Wednesday, May 6, 2015

A Warm Gas Giant Planet Circling a Fast-Spinning Star

Bourrier et al. (2015) present the discovery of a transiting warm gas giant planet identified as KOI-12b. It was first detected by NASA’s Kepler space telescope. Follow-up observations were performed using the SOPHIE (Spectrographe pour l’Observation des Phénomènes des Intérieurs stellaires et des Exoplanètes) échelle spectrograph on the 1.93 m reflector telescope at the Haute-Provence Observatory in south-eastern France. KOI-12b has 1.43 ± 0.13 times the diameter of Jupiter and a 3σ upper mass limit of about 10 times the mass of Jupiter. The diameter of KOI-12b was directly measured from the amount of starlight the planet blocks as it transits in front of its host star.

Figure 1: Artist’s rendition of a gas giant planet.

KOI-12b orbits its host star at a distance of about 0.14 AU, with an orbital period of 17.86 days. The host star of KOI-12b is a fast-rotating star (spins with a speed of at least 60 km/s) with 1.45 ± 0.09 times the mass and 1.63 ± 0.15 times the diameter of the Sun. Furthermore, it is a relatively hot star with an effective surface temperature of 6820 ± 120 K. For comparison, the Sun has an effective temperature of 5778 K. The larger size and higher surface temperature means that the host star of KOI-12b is at least ~5 times more luminous than the Sun.

The distance of KOI-12b from its host star suggests that it is a moderately irradiated warm gas giant planet with an estimated temperature of just over 1000 K. However, the diameter of KOI-12b appears inflated given the moderate stellar irradiation it receives. KOI-12b is the largest exoplanet known that has an orbital distance greater than 0.1 AU and the host star of KOI-12b is one of the hottest known to host an exoplanet. Both KOI-12b and its host star are located at a distance of about 1400 light-years away.

Figure 2: Transit light curve of KOI-12b. The best fit to the data is displayed as a red line, with residuals in the lower panel. Bourrier et al. (2015).

Bourrier et al. (2015), “SOPHIE velocimetry of Kepler transit candidates XVI. Tomographic measurement of the low obliquity of KOI-12b, a warm Jupiter transiting a fast rotator”, arXiv:1504.04130 [astro-ph.EP]