Title: KELT-18b: Puffy Planet, Hot Host, Probably Perturbed,
Abstract: 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 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
(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.