TESS unveils the optical phase curve of KELT-1b. Thermal emission and ellipsoidal variation from the brown dwarf companion, and activity from the star

Abstract

We present the detection and analysis of the phase curve of KELT-1b at optical wavelengths, analyzing data taken by the Transiting Exoplanet Survey Satellite (TESS) during cycle 2 and sector 17. With a mass of ∼27 M Jup , KELT-1b is a low-mass brown dwarf. Due to the high mass and close proximity of its companion, the host star has a TESS light curve which shows clear ellipsoidal variations. We model the data with a six-component model: secondary eclipse, phase curve accounting for reflected light and thermal emission, Doppler beaming, ellipsoidal variations, stellar activity and the primary transit. We determine the secondary eclipse depth in the TESS bandpass to be 304 ± 75 parts-per-million (ppm), the most accurate eclipse depth determined so far for KELT-1b. In addition, we measure the amplitude of the phase curve to be 128 ± 27 ppm, with a corresponding eastward offset between the region of maximum brightness and the substellar point of 19.2 ± 9.6 degrees, the latter in good agreement with Spitzer measurements. We determine day and night brightness temperatures of 3201 ± 147 K and 1484 ± 110 K, respectively, slightly higher than those determined from Spitzer 3.6 and 4.5 µm data. By combining TESS and Spitzer eclipse depths, we derive a day side effective temperature of T eff = 3010 ± 78 K. Previously published eclipse depths in the near-infrared suggest a much higher brightness temperature and cannot be explained by spectral models together with current data. We attribute those large eclipse depths to unmodeled ellipsoidal variations, which would manifest themselves as a deeper secondary eclipse in observations with insufficient phase coverage. A one-dimensional self-consistent atmospheric model can explain the TESS and Spitzer day side brightness temperatures with thermal emission alone and no reflected light. The night side data can be explained by a model with an internal temperature of ∼1100 K, which may be related to the inflated radius. The difference between the TESS and Spitzer brightness temperatures can be explained by stronger molecular opacity in the Spitzer bands. On the night side, this opacity is due primarily to CH 4 and CO while on the day side it is due to H 2 -H 2 and H 2 -He collision-induced absorption.

Aerosols in Exoplanet Atmospheres

“…Several recent estimations of geometric albedoes in the TESS bandpass have yielded high values (>0.2) for ultra-hot (T eq > 2000 K) giant exoplanets (Figure 6; I. Wong, Shporer, et al, 2020;von Essen et al, 2020). However, because the measured flux in the TESS bandpass for ultra hot Jupiters is dominated by thermal emission rather than reflected light, the estimated albedoes are very sensitive to the assumptions made when estimating the thermal contamination.…”

“…Rowe et al, 2008;Borucki et al, 2009;Snellen et al, 2009;Kipping & Spiegel, 2011;Esteves et al, 2013;Shporer et al, 2014Shporer et al, , 2019I. Wong, Shporer, et al, 2020;Bourrier et al, 2020;Beatty et al, 2020;von Essen et al, 2020;Jansen & Kipping, 2020). We refer the reader to Parmentier and Crossfield (2018); Alonso (2018) for comprehensive reviews.…”

“…For example, dayside near-infrared photometry and spectra can be explained without the need for optically thick aerosols down to the pressure levels probed, suggesting either a lack of aerosols altogether or that aerosols form at pressures higher than the planets' pho- from the daysides of ultra-hot giant exoplanets are emitted rather than reflected. Two planets are not shown here, as they are situated beyond the plot limits: KELT-1b, which has an apparent albedo of 0.7 in the TESS bandpass (von Essen et al, 2020), and WASP-100b, which has a controversial hot spot measurement in the TESS bandpass (see Jansen and Kipping (2020) vs. I.…”

“…However, while many species have been detected for ultra hot Jupiters (e.g. Fossati et al, 2010;Haswell et al, 2012;Hoeijmakers et al, 2018;Yan et al, 2019;von Essen et al, 2019;Sing et al, 2019;Ben-Yami et al, 2020;Cabot et al, 2020;Nugroho et al, 2020), suggesting largely cloud-free atmospheres, efforts at lower temperatures have yielded mixed results due to controversial detections that are difficult to replicate (e.g. Vidal-Madjar et al, 2013;Cubillos et al, 2020;Sedaghati et al, 2017;Espinoza et al, 2019;Chen et al, 2018;Seidel et al, 2020;Sing et al, 2015;Gibson et al, 2017Gibson et al, , 2019McGruder et al, 2020) and aerosol opacity at optical wavelengths that reduce the amplitudes of atomic and molecular absorption features (Charbonneau et al, 2002;Pont et al, 2008;Heng, 2016;Sing et al, 2016).…”

Revisiting the secondary eclipses of KELT-1b using TESS observations

“…Several studies have measured the KELT-1b's primary transit (when an exoplanet passes in front of its host star) and the depth of its secondary eclipse (i.e., when an exoplanet is occulted by its host star) Siverd et al (2012); Beatty et al (2019); Kane et al (2021). Two separate investigations, von Essen et al (2020) and Beatty et al (2020) used TESS data to study the full-orbit phase curve of KELT-1b. The planet-to-star radius ratio and eclipse depth of KELT-1b were measured to 304 +75 −75 ppm and 0.07688 +0.00040 −0.00040 respectively, in the TESS bandpass (von Essen et al, 2020).…”