* Astronomy

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RE: WASP-12b

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Title: Probing the extreme planetary atmosphere of WASP-12b
Authors: Mark Swain, Pieter Deroo, Giovanna Tinetti, Morgan Hollis, Marcell Tessenyi, Michael Line, Hajime Kawahara, Yuka Fujii, Adam Showman, Sergey Yurchenko

We report near-infrared measurements of the terminator region transmission spectrum and dayside emission spectrum of the exoplanet WASP-12b obtained using the HST WFC3 instrument. The disk-average dayside brightness temperature averages about 2900 K, peaking to 3200 K around 1.46 µm. Both the dayside and terminator region spectra can be explained in terms of opacity due to the metal hydrides CrH and TiH together with a dayside temperature inversion with a deep tropopause. Although our measurements do not constrain the C/O ratio, the combination of TiH and high temperatures could imply the atmosphere of WASP-12b may be significantly metal poor. The dayside flux distribution reconstructed from the ingress light-curve shape shows indications of a hotspot. If located along the equatorial plane, the possible hot spot is near the sub-stellar point, indicating the radiative time scale may be shorter than the advection time scale. We also find the near-infrared primary eclipse light curve is consistent with small amounts of prolate distortion. The likely picture of WASP-12b that emerges is that this gas giant is powerfully influenced by the parent star to the extent that the planet's dayside atmosphere is star-like in terms of temperature, opacity, and the relative importance of radiation over advection. As part of the calibration effort for these data, we conducted a detailed study of instrument systematics using 65 orbits of WFC3-IR grims observations. The instrument systematics are dominated by detector-related affects, which vary significantly depending on the detector readout mode. The 256 x 256 subarray observations of WASP-12 produced measurements within 15% of the photon-noise limit using a simple calibration approach. Residual systematics are estimated to be \leq70 parts per million.

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Title: Thermal Phase Variations of WASP-12b: Defying Predictions
Authors: N. B. Cowan (Northwestern), P. Machalek (SETI), B. Croll (MIT), L. M. Shekhtman (Northwestern), A. Burrows (Princeton), D. Deming (UMD), T. Greene (NASA Ames), J. L. Hora (CfA)

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 planet's 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 planet's elongated shape, these variations imply a 3:2 ratio for the planet's longest:shortest axes and gravitational brightening of the 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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Title: A photochemical model for the carbon-rich planet WASP-12b
Authors: Ravi kumar Kopparapu, James F. Kasting, Kevin J. Zahnle

The hot Jupiter WASP-12b is a heavily irradiated exoplanet in a short period orbit around a G0-star with twice the metallicity of the Sun. A recent thermochemical equilibrium analysis based on Spitzer and ground-based infrared observations suggests that the presence of \ch4 in its atmosphere and the lack of \H2O features can only be explained if the carbon-to-oxygen ratio in the planet's atmosphere is much greater than the solar ratio (\CtoO = 0.54). Here, we use a 1-D photochemical model to study the effect of disequilibrium chemistry on the observed abundances of \H2O, \com, \co2 and \ch4 in the WASP-12b atmosphere. We consider two cases: one with solar \CtoO and another with \CtoO = 1.08. The solar case predicts that \h2o and \com are more abundant than \co2 and \ch4, as expected, whereas the high \CtoO model shows that \com, C_{2}H_{2} and HCN are more abundant. This indicates that the extra carbon from the high \CtoO model is in hydrocarbon species. \H2O photolysis is the dominant disequilibrium mechanism that alters the chemistry at higher altitudes in the solar \CtoO case, whereas photodissociation of C_{2}H_{2} and HCN is significant in the super-solar case. Furthermore, our analysis indicates that \c2h2 is the major absorber in the atmosphere of WASP-12b and the absorption features detected near 1.6 and 8 micron may be arising from C_{2}H_{2} rather than \ch4. The Hubble Space Telescope's WFC3 can resolve this discrepancy, as \c2h2 has absorption between 1.51 - 1.54 microns, while \ch4 does not.

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Title: Carbon-rich Giant Planets: Atmospheric Chemistry, Thermal Inversions, Spectra, and Formation conditions
Authors: Nikku Madhusudhan (Princeton), Olivier Mousis (CNRS, France), Torrence V. Johnson (NASA JPL), Jonathan I. Lunine (Cornell)

The recent inference of a carbon-rich atmosphere, with C/O >= 1, in the hot Jupiter WASP-12b motivates the exotic new class of carbon-rich planets (CRPs). We report a detailed study of the atmospheric chemistry and spectroscopic signatures of carbon-rich giant planets (CRGs) and the compositions of icy planetesimals required for their formation, and the apportionment of ices, rock, and volatiles in their envelopes. For C/O >= 1, most of the atmospheric oxygen is occupied by CO for T > 1400 K and pressure (P) < 1 bar, causing a substantial depletion in H2O, and an overabundance of CH4 compared to those obtained by assuming solar abundances (C/O = 0.54) in chemical equilibrium. These differences in chemistry cause distinctly observable signatures in spectra. We also find that a C/O >= 1 strongly depletes the abundances of TiO and VO available to form thermal inversions, which is adequate to rule out thermal inversions due to TiO/VO even in the most highly irradiated hot Jupiters, such as WASP-12b. Adopting stellar abundances (C/O = 0.44) for the primordial disk composition and low-temperature formation conditions (T <= 30 K) for WASP-12b leads to a C/O ratio of 0.27 in accreted planetesimals, and, consequently, in the planet's envelope. In contrast, a C/O ratio of 1 in the envelope of WASP-12b requires a substantial depletion of oxygen in the disk, by a factor of ~0.41 for the same formation conditions. This scenario also satisfies the constraints on the C/H and O/H ratios reported for WASP-12b. If, alternatively, hotter conditions prevailed in a stellar composition disk such that only H2O is condensed, the remaining gas can potentially have a C/O ~ 1. However, a high C/O in WASP-12b caused predominantly by gas accretion would preclude super-stellar C/H ratios which also fit the data.

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Title: The shocking transit of WASP-12b: Modelling the observed early ingress in the near ultraviolet
Authors: J. Llama (University of St Andrews), K. Wood (University of St Andrews), M. Jardine (University of St Andrews), A. A. Vidotto (University of St Andrews), Ch. Helling (University of St Andrews), L. Fossati (Open University), C. A. Haswell (Open University)

Near ultraviolet observations of WASP-12b have revealed an early ingress compared to the optical transit lightcurve. This has been interpreted as due to the presence of a magnetospheric bow shock which forms when the relative velocity of the planetary and stellar material is supersonic. We aim to reproduce this observed early ingress by modelling the stellar wind (or coronal plasma) in order to derive the speed and density of the material at the planetary orbital radius. From this we determine the orientation of the shock and the density of compressed plasma behind it. With this model for the density structure surrounding the planet we perform Monte Carlo radiation transfer simulations of the near UV transits of WASP-12b with and without a bow shock. We find that we can reproduce the transit lightcurves with a wide range of plasma temperatures, shock geometries and optical depths. Our results support the hypothesis that a bow shock could explain the observed early ingress.

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Title: A lower mass for the exoplanet WASP-21b
Authors: S. C. C. Barros (1), D. L. Pollacco (1), N. P. Gibson (1,2), I. D. Howarth (3), F. P. Keenan (1), E. K. Simpson (1), I. Skillen (4), I. A. Steele (5). (1- Queen's University Belfast, 2-University of Oxford, 3-UCL, 4- INT,5-Liverpool John Moores University)

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 ±0.04 Msun which is significantly lower than previously reported (1.01 ±0.03 Msun). Consequently, we find a lower planetary mass of 0.27 ±0.01 Mjup. A lower inclination (87.4 ±0.3 degrees) is also found for the system than previously reported, resulting in a slightly larger stellar (R_* =1.10 ±0.03 Rsun) and planetary radius (R_p = 1.14 ±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 ±0.00020 (HJD) and P=4.3225060 ±0.0000031 days. We found no transit timing variations in WASP-21b.

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Title: High-precision photometry of WASP-12 b transits
Authors: G.Maciejewski, R.Errmann, St.Raetz, M.Seeliger, I.Spaleniak, R.Neuhaeuser

The transiting extrasolar planet WASP-12 b was found to be one of the most intensely irradiated exoplanets. It is unexpectedly bloated and is losing mass that may accrete into the host star. Our aim was to refine the parameters of this intriguing system and search for signs of transit timing variations. We gathered high-precision light curves for two transits of WASP-12 b. Assuming various limb-darkening laws, we generated best-fitting models and redetermined parameters of the system. Error estimates were derived by the prayer bead method and Monte Carlo simulations. System parameters obtained by us are found to agree with previous studies within one sigma. Use of the non-linear limb-darkening laws results in the best-fitting models. With two new mid-transit times, the ephemeris was refined to BJD(TDB)=(2454508.97682 ± 0.00020) + (1.09142245 ± 0.00000033) E. Interestingly, indications of transit timing variation are detected at the level of 3.4 sigma. This signal can be induced by an additional planet in the system. Simplified numerical simulations shows that a perturber could be a terrestrial-type planet if both planets are in a low-order orbital resonance. However, we emphasise that further observations are needed to confirm variation and to constrain properties of the perturber.

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Diamond Studded Planet Discovered by U.S. Indian Astronomer Led Team

Astronomers led by an Indian American have discovered a giant planet with an atmosphere and a core dominated by carbon. Nikku Madhusudan, a Banaras Hindu University (BHU) alumnus who works at Princeton University, New Jersey, and his colleagues have observed that an extremely hot planet discovered last year has a never before seen characteristic of having more carbon than oxygen. They said that this actually raised the prospect that diamond-studded stars may exist.
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Title: On the Orbit of Exoplanet WASP-12b
Authors: Christopher J. Campo, Joseph Harrington, Ryan A. Hardy, Kevin B. Stevenson, Sarah Nymeyer, Darin Ragozzine, Nate B. Lust, David R. Anderson, Andrew Collier-Cameron, Jasmina Blecic, Christopher B. T. Britt, William C. Bowman, Peter J. Wheatley, Thomas J. Loredo, Drake Deming, Leslie Hebb, Coel Hellier, Pierre F. L. Maxted, Don Pollaco, Richard G. West
(Version v2)

We observed two secondary eclipses of the exoplanet WASP-12b using the Infrared Array Camera on the Spitzer Space Telescope. The close proximity of WASP-12b to its G-type star results in extreme tidal forces capable of inducing apsidal precession with a period as short as a few decades. This precession would be measurable if the orbit had a significant eccentricity, leading to an estimate of the tidal Love number and an assessment of the degree of central concentration in the planetary interior. An initial ground-based secondary eclipse phase reported by Lopez-Morales et al. (0.510 ±0.002) implied eccentricity at the 4.5 sigma level. The spectroscopic orbit of Hebb et al. has eccentricity 0.049 ±0.015, a 3 sigma result, implying an eclipse phase of 0.509 ±0.007. However, there is a well documented tendency of spectroscopic data to overestimate small eccentricities. Our eclipse phases are 0.5010 ±0.0006 (3.6 and 5.8 microns) and 0.5006 ±0.0007 (4.5 and 8.0 microns). An unlikely orbital precession scenario invoking an alignment of the orbit during the Spitzer observations could have explained this apparent discrepancy, but the final eclipse phase of Lopez-Morales et al. (0.510 -0.006 / +0.007) is consistent with a circular orbit at better than 2 sigma. An orbit fit to all the available transit, eclipse, and radial-velocity data indicates precession at <1 sigma; a non-precessing solution fits better. We also comment on analysis and reporting for Spitzer exoplanet data in light of recent re-analyses.

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