TOI-257b (HD 19916b): a warm sub-saturn orbiting an evolved F-type star
Brett Addison, D. J. Wright, Belinda Nicholson, Bryson Cale, Teo Močnik, Daniel Huber, Peter Plavchan, Robert A. Wittenmyer, Andrew Vanderburg, W. J. Chaplin, Ashley Chontos, Jake Clark, Jason D. Eastman, Carl Ziegler, Rafael Brahm, Brad Carter, Mathieu Clerté, Néstor Espinoza, Jonathan Horner, John Bentley, Andrés Jordán, Stephen R. Kane, John F. Kielkopf, Emilie Laychock, Matthew W. Mengel, Jack Okumura, Keivan G. Stassun, T. R. Bedding, Brendan P. Bowler, Andrius Burnelis, S. Blanco-Cuaresma, Michaela Collins, Ian J. M. Crossfield, Allen B. Davis, Dag Evensberget, A. Heitzmann, Steve B. Howell, Nicholas M. Law, Andrew W. Mann, S. C. Marsden, Rachel A. Matson, James H O’Connor, Avi Shporer, C. P. Stevens, C. G. Tinney, Christopher Tylor, Songhu Wang, Hui Zhang, Thomas Henning, D. Kossakowski, G. Ricker, P. Sarkis, Martin Schlecker, P. Torres, R. Vanderspek, David W. Latham, Sara Seager, Joshua N. Winn, Jon M. Jenkins, Ismael Mireles, Pamela 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, T. L. Campante, E. Corsaro, L. González-Cuesta, G. R. Davies, Leandro de Almeida, J.-D. do Nascimento, R. A. García, Zhao Guo, R. Handberg, S. Hekker, Daniel Hey, T. Kallinger, S. D. Kawaler, Cenk Kayhan, James S. Kuszlewicz, Mikkel N. Lund, Alexander J Lyttle, S. Mathur, A. Miglio, B. Mosser, M. B. Nielsen, Aldo Serenelli, V. Silva Aguirre, N. Themeßl
Abstract
ABSTRACT 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 MP = 0.138 ± 0.023 $\rm {M_J}$ (43.9 ± 7.3 $\, M_{\rm \oplus}$), a radius of RP = 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 $\rm {R_{sun}}$, Teff = 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.