The TESS Phase Curve of KELT-1b Suggests a High Dayside Albedo

Abstract

We measured the optical phase curve of the transiting brown dwarf KELT-1b (TOI 1476, Siverd et al. 2012) using data from the TESS spacecraft. We found that KELT-1b shows significant phase variation in the TESS bandpass, with a relatively large phase amplitude of $234_{-44}^{+43}$ ppm and a secondary eclipse depth of $371_{-49}^{+47}$ ppm. We also measured a marginal eastward offset in the dayside hotspot of $18.3^{\circ }\pm 7.4^{\circ }$ relative to the substellar point. We detected a strong phase curve signal attributed to ellipsoidal distortion of the host star, with an amplitude of $399\pm 19$ ppm. Our results are roughly consistent with the Spitzer phase curves of KELT-1b (Beatty et al. 2019), but the TESS eclipse depth is deeper than expected. Our cloud-free 1D models of KELT-1b's dayside emission are unable to fit the full combined eclipse spectrum. Instead, the large TESS eclipse depth suggests that KELT-1b may have a significant dayside geometric albedo of ${\mathrm{A}}_{\mathrm{g}}\sim 0.5$ in the TESS bandpass, which would agree with the tentative trend between equilibrium temperature and geometric albedo recently suggested by Wong et al. 2020. We posit that if KELT-1b has a high dayside albedo, it is likely due to silicate clouds (Gao et al. 2020) that form on KELT-1b's nightside (Beatty et al. 2019, Keating et al. 2019) and are subsequently transported onto the western side of KELT-1b's dayside hemisphere before breaking up.

AJ in press; updated to reflect publication version. Comments welcome

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