McLeod, Kim K.; Rodriguez, Joseph E.; Oelkers, Ryan J.; Collins, Karen A.; ... ; Baştürk, Özgür; Oberst, Thomas E.; ....
The Astronomical Journal, 153, article id. 263
Publication year: 2017

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.