The optical transmission spectrum of the hot Jupiter HAT-P-32b: clouds explain the absence of broad spectral features?

We report Gemini-North GMOS observations of the inflated hot Jupiter HAT-P-32b during two primary transits. We simultaneously observed two comparison stars and used differential spectro-photometry to produce multi-wavelength light curves. White light curves and 29 spectral light curves were extracted for each transit and analysed to refine the system parameters and produce transmission spectra from 520-930nm in ~14nm bins. The light curves contain time-varying white noise as well as time-correlated noise, and we used a Gaussian process model to fit this complex noise model. Common mode corrections derived from the white light curve fits were applied to the spectral light curves which significantly improved our precision, reaching typical uncertainties in the transit depth of ~2x10^-4, corresponding to about half a pressure scale height. The low resolution transmission spectra are consistent with a featureless model, and we can confidently rule out broad features larger than about one scale height. The absence of Na/K wings or prominent TiO/VO features is most easily explained by grey absorption from clouds in the upper atmosphere, masking the spectral features. However, we cannot confidently rule out clear atmosphere models with low abundances (~10^-3 solar) of TiO, VO or even metal hydrides masking the Na and K wings. A smaller scale height or ionisation could also contribute to muted spectral features, but alone are unable to to account for the absence of features reported here.

Transit timing variations in WASP-10b induced by stellar activity?

The hot-Jupiter WASP-10b was reported by Maciejewski et al. (2011a,b) to show transit timing variations (TTV) with an amplitude of ~ 3.5 minutes. These authors proposed that the observed TTVs were caused by a 0.1 MJup perturbing companion with an orbital period of ~ 5.23 d, and hence, close to the outer 5:3 mean motion resonance with WASP-10b. To test this scenario, we present eight new transit light curves of WASP-10b obtained with the Faulkes Telescope North and the Liverpool Telescope. The new light curves, together with 22 previously published ones, were modelled with a Markov-Chain Monte-Carlo transit fitting code. (...) Our homogeneously derived transit times do not support the previous claimed TTV signal, which was strongly dependent on 2 previously published transits that have been incorrectly normalised. Nevertheless, a linear ephemeris is not a statistically good fit to the transit times of WASP-10b. We show that the observed transit time variations are due to spot occultation features or systematics. We discuss and exemplify the effects of occultation spot features in the measured transit times and show that despite spot occultation during egress and ingress being difficult to distinguish in the transit light curves, they have a significant effect in the measured transit times. We conclude that if we account for spot features, the transit times of WASP-10 are consistent with a linear ephemeris with the exception of one transit (epoch 143) which is a partial transit. Therefore, there is currently no evidence for the existence of a companion to WASP-10b. Our results support the lack of TTVs of hot-Jupiters reported for the Kepler sample.

The prevalence of dust on the exoplanet HD 189733b from Hubble and Spitzer observations

The hot Jupiter HD189733b is the most extensively observed exoplanet. Its atmosphere has been detected and characterised in transmission and eclipse spectroscopy, and its phase curve measured at several wavelengths. This paper brings together results of our campaign to obtain the complete transmission spectrum of the atmosphere of this planet from UV to IR with HST, using STIS, ACS and WFC3. We provide a new tabulation of the transmission spectrum across the entire visible and IR range. The radius ratio in each wavelength band was rederived to ensure a consistent treatment of the bulk transit parameters and stellar limb-darkening. Special care was taken to correct for, and derive realistic estimates of the uncertainties due to, both occulted and unocculted star spots. The combined spectrum is very different from the predictions of cloud-free models: it is dominated by Rayleigh scattering over the whole visible and near infrared range, the only detected features being narrow Na and K lines. We interpret this as the signature of a haze of condensate grains extending over at least 5 scale heights. We show that a dust-dominated atmosphere could also explain several puzzling features of the emission spectrum and phase curves, including the large amplitude of the phase curve at 3.6um, the small hot-spot longitude shift and the hot mid-infrared emission spectrum. We discuss possible compositions and derive some first-order estimates for the properties of the putative condensate haze/clouds. We finish by speculating that the dichotomy between the two observationally defined classes of hot Jupiter atmospheres, of which HD189733b and HD209458b are the prototypes, might not be whether they possess a temperature inversion, but whether they are clear or dusty. We also consider the possibility of a continuum of cloud properties between hot Jupiters, young Jupiters and L-type brown dwarfs.

High precision transit observations of the exoplanet WASP-13b with the RISE instrument

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.

A lower mass for the exoplanet WASP-21b

We present high precision transit observations of the exoplanet WASP-21b, obtained with the RISE instrument mounted on 2.0m Liverpool Telescope. A transit model is fitted, coupled with an MCMC routine to derive accurate system parameters. The two new high precision transits allow to estimate the stellar density directly from the light curve. Our analysis suggests that WASP-21 is evolving off the main sequence which led to a previous overestimation of the stellar density. Using isochrone interpolation, we find a stellar mass of 0.86 pm 0.04 Msun which is significantly lower than previously reported (1.01 pm 0.03 Msun). Consequently, we find a lower planetary mass of $0.27 pm 0.01 Mjup$. A lower inclination (87.4 pm 0.3 degrees) is also found for the system than previously reported, resulting in a slightly larger stellar (R_* =1.10 pm 0.03 Rsun) and planetary radius (R_p = 1.14 pm 0.04 Rjup). The planet radius suggests a hydrogen/helium composition with no core which strengthens the correlation between planetary density and host star metallicity. A new ephemeris is determined for the system, i.e., t0 =2455084.51974 pm 0.00020 (HJD) and P=4.3225060 pm 0.0000031 days. We found no transit timing variations in WASP-21b.

Ground-based detection of thermal emission from the exoplanet WASP-19b

We present an occultation of the newly discovered hot Jupiter system WASP-19, observed with the HAWK-I instrument on the VLT, in order to measure thermal emission from the planets dayside at ~2 um. The light curve was analysed using a Markov-Chain Monte-Carlo method to find the eclipse depth and the central transit time. The transit depth was found to be 0.366+-0.072 %, corresponding to a brightness temperature of 2540+-180 K. This is significantly higher than the calculated (zero-albedo) equilibrium temperature, and indicates that the planet shows poor redistribution of heat to the night side, consistent with models of highly irradiated planets. Further observations are needed to confirm the existence of a temperature inversion, and possibly molecular emission lines. The central eclipse time was found to be consistent with a circular orbit.

A transit timing analysis of seven RISE light curves of the exoplanet system HAT-P-3

We present seven light curves of the exoplanet system HAT-P-3, taken as part of a transit timing program using the RISE instrument on the Liverpool Telescope. The light curves are analysed using a Markov-Chain Monte-Carlo algorithm to update the parameters of the system. The inclination is found to be i = 86.75^{+0.22}_{-0.21} deg, the planet-star radius ratio to be R_p/R_star = 0.1098^{+0.0010}_{-0.0012}, and the stellar radius to be R_star = 0.834^{+0.018}_{-0.026} R_sun, consistent with previous results but with a significant improvement in the precision. Central transit times and uncertainties for each light curve are also determined, and a residual permutation algorithm used as an independent check on the errors. The transit times are found to be consistent with a linear ephemeris, and a new ephemeris is calculated as T_c(0) = 2454856.70118 +- 0.00018 HJD and P = 2.899738 +- 0.000007 days. Model timing residuals are fitted to the measured timing residuals to place upper mass limits for a hypothetical perturbing planet as a function of the period ratio. These show that we have probed for planets with masses as low as 0.33 M_earth and 1.81 M_earth in the interior and exterior 2:1 resonances, respectively, assuming the planets are initially in circular orbits.

A transit timing analysis of nine RISE light curves of the exoplanet system TrES-3

We present nine newly observed transits of TrES-3, taken as part of a transit timing program using the RISE instrument on the Liverpool Telescope. A Markov-Chain Monte-Carlo analysis was used to determine the planet-star radius ratio and inclination of the system, which were found to be Rp/Rstar=0.1664^{+0.0011}_{-0.0018} and i = 81.73^{+0.13}_{-0.04} respectively, consistent with previous results. The central transit times and uncertainties were also calculated, using a residual-permutation algorithm as an independent check on the errors. A re-analysis of eight previously published TrES-3 light curves was conducted to determine the transit times and uncertainties using consistent techniques. Whilst the transit times were not found to be in agreement with a linear ephemeris, giving chi^2 = 35.07 for 15 degrees of freedom, we interpret this to be the result of systematics in the light curves rather than a real transit timing variation. This is because the light curves that show the largest deviation from a constant period either have relatively little out-of-transit coverage, or have clear systematics. A new ephemeris was calculated using the transit times, and was found to be T_c(0) = 2454632.62610 +- 0.00006 HJD and P = 1.3061864 +- 0.0000005 days. The transit times were then used to place upper mass limits as a function of the period ratio of a potential perturbing planet, showing that our data are sufficiently sensitive to have probed for sub-Earth mass planets in both interior and exterior 2:1 resonances, assuming the additional planet is in an initially circular orbit.

Updated parameters for the transiting exoplanet WASP-3b using RISE, a new fast camera for the Liverpool Telescope

Some of the first results are reported from RISE - a new fast camera mounted on the Liverpool Telescope primarily designed to obtain high time resolution light curves of transiting extrasolar planets for the purpose of transit timing. A full and partial transit of WASP-3 are presented, and a Monte Carlo Markov Chain analysis is used to update the parameters from the discovery paper. This results in a planetary radius of 1.29^{+0.05}_{-0.12} R_J and therefore a density of 0.82^{+0.14}_{-0.09} p_J, consistent with previous results. The inclination is 85.07^{+0.15}_{-0.16} deg, in agreement (but with a significant improvement in the precision) with the previously determined value. Central transit times are found to be consistent with the ephemeris given in the discovery paper. However, a new ephemeris calculated using the longer baseline results in T_c(0) = 2454605.55915 +- 0.00023 HJD and P = 1.846835 +- 0.000002 days.