The Broadband and Spectrally Resolved H-band Eclipse of KELT-1b and the Role of Surface Gravity in Stratospheric Inversions in Hot Jupiters

We present a high-precision H-band emission spectrum of the transiting brown dwarf KELT-1b, which we spectrophotometrically observed during a single secondary eclipse using the LUCI1 multiobject spectrograph on the Large Binocular Telescope. Using a Gaussian-process regression model, we are able to clearly measure the broadband eclipse depth as ΔH = 1418 ± 94 ppm. We are also able to spectrally resolve the H band into five separate wave channels and measure the eclipse spectrum of KELT-1b at R ≈ 50 with an average precision of ±135 ppm. We find that the day side has an average brightness temperature of 3250 ± 50 K, with significant variation as a function of wavelength. Based on our observations and previous measurements of KELT-1b's eclipse at other wavelengths, we find that KELT-1b's day side appears identical to an isolated 3200 K brown dwarf, and our modeling of the atmospheric emission shows a monotonically decreasing temperature-pressure profile. This is in contrast to hot Jupiters with similar day-side brightness temperatures near 3000 K, all of which appear to be either isothermal or possess a stratospheric temperature inversion. We hypothesize that the lack of an inversion in KELT-1b is due to its high surface gravity, which we argue could be caused by the increased efficiency of cold-trap processes within its atmosphere.