TOI-257b (HD 19916b): a warm sub-saturn orbiting an evolved F-type star

Brett C. Addison , Duncan J. Wright , Belinda A. Nicholson , Bryson Cale , Teo Mocnik , Daniel Huber , Peter Plavchan , Robert A. Wittenmyer , Andrew Vanderburg , William J. Chaplin , Ashley Chontos , Jake T. Clark , Jason D. Eastman , Carl Ziegler , Rafael Brahm , Bradley D. Carter , Mathieu Clerte , Néstor Espinoza , Jonathan Horner , John Bentley , Andrés Jordán , Stephen R. Kane , John F. Kielkopf , Emilie Laychock , Matthew W. Mengel , Jack Okumura , Keivan G. Stassun , Timothy R. Bedding , Brendan P. Bowler , Andrius Burnelis , Sergi Blanco-Cuaresma , Michaela Collins , Ian Crossfield , Allen B. Davis , Dag Evensberget , Alexis Heitzmann , Steve B. Howell , Nicholas Law , Andrew W. Mann , Stephen C. Marsden , Rachel A. Matson , James H. O'Connor , Avi Shporer , Catherine Stevens , C. G. Tinney , Christopher Tylor , Songhu Wang , Hui Zhang , Thomas Henning , Diana Kossakowski , George Ricker , Paula Sarkis , Martin Schlecker , Pascal Torres , Roland Vanderspek , David W. Latham , Sara Seager , Joshua N. Winn , Jon M. Jenkins , Ismael Mireles , Pam Rowden , Joshua Pepper , Tansu Daylan , Joshua E. Schlieder , Karen A. Collins , Kevin I. Collins , Thiam-Guan Tan , Warrick H. Ball , Sarbani Basu , Derek L. Buzasi , Tiago L. Campante , Enrico Corsaro , L. González-Cuesta , Guy R. Davies , Leandro de Almeida , Jose-Dias Do Nascimento , Rafael A. García , Zhao Guo , Rasmus Handberg , Saskia Hekker , Daniel R. Hey , Thomas Kallinger , Steven D. Kawaler , Cenk Kayhan , James S. Kuszlewicz , Mikkel N. Lund , Alexander Lyttle , Savita Mathur , Andrea Miglio , Benoit Mosser (1) , Martin B. Nielsen , Aldo M. Serenelli , Victor Silva Aguirre , Nathalie Themeßl

1 LESIA - Laboratoire d'études spatiales et d'instrumentation en astrophysique = Laboratory of Space Studies and Instrumentation in Astrophysics

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 ± 0.023 $\rm {M_J}$ (43.9 ± 7.3 $\, M_{\rm \oplus}$ ), a radius of R_P = 0.639 ± 0.013 $\rm {R_J}$ (7.16 ± 0.15 $\, \mathrm{ R}_{\rm \oplus}$ ), 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 (V = 7.612 mag) somewhat evolved late F-type star with M_* = 1.390 ± 0.046 $\rm {M_{sun}}$ , R_* = 1.888 ± 0.033 $\rm {R_{sun}}$ , T_eff = 6075 ± 90 $\rm {K}$ , and vsin i = 11.3 ± 0.5 km s^-1. Additionally, we find hints for a second non-transiting sub-Saturn mass planet on a ∼71 day orbit using the radial velocity data. This system joins the ranks of a small number of exoplanet host stars (∼100) 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.

Keywords: asteroseismology; techniques: photometric; techniques: radial velocities; techniques: spectroscopic; planetary systems; stars: individual (TIC 200723869/TOI-257); Astrophysics - Earth and Planetary Astrophysics; Astrophysics - Solar and Stellar Astrophysics

Domains: Sciences of the Universe [physics]

Brett C. Addison, Duncan J. Wright, Belinda A. Nicholson, Bryson Cale, Teo Mocnik, et al.. TOI-257b (HD 19916b): a warm sub-saturn orbiting an evolved F-type star. Monthly Notices of the Royal Astronomical Society, 2021, 502, pp.3704-3722. ⟨10.1093/mnras/staa3960⟩. ⟨insu-03713781⟩