Wednesday, September 11, 2013

A Brown Dwarf on an Eccentric Orbit

Moutou et al. (2013) report the discovery of a long orbital period, highly eccentric transiting brown dwarf around a slightly evolved Sun-like star. This brown dwarf is identified as KOI-415 b and it was observed to transit 7 times in front of its host star by NASA’s Kepler space telescope over a period of 14 quarters. The transit of KOI-415 b has a period of 166.8 days, depth of 0.5 percent and duration of 6 hours. Transiting brown dwarfs are much less common than transiting giant planets and above 20 Jupiter-masses, only a handful of such objects are known.

Figure 1: Artist’s conception of a brown dwarf / gas giant.

 Figure 2: Top panel: phase-folded transit light curve of KOI-415 b. Bottom panel: the residuals between the observations and the best-fit model. (Moutou et al., 2013)

By measuring how much light from its host star gets blocked as KOI-415 b transits in front, the size of KOI-415 b is determined to be 0.79 times the diameter of Jupiter. As for the mass of KOI-415 b, it is determined by measuring the amount of gravitational tugging the brown dwarf exerts on its host star. The gravitational tugging from KOI-415 b causes its host star to wobble with an observed radial velocity semi-amplitude of 3.346 km/s. From this measurement, the mass of KOI-415 b is determined to be 62 times the mass of Jupiter. KOI-415 b is also estimated to be around 10 billion years old.

With a long orbital period of 166.8 days, the position of KOI-415 b on the mass-radius diagram shows that it more resembles an isolated brown dwarf than a heavily irradiated brown dwarf in a close-in orbit around its host star. This is because the radius of KOI-415 b perfectly fits the predicted radius for an isolated brown dwarf with the same age and mass. KOI-415 b is on a highly eccentric orbit around its host star, coming as close as 0.179 AU and swinging out as far as 1.006 AU. This causes the estimated dayside temperature of the brown dwarf to vary by as much as 400 K due to the changing stellar irradiation.

Figure 3: The mass-radius diagram in the domain between giant planets and low-mass stars. Isochrones for objects with ages of 10, 5, 1 and 0.1 billion years are shown for comparison. Colour symbols indicate the mass range: 15-25 Jupiter-mass (green), 37-40 Jupiter-mass (blue), 59-65 Jupiter-mass (black) and 89-97 Jupiter-mass (pink). (Moutou et al., 2013)

Figure 4: The radius as a function of system’s age for objects from 15 to 100 Jupiter-mass. BT-SETTL isochrones are shown for 0.02 to 0.09 solar-mass objects. An object of a given mass cools and contracts as it ages. Colour symbols indicate the mass range: 15-25 Jupiter-mass (green), 37-40 Jupiter-mass (blue), 59-65 Jupiter-mass (black) and 89-97 Jupiter-mass (pink). (Moutou et al., 2013)

Moutou et al. (2013), “SOPHIE velocimetry of Kepler transit candidates IX. KOI-415 b: a long-period, eccentric transiting brown dwarf to an evolved Sun”, arXiv:1309.0905 [astro-ph.EP]