TOI-257b (HD 19916b): A Warm sub-Saturn Orbiting an Evolved F-type Star
Creators: name: Addison, Brett C.
name: Wright, Duncan J.
name: Nicholson, Belinda A.
name: Cale, Bryson
name: Mocnik, Teo
name: Huber, Daniel
name: Plavchan, Peter
name: Wittenmyer, Robert A.
name: Vanderburg, Andrew
name: Chaplin, William J.
name: Chontos, Ashley
name: Clark, Jake T.
name: Eastman, Jason D.
name: Ziegler, Carl
name: Brahm, Rafael
name: Carter, Bradley D.
name: Clerte, Mathieu
name: Espinoza, Néstor
name: Horner, Jonathan
name: Bentley, John
name: Jordán, Andrés
name: Kane, Stephen R.
name: Kielkopf, John F.
name: Laychock, Emilie
name: Mengel, Matthew W.
name: Okumura, Jack
name: Stassun, Keivan G.
name: Bedding, Timothy R.
name: Bowler, Brendan P.
name: Burnelis, Andrius
name: Blanco-Cuaresma, Sergi
name: Collins, Michaela
name: Crossfield, Ian
name: Davis, Allen B.
name: Evensberget, Dag
name: Heitzmann, Alexis
name: Howell, Steve B.
name: Law, Nicholas
name: Mann, Andrew W.
name: Marsden, Stephen C.
name: Matson, Rachel A.
name: O'Connor, James
name: Shporer, Avi
name: Stevens, Catherine
name: Tinney, C. G.
name: Tylor, Christopher
name: Wang, Songhu
name: Zhang, Hui
name: Henning, Thomas
name: Kossakowski, Diana
name: Ricker, George
name: Sarkis, Paula
name: Schlecker, Martin
name: Torres, Pascal
name: Vanderspek, Roland
name: Latham, David W.
name: Seager, Sara
name: Winn, Joshua N.
name: Jenkins, Jon M.
name: Mireles, Ismael
name: Rowden, Pam
name: Pepper, Joshua
name: Daylan, Tansu
name: Schlieder, Joshua E.
name: Collins, Karen A.
name: Collins, Kevin I.
name: Tan, Thiam-Guan
name: Ball, Warrick H.
name: Basu, Sarbani
name: Buzasi, Derek L.
name: Campante, Tiago L.
name: Corsaro, Enrico
name: González-Cuesta, Lucía
name: Davies, Guy R.
name: de Almeida, Leandro
name: Nascimento,, Jose-Dias do
name: a, Rafael A. Garcí
name: Guo, Zhao
name: Handberg, Rasmus
name: Hekker, Saskia
name: Hey, Daniel R.
name: Kallinger, Thomas
name: Kawaler, Steven D.
name: Kayhan, Cenk
name: Kuszlewicz, James S.
name: Lund, Mikkel N.
name: Lyttle, Alexander
name: Mathur, Savita
name: Miglio, Andrea
name: Mosser, Benoit
name: Nielsen, Martin B.
name: Serenelli, Aldo M.
name: Aguirre, Victor Silva
name: Themeßl, Nathalie
Publisher: name: arXiv
Year: 2020
Subjects: subject: Earth and Planetary Astrophysics (astro-ph.EP)
subject: Solar and Stellar Astrophysics (astro-ph.SR)
subject: FOS: Physical sciences
subject: FOS: Physical sciences

description: We report the discovery of a warm sub-Saturn, TOI-257b (HD 19916b), based on data from NASA's Transiting Exoplanet Survey Satellite (TESS). The transit signal was detected by TESS and confirmed to be of planetary origin based on radial velocity observations. An analysis of the TESS photometry, the Minerva-Australis, FEROS, and HARPS radial velocities, and the asteroseismic data of the stellar oscillations reveals that TOI-257b has a mass of $M_P=0.138\pm0.023$\,$\rm{M_J}$ ($43.9\pm7.3$\,$M_{\rm \oplus}$), a radius of $R_P=0.639\pm0.013$\,$\rm{R_J}$ ($7.16\pm0.15$\,$R_{\rm \oplus}$), bulk density of $0.65^{+0.12}_{-0.11}$ (cgs), and period $18.38818^{+0.00085}_{-0.00084}$\,$\rm{days}$. TOI-257b orbits a bright ($\mathrm{V}=7.612$\,mag) somewhat evolved late F-type star with $M_*=1.390\pm0.046$\,$\rm{M_{\odot}}$, $R_*=1.888\pm0.033$\,$\rm{R_{\odot}}$, $T_{\rm eff}=6075\pm90$\,$\rm{K}$, and $v\sin{i}=11.3\pm0.5$\,km\,s$^{-1}$. Additionally, we find hints for a second non-transiting sub-Saturn mass planet on a $\sim71$\,day orbit using the radial velocity data. This system joins the ranks of a small number of exoplanet host stars ($\sim100$) that have been characterized with asteroseismology. Warm sub-Saturns are rare in the known sample of exoplanets, and thus the discovery of TOI-257b is important in the context of future work studying the formation and migration history of similar planetary systems.
description: 24 pages, 20 figures, 6 tables. Published in MNRAS