M ar 2 00 8 XO-3 b : A Massive Planet in an Eccentric Orbit Transiting an F 5 V Star

We report the discovery of a massive (Mpsini= 13.02± 0.64 MJ; total mass 13.25± 0.64 MJ), large (1.95 ± 0.16 RJ) planet in a transiting, eccentric orbit (e = 0.260 ± 0.017) around a 10 magnitude F5V star in the constellation Camelopardalis. We designate the planet XO-3b, and the star XO-3, also known as GSC 03727-01064. The orbital period of XO-3b is 3.1915426± 0.00014 days. XO-3 lacks a trigonometric distance; we estimate its distance to be 260±23 pc. The radius of XO-3 is 2.13±0.21 R⊙, its mass is 1.41±0.08 M⊙, its vsini = 18.54 ± 0.17 km s, and its metallicity is [Fe/H] = −0.177± 0.027. This system is unusual for a number of reasons. XO-3b is one of the most massive planets discovered around any star for which the orbital period is less than 10 days. The mass is near the deuterium burning limit of 13 MJ, which is a proposed boundary between planets and brown dwarfs. Although Burrows et al. (2001) propose that formation in a disk or formation in the interstellar medium in a manner similar to stars is a more logical way to differentiate planets and brown dwarfs, our current observations are not adequate to address this distinction. XO-3b is also unusual in that its eccentricity is large given its relatively short orbital period. Both the planetary radius and the inclination are functions of the spectroscopically determined stellar radius. Analysis of the transit light curve of XO-3b suggests that the spectroscopically derived parameters may be over estimated. Though relatively noisy, the light curves favor a smaller radius in order to better match the steepness of the ingress and egress. The light curve fits imply a planetary radius of 1.25±0.15 RJ, which would correspond to a mass of 12.03± 0.46 MJ. A precise trigonometric parallax measurement or a very accurate light curve is needed to resolve the uncertainty in the planetary mass and radius. Subject headings: binaries: eclipsing – planetary systems – stars: individual (GSC 03727-01064) – techniques: photometric – techniques: radial velocities Dept. of Physics and Astronomy, Rice University, 6100 Main Street, MS-108, Houston, TX 77005 Visiting Astronomer, McDonald Observatory, which is operated by the University of Texas at Austin. Space Telescope Science Institute, 3700 San Martin Dr., Baltimore MD 21218 Boston University, Astronomy Dept., 725 Commonwealth Ave., Boston, MA 02215 University of Hawaii, Inst. for Astronomy, 2680 Woodlawn Dr., Honolulu, HI 96822 Lowell Observatory, 1400 West Mars Hill Road, Flagstaff, AZ, 86001 Racoon Run Observatory, 1142 Mataro Court, Pleasanton, CA 94566 Volunteer Observatory, 10305 Mantooth Lane, Knoxville, TN 37932 Vermillion Cliffs Observatory, 4175 E. Red Cliffs Drive, Kanab, UT 84741 Esteve Duran Observatory, El Montanya, Seva, 08553 Seva, Barcelona, Spain Hereford Arizona Observatory, 5320 E. Calle Manzana, Hereford, AZ 85615 Campo Catino Astronomical Observatory, P.O. BOX