KELT-18b: Puffy Planet, Hot Host, Probably Perturbed
Bieryla, Allyson; Latham, David W.; Berlind, Perry; Calkins, Michael L.; Esquerdo, Gilbert A.; Pepper, Joshua; Stassun, Keivan G.; Siverd, Robert J.; Fulton, Benjamin J.; Howard, Andrew W.; Weiss, Lauren M.; Pogge, Richard W.; Collins, Karen A.; Fukui, Akihiko; Narita, Norio; Lund, Michael B.; Rodriguez, Joseph E.; Oelkers, Ryan J.; Stevens, Daniel J.; Beatty, Thomas G.; Gaudi, B. Scott; Labadie-Bartz, Jonathan; Penev, Kaloyan; Colón, Knicole D.; Relles, Howard; Stockdale, Chris; Eastman, Jason D.; Cohen, David H.; Curtis, Ivan A.; Gregorio, Joao; Jensen, Eric L. N.; Joner, Michael D.; Kielkopf, John F.; Manner, Mark; McLeod, Kim K.; Oberst, Thomas E.; Penny, Matthew T.; Reed, Phillip A.; Stephens, Denise C.; Zambelli, Roberto; Kempton, Eliza M. -R.; Isaacson, Howard; DePoy, D. L.; Zhou, G.; Tan, T. G.; Baştürk, Özgür; Benni, Paul; Marshall, Jennifer L.; Trueblood, Mark; Scarpetta, Gaetano; Trueblood, Patricia; Tsuguru, Ryu; Tirrell, Bethany; Visgaitis, Tiffany; Melton, Casey; Baldrige, Andrew; Zhao, Y. Sunny
United States, Japan, Portugal, Turkey, Italy, Canada, Australia
Abstract
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 6670+/- 120 K and a mass of {1.524}-0.068+0.069 {M}⊙ , 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 {M}{{J}}, a radius of {1.570}-0.036+0.042 {R}{{J}}, 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 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.