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KELT-18b: Puffy Planet, Hot Host, Probably Perturbed

McLeod, Kim K. and Howard, Andrew W. and Fulton, Benjamin J. (2017) KELT-18b: Puffy Planet, Hot Host, Probably Perturbed. Astronomical Journal, 153 (6). Art. No. 263. ISSN 1538-3881. doi:10.3847/1538-3881/aa6d5d.

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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 6670 ± 120 K and a mass of 1.524^(+0.069)_(-0.068) 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^(+0.042)_(-0.036) 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 (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.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Howard, Andrew W.0000-0001-8638-0320
Fulton, Benjamin J.0000-0003-3504-5316
Additional Information:© 2017 The American Astronomical Society. Received 2017 January 28; revised 2017 April 7; accepted 2017 April 10; published 2017 May 22. K.K.M. acknowledges the support of the Theodore Dunham, Jr. Fund for Astronomical Research and the NASA Massachusetts Space Grant consortium. Work performed by J.E.R. was supported by the Harvard Future Faculty Leaders Postdoctoral fellowship. D.J.S. and B.S.G. were partially supported by NSF CAREER Grant AST-1056524. K.P. acknowledges support from NASA grant NNX13AQ62G. N.N. acknowledges support by a Grant-in-Aid for Scientific Research (A) (JSPS KAKENHI Grant Number 25247026). O.B. would like to acknowledge the support by the research fund of Ankara University (BAP) through the project 13B4240006. E.M.R.K., A.B., and Y.S.Z. were supported by the Research Corporation for Science Advancement through the Cottrell College program. A.B. and Y.S.Z. acknowledge funding from the Grinnell College Mentored Advanced Project (MAP) program. We acknowledge Noriyuki Matsunaga for providing time with the Subaru IRCS. This work has made use of NASAs Astrophysics Data System, the Extrasolar Planet Encyclopedia at, the SIMBAD database operated at CDS, Strasbourg, France, and the VizieR catalog access tool, CDS, Strasbourg, France. We also used data products from the Widefield Infrared Survey Explorer, which is a joint project of the University of California, Los Angeles; the Jet Propulsion Laboratory/California Institute of Technology, which is funded by the National Aeronautics and Space Administration; the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation; the American Association of Variable Star Observers (AAVSO) Photometric All-Sky Survey (APASS), whose funding is provided by the Robert Martin Ayers Sciences Fund and the AAVSO Endowment (; and the European Space Agency (ESA) mission Gaia (, processed by the Gaia Data Processing and Analysis Consortium (DPAC, Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. Facilities: KELT (North), APF (Levy), Keck:I (HIRES), FLWO:1.5 m (TRES), Subaru (IRCS), WCWO:0.6m MO:0.6m.
Group:Astronomy Department
Funding AgencyGrant Number
Theodore Dunham, Jr. FundUNSPECIFIED
Massachusetts Space Grant ConsortiumUNSPECIFIED
Harvard Future Faculty Leaders Postdoctoral FellowshipUNSPECIFIED
Japan Society for the Promotion of Science (JSPS)25247026
Ankara University13B4240006
Cottrell Scholar of Research CorporationUNSPECIFIED
Grinnell CollegeUNSPECIFIED
Subject Keywords:methods: observational; planets and satellites: detection; planets and satellites: gaseous planets; stars: individual (BD+60 1538) ; techniques: photometric; techniques: radial velocities
Issue or Number:6
Record Number:CaltechAUTHORS:20170619-144056463
Persistent URL:
Official Citation:Kim K. McLeod et al 2017 AJ 153 263
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:78339
Deposited By: Tony Diaz
Deposited On:19 Jun 2017 22:29
Last Modified:15 Nov 2021 17:38

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