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We present the detection and analysis of the phase curve of KELT-1b atoptical wavelengths, analyzing data taken by the Transiting Exoplanet SurveySatellite (TESS). With a mass of ~27 M_J, KELT-1b is a low-mass brown dwarf.Due to the high mass and close proximity of its companion, the host star has aTESS light curve which shows clear ellipsoidal variations. We model the datawith a six-component model: secondary eclipse, phase curve accounting forreflected light and thermal emission, Doppler beaming, ellipsoidal variations,stellar activity and the primary transit. We determine the secondary eclipsedepth in the TESS bandpass to be 304 +/- 75 parts-per-million (ppm), the mostaccurate eclipse depth determined so far for KELT-1b. We measure the amplitudeof the phase curve to be 128 +/- 27 ppm, with a corresponding eastward offsetbetween the region of maximum brightness and the substellar point of 19.2 +/-9.6 degrees, in good agreement with Spitzer measurements. We determine day andnight brightness temperatures of 3201 +/- 147 K and 1484 +/- 110 K,respectively, slightly higher than those from Spitzer 3.6 and 4.5 micrometerdata. A one-dimensional self-consistent atmospheric model can explain the TESSand Spitzer day side brightness temperatures with thermal emission alone and noreflected light. The night side data can be explained by a model with aninternal temperature of ~1100 K, which may be related to the inflated radius.The difference between the TESS and Spitzer brightness temperatures can beexplained by stronger molecular opacity in the Spitzer bands. On the nightside, this opacity is due primarily to CH4 and CO while on the day side it isdue to H2-H2 and H2-He collision-induced absorption.

astronomy and astrophysics

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