KELT-21b: A Hot Jupiter Transiting the Rapidly-Rotating Metal-Poor Late-A Primary of a Likely Hierarchical Triple System
Creators: name: Johnson, Marshall C.
name: Rodriguez, Joseph E.
name: Zhou, George
name: Gonzales, Erica J.
name: Cargile, Phillip A.
name: Crepp, Justin R.
name: Penev, Kaloyan
name: Stassun, Keivan G.
name: Gaudi, B. Scott
name: Colón, Knicole D.
name: Stevens, Daniel J.
name: Strassmeier, Klaus G.
name: Ilyin, Ilya
name: Collins, Karen A.
name: Kielkopf, John F.
name: Oberst, Thomas E.
name: Maritch, Luke
name: Reed, Phillip A.
name: Gregorio, Joao
name: Bozza, Valerio
name: Novati, Sebastiano Calchi
name: D'Ago, Giuseppe
name: Scarpetta, Gaetano
name: Zambelli, Roberto
name: Latham, David W.
name: Bieryla, Allyson
name: Cochran, William D.
name: Endl, Michael
name: Tayar, Jamie
name: Serenelli, Aldo
name: Aguirre, Victor Silva
name: Clarke, Seth P.
name: Martinez, Maria
name: Spencer, Michelle
name: Trump, Jason
name: Joner, Michael D.
name: Bugg, Adam G.
name: Hintz, Eric G.
name: Stephens, Denise C.
name: Arredondo, Anicia
name: Benzaid, Anissa
name: Yazdi, Sormeh
name: McLeod, Kim K.
name: Jensen, Eric L. N.
name: Hancock, Daniel A.
name: Sorber, Rebecca L.
name: Kasper, David H.
name: Jang-Condell, Hannah
name: Beatty, Thomas G.
name: Carroll, Thorsten
name: Eastman, Jason
name: James, David
name: Kuhn, Rudolf B.
name: Labadie-Bartz, Jonathan
name: Lund, Michael B.
name: Mallonn, Matthias
name: Pepper, Joshua
name: Siverd, Robert J.
name: Yao, Xinyu
name: Cohen, David H.
name: Curtis, Ivan A.
name: DePoy, D. L.
name: Fulton, Benjamin J.
name: Penny, Matthew T.
name: Relles, Howard
name: Stockdale, Christopher
name: Tan, Thiam-Guan
name: Villanueva, Steven
Publisher: name: arXiv
Year: 2017
Subjects: subject: Earth and Planetary Astrophysics (astro-ph.EP)
subject: Astrophysics of Galaxies (astro-ph.GA)
subject: FOS: Physical sciences
subject: FOS: Physical sciences

description: 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 $P=3.6127647\pm0.0000033$ days and a radius of $1.586_{-0.040}^{+0.039}$ $R_J$. We set an upper limit on the planetary mass of $M_P&lt;3.91$ $M_J$ 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 $λ=-5.6_{-1.9}^{+1.7 \circ}$. The star has $T_{\mathrm{eff}}=7598_{-84}^{+81}$ K, $M_*=1.458_{-0.028}^{+0.029}$ $M_{\odot}$, $R_*=1.638\pm0.034$ $R_{\odot}$, 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]$=-0.405_{-0.033}^{+0.032}$ and [$α$/Fe]$=0.145 \pm 0.053$; 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 $ΔK_S=6.39 \pm 0.06$ 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 $\sim0.12$ $M_{\odot}$, a projected mutual separation of $\sim20$ AU, and a projected separation of $\sim500$ AU from KELT-21. KELT-21b may be one of only a handful of known transiting planets in hierarchical triple stellar systems.
description: Accepted for publication in AJ. Updated to match accepted version. 25 pages, 14 figures