Abstract
The hot Jupiter HD 209458b is particularly amenable to detailed study as it is among the brightest transiting exoplanet systems currently known (V-mag = 7.65; K-mag = 6.308) and has a large planet-to-star contrast ratio. HD 209458b is predicted to be in synchronous rotation about its host star with a hot spot that is shifted eastward of the substellar point by superrotating equatorial winds. Here we present the first full-orbit observations of HD 209458b, in which its 4.5 $\mu$m emission was recorded with $Spitzer$/IRAC. Our study revises the previous 4.5 $\mu$m measurement of HD 209458b's secondary eclipse emission downward by $\sim$35% to $0.1391%^{+0.0072%}_{-0.0069%}$, changing our interpretation of the properties of its dayside atmosphere. We find that the hot spot on the planet's dayside is shifted eastward of the substellar point by $40.9^{\circ}\pm{6.0^{\circ}}$, in agreement with circulation models predicting equatorial superrotation. HD 209458b's dayside (T$_{bright}$ = 1499 $\pm$ 15 K) and nightside (T$_{bright}$ = 972 $\pm$ 44 K) emission indicates a day-to-night brightness temperature contrast smaller than that observed for more highly irradiated exoplanets, suggesting that the day-to-night temperature contrast may be partially a function of the incident stellar radiation. The observed phase curve shape deviates modestly from global circulation model predictions potentially due to disequilibrium chemistry or deficiencies in the current hot CH$_{4}$ line lists used in these models. Observations of the phase curve at additional wavelengths are needed in order to determine the possible presence and spatial extent of a dayside temperature inversion, as well as to improve our overall understanding of this planet's atmospheric circulation.
Highlights
Of the more than 1100 transiting exoplanets discovered to date, over 150 are gas giant planets known as “hot Jupiters” that have near-Jupiter masses (0.5 MJupiter M 5 MJupiter) and that orbit very close to their host stars
We explore apertures that scale according to the noise pixel parameter β, which is defined in Section 2.2.2 (IRAC Image Quality) of the IRAC instrument handbook as β = (
We leave a full exploration of these features for a future study and note that they do not significantly affect the overall shape of the phase curve, the primary transit, or the second secondary eclipse. The success of this reduction method is indicated by the decorrelated data (Figures 2 and 3) having a standard deviation of the normalized residuals (SDNR) of 0.0032 which is within 14% (1.14 times) the photon noise limit
Summary
Of the more than 1100 transiting exoplanets discovered to date, over 150 are gas giant planets known as “hot Jupiters” that have near-Jupiter masses (0.5 MJupiter M 5 MJupiter) and that orbit very close to their host stars (semi-major axis a 0.1 AU) These transiting exoplanets are predicted to be tidally locked (e.g., MacDonald 1964; Peale 1974) so that one hemisphere always points toward its host star while the other is in perpetual night. One of the best methods to directly constrain the nature of the atmospheric circulation patterns on these planets is to continuously monitor their infrared (IR) emission to characterize the full-orbit phase curve Such observations yield longitudinal disk variations, which can be transformed into a 11 Sagan Fellow. The Spitzer Space Telescope is the only platform currently capable of making mid- to far-IR full-orbit observations due to its stability, continuous viewing capability, and access to longer wavelengths than the Hubble Space Telescope (Harrington et al 2006; Cowan et al 2007, 2012; Knutson et al 2007, 2012; Lewis et al 2013; Maxted et al 2013)
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