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Register a new userInvestigating Trends in Atmospheric Compositions of Cool Gas Giant Planets Using Spitzer Secondary Eclipses
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We present new 3.6 and 4.5 micron secondary eclipse measurements for five cool (less than approximately 1000 K) transiting gas giant planets: HAT-P-15b, HAT-P-17b, HAT-P-18b, HAT-P-26b, and WASP-69b. We detect eclipses in at least one bandpass for all planets except HAT-P-15b. We confirm and refine the orbital eccentricity of HAT-P-17b, which is also the only planet in our sample with a known outer companion. We compare our measured eclipse depths in these two bands, which are sensitive to the relative abundances of methane versus carbon monoxide and carbon dioxide, respectively, to predictions from 1D atmosphere models for each planet. For planets with hydrogen-dominated atmospheres and equilibrium temperatures cooler than approximately 1000 K, this ratio should vary as a function of both atmospheric metallicity and the carbon-to-oxygen ratio. For HAT-P-26b, our observations are in good agreement with the low atmospheric metallicity inferred from transmission spectroscopy. We find that all four of the planets with detected eclipses are best matched by models with relatively efficient circulation of energy to the nightside. We see no evidence for a solar-system-like correlation between planet mass and atmospheric metallicity, but instead identify a potential (1.9 sigma) correlation between the inferred methane/(carbon monoxide + carbon dioxide) ratio and stellar metallicity. Our ability to characterize this potential trend is limited by the relatively large uncertainties in the stellar metallicity values. Our observations provide a first look at the brightness of these planets at wavelengths accessible to the James Webb Space Telescope, which will be able to resolve individual methane, carbon monoxide, and carbon dioxide bands and provide much stronger constraints on their atmospheric compositions.
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It is well-established that the magnitude of the incident stellar flux is the single most important factor in determining the day-night temperature gradients and atmospheric chemistries of short-period gas giant planets. However it is likely that other factors, such as planet-to-planet variations in atmospheric metallicity, C/O ratio, and cloud properties, also contribute to the observed diversity of infrared spectra for this population of planets. In this study we present new 3.6 and 4.5 micron secondary eclipse measurements for five transiting gas giant planets: HAT-P-5b, HAT-P-38b, WASP-7b, WASP-72b, and WASP-127b. We detect eclipses in at least one bandpass for all five planets and confirm circular orbits for all planets except for WASP-7b, which shows evidence for a non-zero eccentricity. Building on the work of Garhart et al. (2020), we place these new planets into a broader context by comparing them with the sample of all planets with measured Spitzer secondary eclipses. We find that incident flux is the single most important factor for determining the atmospheric chemistry and circulation patterns of short-period gas giant planets. Although we might also expect surface gravity and host star metallicity to play a secondary role, we find no evidence for correlations with either of these two variables.
In this work we present Spitzer 3.6 and 4.5 micron secondary eclipse observations of five new cool (<1200 K) transiting gas giant planets: HAT-P-19b, WASP-6b, WASP-10b, WASP-39b, and WASP-67b. We compare our measured eclipse depths to the predictions of a suite of atmosphere models and to eclipse depths for planets with previously published observations in order to constrain the temperature- and mass-dependent properties of gas giant planet atmospheres. We find that the dayside emission spectra of planets less massive than Jupiter require models with efficient circulation of energy to the night side and/or increased albedos, while those with masses greater than that of Jupiter are consistently best-matched by models with inefficient circulation and low albedos. At these relatively low temperatures we expect the atmospheric methane to CO ratio to vary as a function of metallicity, and we therefore use our observations of these planets to constrain their atmospheric metallicities. We find that the most massive planets have dayside emission spectra that are best-matched by solar metallicity atmosphere models, but we are not able to place strong constraints on metallicities of the smaller planets in our sample. Interestingly, we find that the ratio of the 3.6 and 4.5 micron brightness temperatures for these cool transiting planets is independent of planet temperature, and instead exhibits a tentative correlation with planet mass. If this trend can be confirmed, it would suggest that the shape of these planets emission spectra depends primarily on their masses, consistent with the hypothesis that lower-mass planets are more likely to have metal-rich atmospheres.
The transiting exoplanet WASP-18b was discovered in 2008 by the Wide Angle Search for Planets (WASP) project. The Spitzer Exoplanet Target of Opportunity Program observed secondary eclipses of WASP-18b using Spitzers Infrared Array Camera (IRAC) in the 3.6 micron and 5.8 micron bands on 2008 December 20, and in the 4.5 micron and 8.0 micron bands on 2008 December 24. We report eclipse depths of 0.30 +/- 0.02%, 0.39 +/- 0.02%, 0.37 +/- 0.03%, 0.41 +/- 0.02%, and brightness temperatures of 3100 +/- 90, 3310 +/- 130, 3080 +/- 140 and 3120 +/- 110 K in order of increasing wavelength. WASP-18b is one of the hottest planets yet discovered - as hot as an M-class star. The planets pressure-temperature profile most likely features a thermal inversion. The observations also require WASP-18b to have near-zero albedo and almost no redistribution of energy from the day-side to the night side of the planet.
Previous secondary eclipse observations of the hot Jupiter Qatar-1b in the Ks band suggest that it may have an unusually high day side temperature, indicative of minimal heat redistribution. There have also been indications that the orbit may be slightly eccentric, possibly forced by another planet in the system. We investigate the day side temperature and orbital eccentricity using secondary eclipse observations with Spitzer. We observed the secondary eclipse with Spitzer/IRAC in subarray mode, in both 3.6 and 4.5 micron wavelengths. We used pixel-level decorrelation to correct for Spitzers intra-pixel sensitivity variations and thereby obtain accurate eclipse depths and central phases. Our 3.6 micron eclipse depth is 0.149 +/- 0.051% and the 4.5 micron depth is 0.273 +/- 0.049%. Fitting a blackbody planet to our data and two recent Ks band eclipse depths indicates a brightness temperature of 1506 +/- 71K. Comparison to model atmospheres for the planet indicates that its degree of longitudinal heat redistribution is intermediate between fully uniform and day side only. The day side temperature of the planet is unlikely to be as high (1885K) as indicated by the ground-based eclipses in the Ks band, unless the planets emergent spectrum deviates strongly from model atmosphere predictions. The average central phase for our Spitzer eclipses is 0.4984 +/- 0.0017, yielding e cos(omega) = -0.0028 +/- 0.0027. Our results are consistent with a circular orbit, and we constrain e cos(omega) much more strongly than has been possible with previous observations.
We present {em Spitzer} secondary-eclipse observations of the hot Jupiter HAT-P-13 b in the 3.6 {micron} and 4.5 {micron} bands. HAT-P-13 b inhabits a two-planet system with a configuration that enables constraints on the planets second Love number, math{ksb{2}}, from precise eccentricity measurements, which in turn constrains models of the planets interior structure. We exploit the direct measurements of math{e cos omega} from our secondary-eclipse data and combine them with previously published radial velocity data to generate a refined model of the planets orbit and thus an improved estimate on the possible interval for math{ksb{2}}. We report eclipse phases of math{0.49154 pm 0.00080} and math{0.49711 pm 0.00083} and corresponding math{e cos omega} estimates of math{-0.0136 pm 0.0013} and math{-0.0048 pm 0.0013}. Under the assumptions of previous work, our estimate of math{ksb{2}} of 0.81 {pm} 0.10 is consistent with the lower extremes of possible core masses found by previous models, including models with no solid core. This anomalous result challenges both interior models and the dynamical assumptions that enable them, including the essential assumption of apsidal alignment. We also report eclipse depths of 0.081% {pm} 0.008% in the 3.6 {micron} channel and 0.088 % {pm} 0.028 % in the 4.5 {micron} channel. These photometric results are non-uniquely consistent with solar-abundance composition without any thermal inversion.
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