Kim K. McLeod, Wellesley College
Joseph E. Rodriguez, Harvard-Smithsonian Center for Astrophysics
Ryan J. Oelkers, Vanderbilt University
Karen A. Collins, Vanderbilt University
Allyson Bieryla, Harvard-Smithsonian Center for Astrophysics
Benjamin J. Fulton, University Hawaii Institute for Astronomy
Keivan G. Stassun, Vanderbilt University
B. Scott Gaudi, The Ohio State University
Kaloyan Penev, Princeton University
Daniel J. Stevens, The Ohio State University
Knicole D. Colón, NASA Ames Research Center
Joshua Pepper, Lehigh University
Norio Narita, The University of Tokyo
Ryu Tsuguru, National Institutes of Natural Sciences - National Astronomical Observatory of Japan
Akihiko Fukui, National Institutes of Natural Sciences - National Astronomical Observatory of Japan
Phillip A. Reed, Kutztown University
Bethany Tirrell, Kutztown University
Tiffany Visgaitis, Kutztown University
John F. Kielkopf, University of Louisville
David H. Cohen, Swarthmore College
Eric L.N. Jensen, Swarthmore College
Joao Gregorio, Atalaia Group and CROW Observatory
Özgür Baştürk, Ankara Üniversitesi
Thomas E. Oberst, Westminster College, New Wilmington
Casey Melton, Wellesley College
Eliza M.R. Kempton, Grinnell College
Andrew Baldrige, Grinnell College
Y. Sunny Zhao, Grinnell College
Roberto Zambelli, Società Astronomica Lunae
David W. Latham, Harvard-Smithsonian Center for Astrophysics
Gilbert A. Esquerdo, Harvard-Smithsonian Center for Astrophysics
Perry Berlind, Harvard-Smithsonian Center for Astrophysics
Michael L. Calkins, Harvard-Smithsonian Center for Astrophysics

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We report the discovery of KELT-18b, a transiting hot Jupiter in a 2.87-day 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 K and a mass of, 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, a radius of, and a density of, making it one of the most inflated planets known around a hot star. We argue that KELT-18b's high temperature and low surface gravity, which yield an estimated ∼600 km atmospheric scale height, combined with its hot, bright host, make it an excellent candidate for observations aimed at atmospheric characterization. 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 (BJDTDB) 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.

Publication Source (Journal or Book title)

Astronomical Journal