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Register a new userThe 0.5M$_J$ transiting exoplanet WASP-13b
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We report the discovery of WASP-13b, a low-mass $ M_p = 0.46 ^{+ 0.06}_{- 0.05} M_J$ transiting exoplanet with an orbital period of $4.35298 pm 0.00004$ days. The transit has a depth of 9 mmag, and although our follow-up photometry does not allow us to constrain the impact parameter well ($0 < b < 0.46$), with radius in the range $R_p sim 1.06 - 1.21 R_J$ the location of WASP-13b in the mass-radius plane is nevertheless consistent with H/He-dominated, irradiated, low core mass and core-free theoretical models. The G1V host star is similar to the Sun in mass (M$_{*} = 1.03^{+0.11}_ {- 0.09} M_{odot}$) and metallicity ([M/H]=$0.0pm0.2$), but is possibly older ($8.5^{+ 5.5}_{- 4.9}$ Gyr).
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WASP-13b is a sub-Jupiter mass exoplanet orbiting a G1V type star with a period of 4.35 days. The current uncertainty in its impact parameter (0 < b < 0.46) resulted in poorly defined stellar and planetary radii. To better constrain the impact parameter we have obtained high precision transit observations with the RISE instrument mounted on 2.0 m Liverpool Telescope. We present four new transits which are fitted with an MCMC routine to derive accurate system parameters. We found an orbital inclination of 85.2 pm 0.3 degrees resulting in stellar and planetary radii of 1.56 pm 0.04 Rodot and 1.39 pm 0.05 RJup, respectively. This suggests that the host star has evolved off the main-sequence and is in the shell hydrogen-burning phase. We also discuss how the limb darkening affects the derived system parameters. With a density of 0.17{rho}J, WASP-13b joins the group of low density planets whose radii are too large to be explained by standard irradiation models. We derive a new ephemeris for the system, T0 = 2455575.5136 pm 0.0016 (HJD) and P = 4.353011 pm 0.000013 days. The planet equilibrium temperature (Tequ = 1500 K) and the bright host star (V = 10.4 mag) make it a good candidate for follow-up atmospheric studies.
We report the discovery of a low-density exoplanet transiting an 11th magnitude star in the Southern hemisphere. WASP-15b, which orbits its host star with a period P=3.7520656+-0.0000028d has a mass M_p=0.542+-0.050M_J and radius R_p=1.428+-0.077R_J, and is therefore the one of least dense transiting exoplanets so far discovered (rho_p=0.247+-0.035g cm^-3). An analysis of the spectrum of the host star shows it to be of spectral type around F5, with an effective temperature T_eff=6300+-100K and [Fe/H]=-0.17+-0.11.
We present Rossiter-McLaughlin observations of WASP-13b and WASP-32b and determine the sky-projected angle between the normal of the planetary orbit and the stellar rotation axis ($lambda$). WASP-13b and WASP-32b both have prograde orbits and are consistent with alignment with measured sky-projected angles of $lambda={8^{circ}}^{+13}_{-12}$ and $lambda={-2^{circ}}^{+17}_{-19}$, respectively. Both WASP-13 and WASP-32 have $T_{mathrm{eff}}<6250$K and therefore these systems support the general trend that aligned planetary systems are preferentially found orbiting cool host stars. A Lomb-Scargle periodogram analysis was carried out on archival SuperWASP data for both systems. A statistically significant stellar rotation period detection (above 99.9% confidence) was identified for the WASP-32 system with $P_{mathrm{rot}}=11.6 pm 1.0 $ days. This rotation period is in agreement with the predicted stellar rotation period calculated from the stellar radius, $R_{star}$, and $v sin i$ if a stellar inclination of $i_{star}=90^{circ}$ is assumed. With the determined rotation period, the true 3D angle between the stellar rotation axis and the planetary orbit, $psi$, was found to be $psi=11^{circ} pm 14$. We conclude with a discussion on the alignment of systems around cool host stars with $T_{mathrm{eff}}<6150$K by calculating the tidal dissipation timescale. We find that systems with short tidal dissipation timescales are preferentially aligned and systems with long tidal dissipation timescales have a broad range of obliquities.
Secondary eclipses are a powerful tool to measure directly the thermal emission from extrasolar planets, and to constrain their type and physical parameters. We started a project to obtain reliable broad-band measurements of the thermal emission of transiting exoplanets. Ground-based high-cadence near-infrared relative photometry was used to obtain sub-millimagnitude precision light curve of a secondary eclipse of WASP-4b -- a 1.12 M_J hot Jupiter on a 1.34 day orbit around G7V star. The data show a clear ~10-sigma detection of the planets thermal emission at 2.2 mu m. The calculated thermal emission corresponds to a fractional eclipse depth of 0.185^{+0.014}_{-0.013}%, with a related brightness temperature in Ks of T_B = 1995 pm 40 K, centered at T_C = 2455102.61162^{+0.00071}_{-0.00077} HJD. We could set a limit on the eccentricity of e cos omega=0.0027 pm 0.0018, compatible with a near-circular orbit. The calculated brightness temperature, as well as the specific models suggest a highly inefficient redistribution of heat from the day-side to the night-side of the planet, and a consequent emission mainly from the day-side. The high-cadence ground-based technique is capable of detecting the faint signal of the secondary eclipse of extrasolar planets, making it a valuable complement to space-based mid-IR observations.
The Transiting Exoplanet Survey Satellite (TESS) will search for planets transiting bright and nearby stars. TESS has been selected by NASA for launch in 2017 as an Astrophysics Explorer mission. The spacecraft will be placed into a highly elliptical 13.7-day orbit around the Earth. During its two-year mission, TESS will employ four wide-field optical CCD cameras to monitor at least 200,000 main-sequence dwarf stars with I = 4-13 for temporary drops in brightness caused by planetary transits. Each star will be observed for an interval ranging from one month to one year, depending mainly on the stars ecliptic latitude. The longest observing intervals will be for stars near the ecliptic poles, which are the optimal locations for follow-up observations with the James Webb Space Telescope. Brightness measurements of preselected target stars will be recorded every 2 min, and full frame images will be recorded every 30 min. TESS stars will be 10-100 times brighter than those surveyed by the pioneering Kepler mission. This will make TESS planets easier to characterize with follow-up observations. TESS is expected to find more than a thousand planets smaller than Neptune, including dozens that are comparable in size to the Earth. Public data releases will occur every four months, inviting immediate community-wide efforts to study the new planets. The TESS legacy will be a catalog of the nearest and brightest stars hosting transiting planets, which will endure as highly favorable targets for detailed investigations.
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