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A Hot Saturn Orbiting an Oscillating Late Subgiant Discovered by TESS

D. Huber, W. J. Chaplin, A. Chontos, H. Kjeldsen, J. Christensen-Dalsgaard, T. R. Bedding, W. Ball, R. Brahm, N. Espinoza, T. Henning, A. Jordán, P. Sarkis, E. Knudstrup, S. Albrecht, F. Grundahl, M. Fredslund Andersen, P. L. Pallé, I. J. M. Crossfield, B. J. Fulton, A. W. Howard, H. Isaacson, L. M. Weiss, R. Handberg, M. N. Lund, A. M. Serenelli, J. Rřrsted Mosumgaard, A. Stokholm, A. Bieryla, L. A. Buchhave, D. W. Latham, S. N. Quinn, E. Gaidos, T. Hirano, G. R. Ricker, R. K. Vanderspek, S. Seager, J. M. Jenkins, J. Winn, H. M. Antia, T. Appourchaux, S. Basu, K. J. Bell, O. Benomar, A. Bonanno, D. L. Buzasi, T. L. Campante, Z. Çelik, E. Corsaro, M. S. Cunha, G. R. Davies, S. Deheuvels, S. K. Grunblatt, A. Hasanzadeh, M. P. Di Mauro, R. A. García, P. Gaulme, L. Girardi, J. A. Guzik, M. Hon, C. Jiang, T. Kallinger, S. D. Kawaler, J. S. Kuszlewicz, Y. Lebreton, T. Li, M. Lucas, M. Lundkvist, A. W. Mann, S. Mathis, S. Mathur, A. Mazumdar, T. S. Metcalfe, A. Miglio, M. J. P. F. G. Monteiro, B. Mosser, A. Noll, B. Nsamba, J. M. J. Ong, S. Örtel, L. F. Pereira, P. Ranadive, C. Régulo, T. Rodrigues, I. W. Roxburgh, V. Silva Aguirre, B. Smalley, Mathew Schofield, S. G. Sousa, Stassun K. G., D. Stello, J. Tayar, T. R. White, K. Verma, M. Vrard, M. Yildiz, D. Baker, M. Bazot, C. Beichman, C. Bergmann, L. Bugnet, B. Cale, R. Carlino, S. M. Cartwright, J. L. Christiansen, D. Ciardi, O. L. Creevey, J. A. Dittman, J. D. do Nascimento Jr., V. Van Eylen, G. Fürész, J. Gagné, P. Gao, K. Gazeas, F. Giddens, O. J. Hall, S. Hekker, M. J. Ireland, N. Latouf, D. LeBrun, A. M. Levine, W. Matzko, E. Natinsky, E. Page, P. P. Plavchan, M. Mansouri-Samani, S. McCauliff, S. E. Mullally, B. Orenstein, A. Garcia Soto, M. Paegert, J. L. Van Saders, C. Schnaible, D. R. Soderblom, R. Szabó, A. Tanner, C. G. Tinney, J. Teske, A. Thomas, R. Trampedach, D. Wright, T. T. Yuan, F. Zohrabi

Abstract
We present the discovery of HD 221416 b, the first transiting planet identified by the Transiting Exoplanet Survey Satellite (TESS) for which asteroseismology of the host star is possible. HD 221416 b (HIP 116158, TOI-197) is a bright (V = 8.2 mag), spectroscopically classified subgiant that oscillates with an average frequency of about 430 μHz and displays a clear signature of mixed modes. The oscillation amplitude confirms that the redder TESS bandpass compared to Kepler has a small effect on the oscillations, supporting the expected yield of thousands of solar-like oscillators with TESS 2 minute cadence observations. Asteroseismic modeling yields a robust determination of the host star radius (R = 2.943 ± 0.064 R), mass (M = 1.212 ± 0.074 M⊙), and age (4.9 ± 1.1 Gyr), and demonstrates that it has just started ascending the red-giant branch. Combining asteroseismology with transit modeling and radial-velocity observations, we show that the planet is a "hot Saturn" (Rp = 9.17 ± 0.33 R) with an orbital period of ~14.3 days, irradiance of F = 343 ± 24 F, and moderate mass (Mp = 60.5 ± 5.7 M) and density (ρp = 0.431 ± 0.062 g cm−3). The properties of HD 221416 b show that the host-star metallicity–planet mass correlation found in sub-Saturns (4–8 R) does not extend to larger radii, indicating that planets in the transition between sub-Saturns and Jupiters follow a relatively narrow range of densities. With a density measured to ~15%, HD 221416 b is one of the best characterized Saturn-size planets to date, augmenting the small number of known transiting planets around evolved stars and demonstrating the power of TESS to characterize exoplanets and their host stars using asteroseismology.

Keywords
asteroseismology; techniques: photometric; planets and satellites: individual: HD 221416 b; stars: fundamental parameters; planets and satellites: fundamental parameters

The Astronomical Journal
Volume 157, Number 6
2019 May

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Instituto de AstrofĂ­sica e CiĂŞncias do Espaço Universidade do Porto Faculdade de Ciências da Universidade de Lisboa
Fundação para a Ciência e a Tecnologia COMPETE 2020 PORTUGAL 2020 União Europeia