THE 4.5 μm FULL-ORBIT PHASE CURVE OF THE HOT JUPITER HD 209458b

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 μm emission was recorded with Spitzer/IRAC. Our study revises the previous 4.5 μm measurement of HD 209458b's secondary eclipse emission downward by ∼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 ± 6.• 0, in agreement with circulation models predicting equatorial superrotation. HD 209458b's dayside (T bright = 1499 ± 15 K) and nightside (T bright = 972 ± 44 K) emission indicate a day-tonight 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.

Atmospheric Circulation of Hot Jupiters: Dayside–Nightside Temperature Differences. II. Comparison with Observations

“…There is a general trend of increasing A obs with increasing T eq , both for the band-averaged values and in each individual wavelength band. Observational data are taken from Knutson et al (2007Knutson et al ( , 2009aKnutson et al ( , 2009b, Nymeyer et al (2011), Cowan et al (2012, Crossfield et al (2012), Knutson et al (2012), Maxted et al (2013), Stevenson et al (2014), Zellem et al (2014), Wong et al (2016Wong et al ( , 2015, and Stevenson et al (2016).…”

SPITZERSECONDARY ECLIPSES OF THE DENSE, MODESTLY-IRRADIATED, GIANT EXOPLANET HAT-P-$20{\rm b}$ USING PIXEL-LEVEL DECORRELATION

“…The earliest such decorrelations (e.g., Charbonneau et al 2005;Knutson et al 2008;Machalek et al 2008) modeled the intensity fluctuations as polynomial (typically quadratic) functions of the Y-coordinate, sometimes with a weaker (e.g., linear) dependence on the X-coordinate. Polynomial decorrelations are still used (e.g., Shporer et al 2014), but methods have evolved to include very powerful implementations such as Bi-linear Interpolated Subpixel Sensitivity (BLISS) mapping (Stevenson et al 2012), and novel variants such as a spatial weighting-function approach (Ballard et al 2010;, and modifications thereof (Lewis et al 2013;Lanotte et al 2014;Zellem et al 2014). These decorrelations have been largely but not entirely successful.…”

Results from a set of three-dimensional numerical experiments of a hot Jupiter atmosphere

“…Explicit damping is often applied to account for these effects. However, we currently have no way of constraining the strength of the dissipation (Li & Goodman 2010;Heng et al 2011), except perhaps by matching simulated offsets in hot Jupiter hotspots from the substellar point with observed phase curve offsets, although processes not explicitly modeled complicate this (Zellem et al 2014). Therefore, such damping and dissipation parametrisation are primarily used to achieve numerical stability in a physically plausible way, but are not robustly constrained.…”