* Astronomy

Title: KELT-18b: Puffy Planet, Hot Host, Probably Perturbed
Author: Kim K. McLeod, Joseph E. Rodriguez, Ryan J. Oelkers, Karen A. Collins, Allyson Bieryla, Benjamin J. Fulton, Keivan G. Stassun, B. Scott Gaudi, Kaloyan Penev, Daniel J. Stevens, Knicole D. Colón, Joshua Pepper, Norio Narita, Ryu Tsuguru, Akihiko Fukui, Phillip A. Reed, Bethany Tirrell, Tiffany Visgaitis, John F. Kielkopf, David H. Cohen, Eric L. N. Jensen, Joao Gregorio, Ozgür Bastürk, Thomas E. Oberst, Casey Melton, Eliza M.-R. Kempton, Andrew Baldridge, Y. Sunny Zhao, Roberto Zambelli, David W. Latham, Gilbert A. Esquerdo, Perry Berlind, Michael L. Calkins, Andrew W. Howard, Howard Isaacson, Lauren M. Weiss, Thomas G. Beatty, Jason D. Eastman, Matthew T. Penny, Robert J. Siverd, Michael B. Lund, Jonathan Labadie-Bartz, G. Zhao, Ivan A. Curtis, Michael D. Joner, et al. (11 additional authors not shown)

We report the discovery of KELT-18b, a transiting hot Jupiter in a 2.87d orbit around the bright (V=10.1), hot, F4V star BD+60 1538 (TYC 3865-1173-1). We present follow-up photometry, spectroscopy, and adaptive optics imaging that allow a detailed characterization of the system. Our preferred model fits yield a host stellar temperature of 6670+/-120 K and a mass of 1.524±0.069 Msun, situating it as one of only a handful of known transiting planets with hosts that are as hot, massive, and bright. The planet has a mass of 1.18±0.11 Mjup, a radius of 1.57±0.04 Rjup, and a density of 0.377±0.040 g/cm^3, making it one of the most inflated planets known around a hot star. We argue that KELT-18b's high temperature, low surface gravity, and hot, bright host make it an excellent candidate for observations aimed at atmospheric characterisation. We also present evidence for a bound stellar companion at a projected separation of ~1100 AU, and speculate that it may have contributed to the strong misalignment we suspect between KELT-18's spin axis and its planet's orbital axis. The inferior conjunction time is 2457542.524998 ±0.000416 (BJD_TDB) and the orbital period is 2.8717510 ± 0.0000029 days. We encourage Rossiter-McLaughlin measurements in the near future to confirm the suspected spin-orbit misalignment of this system.

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