substellar Companion

Figure 1: Artist’s impression of a brown dwarf as seen from a hypothetical planet in a close-in orbit around the brown dwarf.

KIC 5621294 is an eclipsing binary system consisting of two stars in a tight orbit that causes them to partially eclipse one another. The primary star has 1.95 times the mass and 0.99 times the radius of the Sun, and the secondary star has 0.43 times the mass and 1.39 times the radius of the Sun. Both stars orbit around each other every 0.9389 days. Given enough time, the eclipsing binary system of KIC 5621294 will evolve into a contact configuration as the system continues to lose angular momentum. A contact configuration is one where both stars are in direct contact with each other. Photometric observations by NASA’s Kepler space telescope reveal the presence of variations in the eclipse timings. These timing variations indicate the presence of a third-body in a circumbinary orbit around KIC 5621294. Gravitational effects from the third-body perturb the central binary, resulting in the eclipse timing variations.

The third-body is estimated to have at least 46.9 times the mass of Jupiter and the effect it has on the central binary indicates an orbital period of 961 days. However, the actual mass of the third-body will depend on its orbital inclination. A face-on orbit has an inclination of zero degrees and an edge-on orbit has an inclination of 90 degrees. As long as the orbital inclination of the third-body is more than about 40 degrees, it will be in the mass regime of brown dwarfs. However, if its orbital inclination is less than about 40 degrees, then its mass will be large enough for it to be a low-mass hydrogen-burning star. As seen from the third-body, the luminosity of the central binary will be dominated by the primary star which is over 30 times more luminous than the secondary star.

Jae Woo Lee et al. (2014), “The Kepler Eclipsing System KIC 5621294 and its Substellar Companion”, arXiv:1412.7258 [astro-ph.SR]