P - 9 b : A LOW DENSITY PLANET TRANSITING A MODERATELY FAINT F STAR 1

We report the discovery of a planet transiting a moderately faint (V = 12.3 mag) late F star, with an orbital period of 3.92289 ± 0.00004 days. From the transit light curve and radial velocity measurements we determine that the radius of the planet is R p = 1.40 ± 0.06 R Jup and that the mass is M p = 0.78 ± 0.09 M Jup. The density of the new planet, ρ p = 0.35 ± 0.06 g cm −3 , fits to the low-density tail of the currently known transiting planets. We find that the center of transit is at Transiting extra-solar planets are important astrophys-ical objects as they allow to test planetary structure and evolution theory (e.g., Fortney 2008a; Burrows et al. 2008) especially because they yield a measurement of the planetary mass and radius. Over the last few years, the sample of known transiting planets has grown substantially , leading to an improved theoretical understanding of their physical nature (e.g., Guillot et al. 2006; Burrows et al. 2007; Fortney et al. 2008b). In addition , the increasing sample of transiting planets enabled the discovery of some interesting correlations, such as the mass-period relation (Mazeh, Zucker, & Pont 2005; Gaudi et al. 2005). The astrophysics behind these correlations is not fully understood, and a clear way towards progress is the discovery of many more transiting planets. We report here the discovery of another transiting planet detected by the HATNet project 11 (Bakos et al. 2002, 2004), labeled HAT-P-9b, and our determination of its parameters, including mass, radius, density and surface gravity. In § 2 we describe our photometric and spectroscopic observations and in § 3 we derive the stellar parameters. The orbital solution is performed in § 4 and a discussion of possible blend scenarios is brought 1 Based in part on radial velocities obtained with the SOPHIE spectrograph mounted on the 1.93m telescope at the Observatory of Haute Provance (runs 07A.PNP.MAZE, 07B.PNP.MAZE, 08A.PNP.MAZE). Fig. 1.— The top panel shows the unbinned HATNet light curve with 6884 data points, phased with the orbital period of P = 3.92289 d. The binned light curve is over-plotted. The center of the transit is at phase zero. The bottom panel shows a zoom-in around the transit. The HATNet light curve is folded with the ephemeris obtained here, based on the follow-up light curves (see § 2.4). The small offset between …