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

B. C. Addison, D. Wright, P. D. Nicholson, B. Cale, T. Mocnik, D. Huber, P. P. Plavchan, R. A. Wittenmyer, A. Vanderburg, W. J. Chaplin, A. Chontos, J. T. Clark, J. D. Eastman, C. Ziegler, R. Brahm, B. D. Carter, Mathieu Clerte, N. Espinoza, J. Horner, John Bentley, A. Jordán, S. R. Kane, J. F. Kielkopf, Emilie Laychock, M. W. Mengel, J. Okumura, K. G. Stassun, T. R. Bedding, B. P. Bowler, Andrius Burnelis, S. Blanco-Cuaresma, M. Collins, I. J. M. Crossfield, Allen B. Davis, Dag Evensberget, Alexis Heitzmann, S. B. Howell, Nicholas Law, Andrew W. Mann, S. Marsden, R. A. Matson, James H. O'Connor, A. Shporer, C. Stevens, C. G. Tinney, Christopher Tylor, Songhu Wang, H. Zhang, T. Henning, D. Kossakowski, G. R. Ricker, P. Sarkis, M. Schlecker, Pascal Torres, R. K. Vanderspek, D. W. Latham, S. Seager, J. N. Winn, J. M. Jenkins, I. Mireles, P. Rowden, J. Pepper, T. Daylan, J. E. Schlieder, K. A. Collins, K. I. Collins, T.-G. Tan, W. H. Ball, S. Basu, D. L. Buzasi, T. L. Campante, E. Corsaro, L. González-Cuesta, G. R. Davies, Leandro de Almeida, J. D. do Nascimento Jr., R. A. García, Z. Guo, R. Handberg, S. Hekker, D. R. Hey, T. Kallinger, S. D. Kawaler, C. Kayhan, J. S. Kuszlewicz, M. N. Lund, Alexander Lyttle, S. Mathur, A. Miglio, B. Mosser, M. B. Nielsen, A. M. Serenelli, V. Silva Aguirre, N. Themeßl

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 MJ (43.9 ± 7.3 M⁠), a radius of RP = 0.639 ± 0.013 RJ (7.16 ± 0.15 R⁠), bulk density of 0.65+0.12−0.11 (cgs), and period 18.38818+0.00085−0.00084 days⁠. TOI-257b orbits a bright (V = 7.612 mag) somewhat evolved late F-type star with M* = 1.390 ± 0.046 Msun⁠, R* = 1.888 ± 0.033 Rsun⁠, Teff = 6075 ± 90 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 Astrophysi

Monthly Notices of the Royal Astronomical Society
Volume 502, Issue 3, Page 3704
2021 April

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Faculdade de Ciências da Universidade de Lisboa Universidade do Porto Faculdade de Ciências e Tecnologia da Universidade de Coimbra
Fundação para a Ciência e a Tecnologia COMPETE 2020 PORTUGAL 2020 União Europeia