TOI-1518b: A Misaligned Ultra-hot Jupiter with Iron in its Atmosphere

Abstract

We present the discovery of TOI-1518b -- an ultra-hot Jupiter orbiting a bright star $V=8.95$. The transiting planet is confirmed using high-resolution optical transmission spectra from EXPRES. It is inflated, with $R_p=1.875\pm 0.053R_{\mathrm{J}}$, and exhibits several interesting properties, including a misaligned orbit ($240.34_{-0.98}^{+0.93}$ degrees) and nearly grazing transit ($b=0.9036_{-0.0053}^{+0.0061}$). The planet orbits a fast-rotating F0 host star ($T_{\mathrm{eff}}\simeq 7300$ K) in 1.9 days and experiences intense irradiation. Notably, the TESS data show a clear secondary eclipse with a depth of $364\pm 28$ ppm and a significant phase curve signal, from which we obtain a relative day-night planetary flux difference of roughly 320 ppm and a 5.2$\sigma$ detection of ellipsoidal distortion on the host star. Prompted by recent detections of atomic and ionized species in ultra-hot Jupiter atmospheres, we conduct an atmospheric cross-correlation analysis. We detect neutral iron ($5.2\sigma$), at $K_p=157_{-44}^{+68}$ km s${}^{-1}$ and $V_{\mathrm{sys}}=-16_{-4}^{+2}$ km s${}^{-1}$, adding another object to the small sample of highly irradiated gas-giant planets with Fe detections in transmission. Detections so far favor particularly inflated gas giants with radii $\gtrsim 1.78R_{\mathrm{J}}$; although this may be due to observational bias. With an equilibrium temperature of $T_{\mathrm{eq}}=2492\pm 38$ K and a measured dayside brightness temperature of $3237\pm 59$ K (assuming zero geometric albedo), TOI-1518b is a promising candidate for future emission spectroscopy to probe for a thermal inversion.

25 pages, 11 figures, accepted to AJ

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