SOPHIE velocimetry ofKeplertransit candidates

Abstract

We report the discovery of a hot Jupiter transiting a subgiant star with an orbital period of 6.87 days. This discovery was aided by public photometric data from the Kepler space mission and new radial velocity observations obtained by the SOPHIE spectrograph. The planet KOI-428b, with a radius of 1.17 ± 0.04 R Jup and a mass of 2.2 ± 0.4 M Jup , orbits an F5IV star with R = 2.13 ± 0.06 R , M = 1.48 ± 0.06 M , and T eff = 6510 ± 100 K. The star KOI-428 is the largest and the most evolved star discovered so far with a transiting planet.

SOPHIE velocimetry ofKeplertransit candidates

“…(3) in Casagrande et al (2010) assuming [Fe/H] = −0.3 dex, we derived an effective temperature T eff = 6380 ± 150 K. This estimate agrees with our spectroscopic determination. As already mentioned for KOI-428 (Santerne et al 2011a), the infrared determination of the effective temperature from Casagrande et al (2010), based on stars of luminosity class IV and V, seems to be more reliable than the one used for the Kepler Input Catalog.…”

“…The SOPHIE follow-up of four Kepler transiting candidates, selected with the aim of privileging massive planets and brown dwarfs, revealed at least three real massive transiting companions with Jupiter sizes: KOI-428b, a 2.2 M Jup transiting planet (Santerne et al 2011a); KOI-552b, a ∼0.17 M eclipsing lowmass star; and KOI-423b, a 18.0 M Jup transiting companion. The status of the fourth candidate has not yet been solved, but we can exclude a companion heavier than 3.4 M Jup , and we still may suspect a possible background eclipsing binary.…”

“…Our first target KOI-428 was revealed as a transiting hot Jupiter with a radius of 1.17 ± 0.04 R Jup and a mass of 2.2 ± 0.4 M Jup orbiting the largest and the most evolved star (2.13 R and 1.48 M ) discovered so far with a transiting exoplanet. This planet KOI428b, which also corresponds to the first Kepler planet established from the public data, was described by Santerne et al (2011a).…”

“…Our two SOPHIE spectra unfortunately have too low a signalto-noise ratio (S/N) to determine the spectral type of KOI-552. From J & K magnitude and following the same procedure as for KOI-428b (Santerne et al 2011a) and KOI-423b (see Sect. 6.2 in this paper), we estimate T eff = 6560 ± 150 K assuming an [Fe/H] = −0.47 as provided in the KIC catalog, a value significantly higher than the estimation given by Borucki et al (2011), T eff = 6018 K. Assuming a solar metallicity changes the estimated T eff within the uncertainties.…”

“…Following Kipping & Bakos (2011) and Santerne et al (2011a), the transit modeling was performed with a temporal sampling five times denser than that of Kepler, i.e. one point every 5.88 min, and then binning the model samples to match the Kepler sampling rate.…”