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TOI-431/HIP 26013: a super-Earth and a sub-Neptune transiting a bright, early K dwarf, with a third RV planet

A. Osborn, D. J. Armstrong, B. Cale, R. Brahm, R. A. Wittenmyer, F. Dai, I. J. M. Crossfield, E. M. Bryant, V. Zh. Adibekyan, R. Cloutier, K. A. Collins, E. Delgado Mena, M. Fridlund, C. Hellier, S. B. Howell, G. W. King, J. Lillo-Box, J. F. Otegi, S. G. Sousa, K. G. Stassun, E. C. Matthews, C. Ziegler, G. R. Ricker, R. K. Vanderspek, D. W. Latham, S. Seager, J. N. Winn, J. M. Jenkins, J. S. Acton, B. C. Addison, D. R. Anderson, S. Ballard, D. Barrado, S. C. C. Barros, N. Batalha, D. Bayliss, T. Barclay, B. Benneke, J. Berberian, F. Bouchy, B. P. Bowler, C. Briceño, C. J. Burke, M. Burleigh, S. Casewell, D. Ciardi, K. I. Collins, B. F. Cooke, O. Demangeon, R. F. Díaz, C. Dorn, D. Dragomir, C. D. Dressing, X. Dumusque, N. Espinoza, P. Figueira, B. J. Fulton, E. Furlan, E. Gaidos, C. Geneser, S. Gill, M. R. Goad, E. J. Gonzales, V. Gorjian, M. N. Günther, R. Helled, B. Henderson, T. Henning, A. E. Hogan, S. Hojjatpanah, J. Horner, A. W. Howard, S. Hoyer, D. Huber, H. Isaacson, J. S. Jenkins, E. L. N. Jensen, A. Jordán, S. R. Kane, R. C. Kidwell, J. F. Kielkopf, N. M. Law, M. Lendl, M. N. Lund, R. A. Matson, A. W. Mann, J. McCormac, M. W. Mengel, F. Y. Morales, L. D. Nielsen, J. Okumura, H. P. Osborn, E. Petigura, P. P. Plavchan, D. Pollacco, E. V. Quintana, L. Raynard, P. Robertson, M. Rose, A. Roy, M. Reefe, A. Santerne, N. C. Santos, P. Sarkis, J. E. Schlieder, R. P. Schwarz, N. J. Scott, A. Shporer, A. M. S. Smith, C. Stibbard, C. Stockdale, P. A. Strøm, J. D. Twicken, T.-G. Tan, A. Tanner, J. Teske, R. H. Tilbrook, C. G. Tinney, S. Udry, J. Villaseñor, J. I. Vines, S. X. Wang, L. M. Weiss, R. G. West, P. J. Wheatley, D. Wright, H. Zhang, F. Zohrabi

Abstract
We present the bright (Vmag = 9.12), multiplanet system TOI-431, characterized with photometry and radial velocities (RVs). We estimate the stellar rotation period to be 30.5 ± 0.7 d using archival photometry and RVs. Transiting Exoplanet Survey Satellite (TESS) objects of Interest (TOI)-431 b is a super-Earth with a period of 0.49 d, a radius of 1.28 ± 0.04 R⊕, a mass of 3.07 ± 0.35 M⊕, and a density of 8.0 ± 1.0 g cm−3; TOI-431 d is a sub-Neptune with a period of 12.46 d, a radius of 3.29 ± 0.09 R, a mass of 9.90+1.53−1.49 M, and a density of 1.36 ± 0.25 g cm−3. We find a third planet, TOI-431 c, in the High Accuracy Radial velocity Planet Searcher RV data, but it is not seen to transit in the TESS light curves. It has an Msin i of 2.83+0.41−0.34 M⊕, and a period of 4.85 d. TOI-431 d likely has an extended atmosphere and is one of the most well-suited TESS discoveries for atmospheric characterization, while the super-Earth TOI-431 b may be a stripped core. These planets straddle the radius gap, presenting an interesting case-study for atmospheric evolution, and TOI-431 b is a prime TESS discovery for the study of rocky planet phase curves.

Keywords
planets and satellites: detection, planets and satellites: fundamental parameters, planets and satellites: individual: (TOI-431, TIC 31374837)

Monthly Notices of the Royal Astronomical Society
Volume 507, Issue 2, Page 2782
2021 October

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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