Phase curve observations provide an opportunity to study the full energy budgets of exoplanets by quantifying the amount of heat redistributed from their daysides to their nightsides. Theories explaining the properties of phase curves for hot Jupiters have focused on the balance between radiation and dynamics as the primary parameter controlling heat redistribution. However, recent phase curves have shown deviations from the trends that emerge from this theory, which has led to work on additional processes that may affect hot Jupiter energy budgets. One such process, molecular hydrogen dissociation and recombination, can enhance energy redistribution on ultra-hot Jupiters with temperatures above ∼ 2000 K. In order to study the impact of H 2 dissociation on ultra-hot Jupiters, we present a phase curve of KELT-9b observed with the Spitzer Space Telescope at 4.5 µm. KELT-9b is the hottest known transiting planet, with a 4.5-µm dayside brightness temperature of 4566 +140 −136 K and a nightside temperature of 2556 +101 −97 K. We observe a phase curve amplitude of 0.609 ± 0.020 and a hot spot offset of 18.7 +2.1 −2.3 • . The observed amplitude is too small to be explained by a simple balance between radiation and advection. General circulation models (GCMs) and an energy balance model that include the effects of H 2 dissociation and recombination provide a better match to the data. The GCMs, however, predict a maximum hot spot offset of 5 • , which disagrees with our observations at > 5σ confidence. This discrepancy may be due to magnetic effects in the planet's highly ionized atmosphere.
Short-period gas giants (hot Jupiters) on circular orbits are expected to be tidally locked into synchronous rotation, with permanent daysides that face their host stars, and permanent nightsides that face the darkness of space 1 . Thermal flux from the nightside of several hot Jupiters has been measured, meaning energy is transported from day to night in some fashion. However, it is not clear exactly what the physical information from these detections reveals about the atmospheric dynamics of hot Jupiters. Here we show that the nightside effective temperatures of a sample of 12 hot Jupiters are clustered around 1100 K, with a slight upward trend as a function of stellar irradiation. The clustering is not predicted by cloud-free atmospheric circulation models 2-4 . This result can be explained if most hot Jupiters have nightside clouds that are optically thick to outgoing longwave radiation and hence radiate at the cloud-top temperature, and progressively disperse for planets receiving greater incident flux. Phase curve observations at a greater range of wavelengths are crucial to determining the extent of cloud coverage, as well as the cloud composition on hot Jupiter nightsides 5, 6 .We collected published full orbit, infrared phase curves for twelve hot Jupiters: CoRoT-2b 7 , HAT-P-7-b 8 , HD 149026b 9 , HD 189733b 10 , HD 209458b 11 , WASP-12b 12 , WASP-14b 13 , WASP-18b 14 , WASP-19b 8 , WASP-33b 9 , WASP-43b [15][16][17][18] . We also included the brown dwarf KELT-1b 20 . We calculated the nightside brightness temperatures from the phase curve parameters, and used Gaussian Process regression to estimate each planet's bolometric flux, and subsequently its disk-integrated nightside effective temperature. Several of the published phase curve fits imply negative nightside disk-integrated flux, which is unphysical, because it implies that the planets have negative brightness at some longitudes on their surface. We explain how we handled these cases in the Methods section. Future phase curve observations should be fit with the constraint that flux is non-negative everywhere on the planet. We also inferred nightside temperatures by considering and modifying negative brightness maps, which is similar in spirit to demanding positive phase curves and brightness maps when fitting the data. The mapping approach 1 arXiv:1809.00002v2 [astro-ph.EP] 23 Aug 2019 yielded a nightside temperature trend consistent with that of the disk-integrated approach.In Figure 1 we show the dayside and nightside effective temperatures plotted against the stellar irradiation temperature, T 0 ≡ T R /a, were T is the stellar effective temperature, R is the stellar radius, and a is semi-major axis. The nightside temperatures are all around 1100K and exhibit a slight upward trend with stellar irradiation. We tabulate the dayside temperature, nightside temperature, Bond Albedo, and heat recirculation efficiency for each planet in Table 1.Various theories have suggested that reradiation 1 , advection, wave propagation 21 , molecular dissocation 2...
Short-period planets exhibit day-night temperature contrasts of hundreds to thousands of degrees K. They also exhibit eastward hotspot offsets whereby the hottest region on the planet is East of the substellar point 1 ; this has been widely interpreted as advection of heat due to eastward winds 2 . We present thermal phase observations of the hot Jupiter CoRoT-2b obtained with the IRAC instrument on the Spitzer Space Telescope. These measurements show the most robust detection to date of a westward hotspot offset of 23 ± 4 degrees, in contrast with the nine other planets with equivalent measurements [3][4][5][6][7][8][9][10] . The peculiar infrared flux map of CoRoT-2b may result from westward winds due to non-synchronous rotation 11 or magnetic effects 12, 13 , or partial cloud coverage, that obscures the emergent flux from the planet's eastern hemisphere [14][15][16][17] . Non-synchronous rotation and magnetic effects may also explain the planet's anomalously large radius 12,18 . On the other hand, partial cloud coverage could explain the featureless dayside emission spectrum of the planet 19,20 . If CoRoT-2b is not tidally locked, then it means that our understanding of star-planet tidal interaction is incomplete. If the westward offset is due to magnetic effects, our result represents an opportunity to study an exoplanet's magnetic field. If it has Eastern clouds, then it means that our understanding of large-scale circulation on tidally locked planets is incomplete. Main TextAmongst the plethora of known hot Jupiters, the CoRoT-2 system stands out from the rest for three reasons: its remarkably active host star, its unusual inflated radius, and its puzzling emission spectrum. In addition to these anomalous features, previous observations of the CoRoT-2 system show a gravitationally bound stellar companion candidate, 2MASS J19270636+0122577.CoRoT-2b's optical phase curve obtained by the CoRoT mission has previously been studied 21,22 and yielded an upper limit on the planet's geometric albedo of 0.12. Later near-infrared (NIR) and mid-infrared (mid-IR) observations, acquired with ground-based 23 and space-based 20, 24, 25 instruments, have shown that the planet's emission spectrum could not be explained by conventional solar composition spectra or by a blackbody. Several scenarios were invoked to interpret the perplexing spectrum including the presence of silicate clouds affecting the mid-IR emission of the planet 19 and optically thick dayside clouds or a vertically isothermal atmosphere to explain the lack of features in the data acquired by the Wide Field Camera 3 (WFC3) on board of the Hubble Space Telescope (HST) 20 .We present new phase observations of the CoRoT-2 system (PID 11073; PI Cowan) . The top panel shows the normalized raw photometry obtained from Spitzer observations of the CoRoT-2 system (gray dots) and the fit with greatest Bayesian Evidence, instrumental systematics modeled as a 2 nd order polynomial and with no stellar variability (red dots). The error on the photometry measu...
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