Spitzer Phase Curves of KELT-1b and the Signatures of Nightside Clouds in Thermal Phase Observations

Abstract

We observed two full orbital phase curves of the transiting brown dwarf KELT-1b, at 3.6 µm and 4.5 µm, using the Spitzer Space Telescope. Combined with previous eclipse data from Beatty et al. (2014), we strongly detect KELT-1b's phase variation as a single sinusoid in both bands, with an amplitude of 964 ± 43 ppm at 3.6 µm and 979 ± 46 ppm at 4.5 µm, and confirm the secondary eclipse depths measured by Beatty et al. (2014). We also measure noticeable Eastward hotspot offsets of 28.4±3.5 degrees at 3.6 µm and 18.6±4.8 degrees at 4.5 µm. Both the day-night temperature contrasts and the hotspot offsets we measure are in line with the trends seen in hot Jupiters (e.g. Crossfield 2015). This general agreement is despite the fact that KELT-1b should have an atmospheric radiative timescale substantially longer than in a typical hot Jupiter. We therefore suggest that nightside clouds are playing a noticeable role in modulating the thermal emission from these objects, based on: 1) the lack of a clear trend in phase offsets with equilibrium temperature, contrary to a recent suggestion of an offset trend by , 2) the sharp day-night transitions required to have non-negative intensity maps, which also resolves the inversion issues raised in Keating & Cowan (2017), 3) the fact that all the nightsides of these objects appear to be at roughly the same temperature of 1050 K, while the dayside temperatures increase linearly with equilibrium temperature, and 4) the trajectories of these objects on a Spitzer color-magnitude-diagram, which show colors only explainable via nightside clouds.

A transition between the hot and the ultra-hot Jupiter atmospheres

“…Schwartz et al (2017) incorporate phase offsets into their energy budget calculations of six planets, which pushes the results toward lower Bond albedos and slightly higher heat transport than before. Keating et al (2019) and Beatty et al (2019) estimate the nightside temperature of several hot Jupiters using Spitzer phase curves and find that despite the different levels of irradiation, they all demonstrate similar nightside temperatures. This suggests that they might all have some chemically similar high optically thick cloud layer that is emitting at the nightside temperature.…”

“…ODRPACK is a weighted orthogonal distance regression function which takes into account errors on x and on y by minimizing the weighted orthogonal distance between the observations and the model. However, as pointed out in Beatty et al (2019), ODRPACK uses relative errors between the data points, meaning that the resulting covariance matrix remains the same even when you multiply all of the individual errors by some factor. This has the potential for producing incorrect uncertainties on the parameters.…”

“…This has the potential for producing incorrect uncertainties on the parameters. Beatty et al (2019) use another package, bivariate correlated errors and intrinsic scatter (BCES) (Akritas & Bershady 1996;Nemmen et al 2012). However, this package only fits a linear model, and so is not suitable for our cases.…”

Uniformly hot nightside temperatures on short-period gas giants

“…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] , and WASP-103b 19 . 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.…”

KELT-24b: A 5MJ Planet on a 5.6 day Well-aligned Orbit around the Young V = 8.3 F-star HD 93148

“…The characterization of the global cloud properties on KELT-24 b therefore could allow us to better understand the dynamical processes behind the L-T transition. In particular, a recent analysis of Spitzer phase-curve results by Beatty et al (2019) has shown that all hot Jupiters appear to posses a nightside cloud deck at a temperature of roughly 1000 K. The relatively low equilibrium temperature of KELT-24 b's atmosphere indicates that even dayside clouds on KELT-24 b would be close in composition to the universal nightside clouds on other hot Jupiters. The spectroscopic measurement of KELT-24 b's emission might, therefore, be able to determine the specific composition of these clouds.…”