P. F. L. Maxted

Thermal emission at 4.5 and 8 μm of WASP-17b, an extremely large planet in a slightly eccentric orbit

We report the detection of thermal emission at 4.5 and 8 mu m from the planet WASP-17b. We used Spitzer to measure the system brightness at each wavelength during two occultations of the planet by its host star. By combining the resulting light curves with existing transit light curves and radial-velocity measurements in a simultaneous analysis, we find the radius of WASP-17b to be 2.0R(Jup), which is 0.2R(Jup) larger than any other known planet and 0.7R(Jup) larger than predicted by the standard cooling theory of irradiated gas giant planets. We find the retrograde orbit of WASP-17b to be slightly eccentric, with 0.0012 < e < 0.070 (3 sigma). Such a low eccentricity suggests that, under current models, tidal heating alone could not have bloated the planet to its current size, so the radius of WASP-17b is currently unexplained. From the measured planet-star flux-density ratios we infer 4.5 and 8 mu m brightness temperatures of 1881 +/- 50 and 1580 +/- 150 K, respectively, consistent with a low-albedo planet that efficiently redistributes heat from its day side to its night side.

WASP-10b: a 3MJ, gas-giant planet transiting a late-type K star

We report the discovery of WASP-10b, a new transiting extrasolar planet (ESP) discovered by the WASP Consortium and confirmed using NOT FIES and SOPHIE radial velocity data. A 3.09 day period, 29 mmag transit depth, and 2.36 hour duration are derived for WASP-10b using WASP and high precision photometric observations. Simultaneous fitting to the photometric and radial velocity data using a Markov-chain Monte Carlo procedure leads to a planet radius of 1.28RJ, a mass of 2.96MJ and eccentricity of �0.06. WASP-10b is one of the more massive transiting ESPs, and we compare its characteristics to the current sample of transiting ESP, where there is currently little information for masses greater than �2MJ and non-zero eccentricities. WASP-10’s host star, GSC 2752-00114 (USNO-B1.0 1214-0586164) is among the fainter stars in the WASP sample, with V=12.7 and a spectral type of K5. This result shows promise for future late-type dwarf star surveys.

The MUCHFUSS project – searching for hot subdwarf binaries with massive unseen companions - Survey, target selection and atmospheric parameters

The project Massive Unseen Companions to Hot Faint Underluminous Stars from SDSS (MUCHFUSS) aims at finding sdBs with compact companions like supermassive white dwarfs (M > 1.0 M� ), neutron stars or black holes. The existence of such systems is predicted by binary evolution theory and recent discoveries indicate that they are likely to exist in our Galaxy. A determination of the orbital parameters is sufficient to put a lower limit on the companion mass by calculating the binary mass function. If this lower limit exceeds the Chandrasekhar mass and no sign of a companion is visible in the spectra, the existence of a massive compact companion is proven without the need for any additional assumptions. We identified about 1100 hot subdwarf stars from the SDSS by colour selection and visual inspection of their spectra. Stars with high velocities have been reobserved and individual SDSS spectra have been analysed. In total 127 radial velocity variable subdwarfs have been discovered. Binaries with high RV shifts and binaries with moderate shifts within short timespans have the highest probability of hosting massive compact companions. Atmospheric parameters of 69 hot subdwarfs in these binary systems have been determined by means of a quantitative spectral analysis. The atmospheric parameter distribution of the selected sample does not differ from previously studied samples of hot subdwarfs. The systems are considered the best candidates to search for massive compact companions by follow-up time resolved spectroscopy.

WASP-10b: A 3M , gas-giant planet transiting a late-type K star

We report the discovery of WASP-10b, a new transiting extrasolar planet (ESP) discovered by the WASP Consortium and confirmed using NOT FIES and SOPHIE radial velocity data. A 3.09 day period, 29 mmag transit depth, and 2.36 hour duration are derived for WASP-10b using WASP and high precision photometric observations. Simultaneous fitting to the photometric and radial velocity data using a Markov-chain Monte Carlo procedure leads to a planet radius of 1.28RJ, a mass of 2.96MJ and eccentricity of �0.06. WASP-10b is one of the more massive transiting ESPs, and we compare its characteristics to the current sample of transiting ESP, where there is currently little information for masses greater than �2MJ and non-zero eccentricities. WASP-10’s host star, GSC 2752-00114 (USNO-B1.0 1214-0586164) is among the fainter stars in the WASP sample, with V=12.7 and a spectral type of K5. This result shows promise for future late-type dwarf star surveys.

Spitzer 3.6 and 4.5 μm full-orbit light curves of WASP-18

We present new light curves of the massive hot Jupiter system WASP-18 obtained with the Spitzer spacecraft covering the entire orbit at 3.6 and 4.5 μm. These light curves are used to measure the amplitude, shape and phase of the thermal phase effect for WASP-18 b. We find that our results for the thermal phase effect are limited to an accuracy of about 0.01 per cent by systematic noise sources of unknown origin. At this level of accuracy we find that the thermal phase effect has a peak-to-peak amplitude approximately equal to the secondary eclipse depth, has a sinusoidal shape and that the maximum brightness occurs at the same phase as mid-occultation to within about 5 ◦ at 3.6 μm and to within about 10 ◦ at 4.5 μm. The shape and amplitude of the thermal phase curve imply very low levels of heat redistribution within the atmosphere of the planet. We also perform a separate analysis to determine the system geometry by fitting a light curve model to the data covering the occultation and the transit. The secondary eclipse depths we measure at 3.6 and 4.5 μm are in good agreement with previous measurements and imply a very low albedo for WASP-18 b. The parameters of the system (masses, radii, etc.) derived from our analysis are also in good agreement with those from previous studies, but with improved precision. We use new high-resolution imaging and published limits on the rate of change of the mean radial velocity to check for the presence of any faint companion stars that may affect our results. We find that there is unlikely to be any significant contribution to the flux at Spitzer wavelengths from a stellar companion to WASP-18. We find that there is no evidence for variations in the times of eclipse from a linear ephemeris greater than about 100 s over 3 years.

WASP‐14b: 7.3 MJ transiting planet in an eccentric orbit

We report the discovery of a 7.3 MJ exoplanet WASP-14b, one of the most massive transiting exoplanets observed to date. The planet orbits the tenth-magnitude F5V star USNO-B1 11118-0262485 with a period of 2.243752 days and orbital eccentricity e = 0.09. A simultaneous fit of the transit light curve and radial velocity measurements yields a planetary mass of 7.3±0.5 MJ and a radius of 1.28±0.08 RJ. This leads to a mean density of about 4.6 gcm 3 making it densest transiting exoplanets yet found at an orbital period less than 3 days. We estimate this system to be at a distance of 160±20 pc. Spectral analysis of the host star reveals a temperature of 6475±100 K, log g = 4.07 cms 2 and v sin i = 4.9±1.0 kms 1 , and also a high lithium abundance, log N(Li) = 2.84±0.05. The stellar density, effective temperature and rotation rate suggest an age for the system of about 0.5–1.0 Gyr.

A survey for low-mass spectroscopic binary stars in the young clusters around σ Orionis and λ Orionis

We have obtained multi-epoch, high-resolution spectroscopy of 218 candidate low-mass stars and brown dwarfs (BDs) in the young clusters around a Ori and \ Ori. We find that 196 targets are cluster members based on their radial velocity, the equivalent width of their Na Ι 8200 lines and the spectral type from their TiO band strength. We have identified 11 new binary stars among the cluster members based on their variable radial velocity and an additional binary from the variation in its linewidth and shape. Of these, six are double-lined spectroscopic binaries (SB2) where the components of the binary are of comparable brightness. The others are single-lined binaries (SB 1) in which the companion is faint or the spectra of the stars are blended. There are three narrow-lined SB 1 binaries in our sample for which the companion is more than 2.5 mag fainter than the primary. This suggests that the mass ratio distribution for the spectroscopic binaries in our sample is broad but that there may be a peak in the distribution near q = 1. The sample covers the magnitude range I C = 14-18.9 (mass ≈0.55-0.03 M ⊙ ), but all of the binary stars are brighter than I C = 16.6 (mass ≈30.12 M ⊙ ) and 10 are brighter than I C = 15.5 (mass ≈0.23 M ⊙ ). There is a significant lack of spectroscopic binaries in our sample at faint magnitudes even when we account for the decrease in sensitivity with increasing magnitude. We can reject the hypothesis that the fraction of spectroscopic binaries is a uniform function of I C magnitude with more than 99 per cent confidence. The spectroscopic binary fraction for stars more massive than about 0.1 M ⊙ (I C < 16.9) is f bright = 0.095 +0.012 -0.028 . The 90 per cent confidence upper limit to the spectroscopic binary fraction for very low-mass (VLM) stars (mass <0.1 M ⊙ ) and BDs is f faint < 7.5 per cent. The hypothesis that f bright and /faint are equal can be rejected with 90 per cent confidence. The average detection probability for our survey is 50 per cent or more for binaries with separations up to 0.28 au for stars with I C < 16.9 and 0.033 au for the fainter stars in our sample. We conclude that we have found strong evidence for a change in the fraction of spectroscopic binaries among young VLM stars and BDs when compared to more massive stars in the same star-forming region. This implies a difference in the total binary fraction between VLM stars and BDs compared to more massive stars or a difference in the distribution of semimajor axes, or both.

WASP‐25b: a 0.6 MJ planet in the Southern hemisphere

We report the detection of a 0.6 MJ extrasolar planet by WASP-South, WASP-25b, transiting its solar-type host star every 3.76 d. A simultaneous analysis of the WASP, FTS and Euler photometry and CORALIE spectroscopy yields a planet of Rp= 1.22 RJ and Mp= 0.58 MJ around a slightly metal-poor solar-type host star, [Fe/H]=− 0.05 ± 0.10, of R*= 0.92 R⊙ and M*= 1.00 M⊙. WASP-25b is found to have a density of ρp= 0.32 ρJ, a low value for a sub-Jupiter mass planet. We investigate the relationship of planetary radius to planetary equilibrium temperature and host star metallicity for transiting exoplanets with a similar mass to WASP-25b, finding that these two parameters explain the radii of most low-mass planets well.

Thermal emission at 3.6–8 μm from WASP-19b: a hot Jupiter without a stratosphere orbiting an active star

We report detection of thermal emission from the exoplanet WASP-19b at 3.6, 4.5, 5.8 and 8.0 µm. We used the InfraRed Array Camera on the Spitzer Space Telescope to observe two occultations of WASP-19b by its host star. We combine our new detections with previous measurements of WASP-19b’s emission at 1.6 and 2.09 µm to construct a spectral energy distribution of the planet’s dayside atmosphere. By comparing this with model-atmosphere spectra, we find that the dayside atmosphere of WASP-19b lacks a strong temperature inversion. As WASP-19 is an active star (logR ′ = 4.50± 0.03), this finding supports the hypothesis of Knutson, Howard & Isaacson (2010) that inversions are suppressed in hot Jupiters orbiting active stars. The available data are unable to differentiate between a carbon-rich and an oxygen-rich atmosphere.

Rotational period of WD 1953 - 011- A magnetic white dwarf with a star-spot

WD 1953-011 is an isolated, cool (7920 +/- 200K) magnetic white dwarf (MWD) with a low average field strength (~70kG), and a higher than average mass (~0.74M solar ). Spectroscopic observations taken by Maxted et al. showed variations of equivalent width in the Balmer lines, unusual in a low-field white dwarf. Here we present V-band photometry of WD 1953-011 taken at seven epochs over a total of 22 months. All of the data sets show a sinusoidal variation of approximately 2 per cent peak-to-peak amplitude. We propose that these variations are due to a star-spot on the MWD, analogous to a sunspot, which is affecting the temperature at the surface, and therefore its photometric magnitude. The variations have a best-fitting period over the entire 22 months of 1.4418 d, which we interpret as the rotational period of the white dwarf.