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KELT-21b: A Hot Jupiter Transiting the Rapidly-Rotating Metal-Poor Late-A Primary of a Likely Hierarchical Triple System

Johnson, Marshall C. and Calchi Novati, Sebastiano and Fulton, Benjamin J. (2018) KELT-21b: A Hot Jupiter Transiting the Rapidly-Rotating Metal-Poor Late-A Primary of a Likely Hierarchical Triple System. Astronomical Journal, 155 (2). Art. No. 100. ISSN 1538-3881.

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We present the discovery of KELT-21b, a hot Jupiter transiting the V=10.5 A8V star HD 332124. The planet has an orbital period of P = 3.6127647 ± 0.0000033 days and a radius of 1.586^(+0.039)_(-0.040) R_J. We set an upper limit on the planetary mass of M_P < 3.91 M_J at 3σ confidence. We confirmed the planetary nature of the transiting companion using this mass limit and Doppler tomographic observations to verify that the companion transits HD 332124. These data also demonstrate that the planetary orbit is well-aligned with the stellar spin, with a sky-projected spin-orbit misalignment of λ=−5.6^(+1.7∘)_(-1.9). The star has T_(eff) = 7598^(+81)_(-84) K, M∗ = 1.458^(+0.029)_(-0.028) M⊙, R∗ = 1.638 ± 0.034 R⊙, and v sin I∗ = 146 km s^(-1), the highest projected rotation velocity of any star known to host a transiting hot Jupiter. The star also appears to be somewhat metal-poor and α-enhanced, with [Fe/H] = −0.405^(+0.032)_(-0.033) and [α/Fe] = 0.145 ± 0.053; these abundances are unusual, but not extraordinary, for a young star with thin-disk kinematics like KELT-21. High-resolution imaging observations revealed the presence of a pair of stellar companions to KELT-21, located at a separation of 1".2 and with a combined contrast of ΔK_S = 6.39 ± 0.06 with respect to the primary. Although these companions are most likely physically associated with KELT-21, we cannot confirm this with our current data. If associated, the candidate companions KELT-21 B and C would each have masses of ∼0.12 M⊙, a projected mutual separation of ∼20 AU, and a projected separation of ∼500AU from KELT-21. KELT-21b may be one of only a handful of known transiting planets in hierarchical triple stellar systems.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Johnson, Marshall C.0000-0002-5099-8185
Calchi Novati, Sebastiano0000-0002-7669-1069
Fulton, Benjamin J.0000-0003-3504-5316
Additional Information:© 2018 The American Astronomical Society. Received 2017 December 7; accepted 2017 December 20; published 2018 February 1. We thank Jennifer Johnson, Marc Pinsonneault, and Dennis Stello for useful discussions on the Galactic context of KELT-21 and the analysis using the APOKASC catalog. Work performed by J.E.R. was supported by the Harvard Future Faculty Leaders Postdoctoral fellowship. Work by G.Z. is provided by NASA through Hubble Fellowship grant HST-HF2-51402.001-A awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS 5-26555. Work performed by P.A.C. was supported by NASA grant NNX13AI46G. K.P. acknowledges support from NASA grant NNX13AQ62G. B.S.G. and D.J.S. were partially supported by NSF CAREER Grant AST-1056524. A.S. is partially supported by grant ESP2015-66134-R. Funding for the Stellar Astrophysics Centre is provided by The Danish National Research Foundation (grant agreement No. DNRF106). V.S.A. acknowledges support from VILLUM FONDEN (research grant 10118). This project makes use of data from the KELT survey, including support from The Ohio State University, Vanderbilt University, and Lehigh University, along with the KELT follow-up collaboration. The LBT is an international collaboration among institutions in the United States, Italy, and Germany. The LBT Corporation partners are The Ohio State University; LBT Beteiligungsgesellschaft, Germany, representing the Max Planck Society, the Astrophysical Institute Potsdam, and Heidelberg University; The University of Arizona on behalf of the Arizona university system; Istituto Nazionale di Astrofisica, Italy; and The Research Corporation, on behalf of The University of Notre Dame, University of Minnesota, and University of Virginia. This paper includes data taken at The McDonald Observatory of The University of Texas at Austin. Some of the data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. This work has made use of NASA's 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 Wide-field Infrared Survey Explorer, which is a joint project of the University of California, Los Angeles, and the Jet Propulsion Laboratory/California Institute of Technology, 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; 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, LBT (PEPSI), FLWO (TRES), Smith (TS23), Keck (NIRC2). Software: Python, IDL, IRAF, TAPIR (Jensen 2013), AstroImageJ (Collins & Kielkopf 2013), SDS4PEPSI (Strassmeier et al. 2017), EXOFAST (Eastman et al. 2013), POET (Penev et al. 2014), TRILEGAL (Girardi et al. 2005), Besançon (Robin et al. 2003), isochrones (Morton 2015), galpy (Bovy 2015).
Group:Infrared Processing and Analysis Center (IPAC)
Funding AgencyGrant Number
Harvard UniversityUNSPECIFIED
NASA Hubble FellowshipHST-HF2-51402.001-A
NASANAS 5-26555
Fondo Europeo de Desarrollo Regional (FEDER)ESP2015-66134-R
Danish National Research FoundationDNRF106
Villum Foundation10118
W. M. Keck FoundationUNSPECIFIED
Subject Keywords:planets and satellites: detection – planets and satellites: gaseous planets – stars: individual (HD 332124) – techniques: photometric – techniques: radial velocities – methods: observational
Issue or Number:2
Record Number:CaltechAUTHORS:20180110-084137823
Persistent URL:
Official Citation:Marshall C. Johnson et al 2018 AJ 155 100
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:84223
Deposited By: Tony Diaz
Deposited On:10 Jan 2018 17:21
Last Modified:09 Mar 2020 13:18

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