The KOI-13 system (Kepler Object of Interest system) comprises a pair of A-type White Dwarf stars 1630 light years from Earth, orbiting each to closely to be well differentiated. The larger of these, KOI-13α, has a mass 2.05 times that of the Sun, the smaller, KOI-13β, has a mass 1.95 times that of the Sun. In 2011 the Kepler Space Telescope discovered an object (KOI-13.01) orbiting one of these stars every 1.76 days. This object was discovered by the dimming it caused as it passed in front of the star, which meant it was possible to calculate its radius (2.2 times that of Jupiter), but not its mass, leaving scientists unsure whether the object was a very large planet or a Brown Dwarf. In February 2012, a team from the University of Cambridge published a model of the KOI-13 system which suggested that KOI-13.01 was likely to be a super-heated (and therefore super-inflated) Hot Jupiter type planet, with a mass 8.3 times that of Jupiter.
An artists impression of the KOI-13 system. Inset is a telescope image of the stars, scale bar is 1 arc-second. Konkoly Observatory.
In a paper published on the online arXiv database at Cornell University Library on 10 August 2012, and in the journal Astronomy & Astrophysics on 13 August 2012, a team of scientists led by Alexandre Santerne of the Laboratoire d’Astrophysique de Marseille and Observatoire de Haute-Provence at the Université d’Aix-Marseille & CNRS, detailing the results of a new study of the KOI-13 system using the SOPHIE Spectrograph at the Observatoire de Haute-Provence. This study used the radial velocity method, which measures the wobble of stars caused by the gravity of objects orbiting around them, to try to determine the mass of KOI-13.01.
Santerne et al. concluded that KOI-13.01 has a maximum mass of 14.8 times that of Jupiter if it orbits KOI-13α and 9.4 times that of Jupiter if it orbits KOI-13β, supporting the theory that this is a large Hot Jupiter type planet rather than a Brown Dwarf.
They also found evidence for an extra stellar-mass object within the system, which they name KOI-13γ. This appears to have a mass between 0.4 and 1.0 that of our Sun, and to orbit one of the stars every 65.8 days in an eccentric orbit that does not cross the plane of the star when seen from Earth. KOI-13γ could be potentially the same star as KOI-13.01 if it is towards the smaller end of this mass range, but the most likely scenario that could be modeled was that KOI-13.01 orbits KOI-13α, while KOI-13γ orbits KOI-13β.
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