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Register a new userThermal Phase Variations of WASP-12b: Defying Predictions
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N. B. Cowan
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[Abridged] We report Warm Spitzer full-orbit phase observations of WASP-12b at 3.6 and 4.5 micron. We are able to measure the transit depths, eclipse depths, thermal and ellipsoidal phase variations at both wavelengths. The large amplitude phase variations, combined with the planets previously-measured day-side spectral energy distribution, is indicative of non-zero Bond albedo and very poor day-night heat redistribution. The transit depths in the mid-infrared indicate that the atmospheric opacity is greater at 3.6 than at 4.5 micron, in disagreement with model predictions, irrespective of C/O ratio. The secondary eclipse depths are consistent with previous studies. We do not detect ellipsoidal variations at 3.6 micron, but our parameter uncertainties -estimated via prayer-bead Monte Carlo- keep this non-detection consistent with model predictions. At 4.5 micron, on the other hand, we detect ellipsoidal variations that are much stronger than predicted. If interpreted as a geometric effect due to the planets elongated shape, these variations imply a 3:2 ratio for the planets longest:shortest axes and a relatively bright day-night terminator. If we instead presume that the 4.5 micron ellipsoidal variations are due to uncorrected systematic noise and we fix the amplitude of the variations to zero, the best fit 4.5 micron transit depth becomes commensurate with the 3.6 micron depth, within the uncertainties. The relative transit depths are then consistent with a Solar composition and short scale height at the terminator. Assuming zero ellipsoidal variations also yields a much deeper 4.5 micron eclipse depth, consistent with a Solar composition and modest temperature inversion. We suggest future observations that could distinguish between these two scenarios.
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We report a new detection of the H-band thermal emission of CoRoT-1b and two confirmation detections of the Ks-band thermal emission of WASP-12b at secondary eclipses. The H-band measurement of CoRoT-1b shows an eclipse depth of 0.145%pm0.049% with a 3-{sigma} percentile between 0.033% - 0.235%. This depth is consistent with the previous conclusions that the planet has an isother- mal region with inefficient heat transport from dayside to nightside, and has a dayside thermal inversion layer at high altitude. The two Ks band detections of WASP-12b show a joint eclipse depth of 0.299%pm0.065%. This result agrees with the measurement of Croll & collaborators, providing independent confirmation of their measurement. The repeatability of the WASP-12b measurements also validates our data analysis method. Our measurements, in addition to a number of previous results made with other telescopes, demonstrate that ground-based observations are becoming widely available for characterization of atmospheres of hot Jupiters.
WASP-12b is a transiting hot Jupiter on a 1.09-day orbit around a late-F star. Since the planets discovery in 2008, the time interval between transits has been decreasing by $29pm 2$ msec year$^{-1}$. This is a possible sign of orbital decay, although the previously available data left open the possibility that the planets orbit is slightly eccentric and is undergoing apsidal precession. Here, we present new transit and occultation observations that provide more decisive evidence for orbital decay, which is favored over apsidal precession by a $Deltamathrm{BIC}$ of 22.3 or Bayes factor of 70,000. We also present new radial-velocity data that rule out the R{o}mer effect as the cause of the period change. This makes WASP-12 the first planetary system for which we can be confident that the orbit is decaying. The decay timescale for the orbit is $P/dot{P} = 3.25pm 0.23$ Myr. Interpreting the decay as the result of tidal dissipation, the modified stellar tidal quality factor is $Q_star = 1.8 times10^{5}$.
The exoplanet WASP-12b is the prototype for the emerging class of ultra-hot, Jupiter-mass exoplanets. Past models have predicted---and near ultra-violet observations have shown---that this planet is losing mass. We present an analysis of two sets of 3.6 $mu$m and 4.5 $mu$m $textit{Spitzer}$ phase curve observations of the system which show clear evidence of infrared radiation from gas stripped from the planet, and the gas appears to be flowing directly toward or away from the host star. This accretion signature is only seen at 4.5 $mu$m, not at 3.6 $mu$m, which is indicative either of CO emission at the longer wavelength or blackbody emission from cool, $lesssim$ 600 K gas. It is unclear why WASP-12b is the only ultra-hot Jupiter to exhibit this mass loss signature, but perhaps WASP-12bs orbit is decaying as some have claimed, while the orbits of other exoplanets may be more stable; alternatively, the high energy irradiation from WASP-12A may be stronger than the other host stars. We also find evidence for phase offset variability at the level of $6.4sigma$ ($46.2^{circ}$) at 3.6 $mu$m.
We report the detection of the eclipse of the very-hot Jupiter WASP-12b via z-band time-series photometry obtained with the 3.5-meter ARC telescope at Apache Point Observatory. We measure a decrease in flux of 0.082+/-0.015% during the passage of the planet behind the star. That planetary flux is equally well reproduced by atmospheric models with and without extra absorbers, and blackbody models with f > 0.585+/-0.080. It is therefore necessary to measure the planet at other wavelengths to further constrain its atmospheric properties. The eclipse appears centered at phase = 0.5100 (+0.0072,-0.0061), consistent with an orbital eccentricity of |e cos w| = 0.016 (+0.011,-0.009) (see note at end of Section 4). If the orbit of the planet is indeed eccentric, the large radius of WASP-12b can be explained by tidal heating.
We analyse emission spectra of WASP-12b from a partial phase curve observed over three epochs with the Hubble Space Telescope, covering eclipse, quadrature, and transit, respectively. As the 1.1-day period phase curve was only partially covered over three epochs, traditional methods to extract the planet flux and instrument systematic errors cannot recover the thermal emission away from the secondary eclipse. To analyse this partial phase curve, we introduce a new method, which corrects for the wavelength-independent component of the systematic errors. Our new method removes the achromatic instrument and stellar variability, and uses the measured stellar spectrum in eclipse to then retrieve a relative planetary spectrum in wavelength at each phase. We are able to extract the emission spectrum of an exoplanet at quadrature outside of a phase curve for the first time; we recover the quadrature spectrum of WASP-12b up to an additive constant. The dayside emission spectrum is extracted in a similar manner, and in both cases we are able to estimate the brightness temperature, albeit at a greatly reduced precision. We estimate the brightness temperature from the dayside (Tday=3186+-677 K) and from the quadrature spectrum (Tquad=2124+-417 K) and combine them to constrain the energy budget of the planet. We compare our extracted relative spectra to global circulation models of this planet, which are generally found to be a good match. However, we do see tentative evidence of a steeper spectral slope in the measured dayside spectrum compared to our models. We find that we cannot match this increased slope by increasing optical opacities in our models. We also find that this spectral slope is unlikely to be explained by a non-equilibrium water abundance, as water advected from the nightside is quickly dissociated on the dayside.
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