Marshall C. Johnson, The Ohio State University
Joseph E. Rodriguez, Harvard-Smithsonian Center for Astrophysics
George Zhou, Harvard-Smithsonian Center for Astrophysics
Erica J. Gonzales, University of California, Santa Cruz
Phillip A. Cargile, Harvard-Smithsonian Center for Astrophysics
Justin R. Crepp, University of Notre Dame
Kaloyan Penev, The University of Texas at Dallas
Keivan G. Stassun, Vanderbilt University
B. Scott Gaudi, The Ohio State University
Knicole D. Colón, NASA Goddard Space Flight Center
Daniel J. Stevens, The Ohio State University
Klaus G. Strassmeier, Leibniz Institute for Astrophysics Potsdam
Ilya Ilyin, Leibniz Institute for Astrophysics Potsdam
Karen A. Collins, Harvard-Smithsonian Center for Astrophysics
John F. Kielkopf, University of Louisville
Thomas E. Oberst, Westminster College, New Wilmington
Luke Maritch, Kutztown University
Phillip A. Reed, Kutztown University
Joao Gregorio, Atalaia Group and CROW Observatory
Valerio Bozza, Università degli Studi di Salerno
Sebastiano Calchi Novati, Università degli Studi di Salerno
Giuseppe D'Ago, Osservatorio Astronomico di Capodimonte
Gaetano Scarpetta, Università degli Studi di Salerno
Roberto Zambelli, Società Astronomica Lunae
David W. Latham, Harvard-Smithsonian Center for Astrophysics
Allyson Bieryla, Harvard-Smithsonian Center for Astrophysics
William D. Cochran, The University of Texas at Austin
Michael Endl, The University of Texas at Austin
Jamie Tayar, The Ohio State University
Aldo Serenelli, Consejo Superior de Investigaciones Científicas
Victor Silva Aguirre, Aarhus Universitet
Seth P. Clarke, Brigham Young University
Maria Martinez, Brigham Young University

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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 1.586-0.040+0.039 RJ. We set an upper limit on the planetary mass of at 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 = -5.6-1.91.7. The star has Teff = 7598-8481 K, Mz.ast; = 1.458-0.028+0.029 M⊙, Rz.ast; = 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] = -405-0.033+0.032 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 ΔKs = 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 ∼500 au from KELT-21. KELT-21b may be one of only a handful of known transiting planets in hierarchical triple stellar systems.

Publication Source (Journal or Book title)

Astronomical Journal