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TESS and CHEOPS discover two warm sub-Neptunes transiting the bright K-dwarf HD 15906

A. Tuson, D. Queloz, H. P. Osborn, T. G. Wilson, M. J. Hooton, M. Beck, M. Lendl, G. Olofsson, A. Fortier, A. Bonfanti, A. Brandeker, L. A. Buchhave, A. Collier Cameron, D. R. Ciardi, K. A. Collins, D. Gandolfi, Z. Garai, S. Giacalone, J. Gomes da Silva, S. B. Howell, J. A. Patel, C. M. Persson, L. M. Serrano, S. G. Sousa, S. Ulmer-Moll, A. Vanderburg, C. Ziegler, Y. Alibert, R. Alonso, G. Anglada, T. Bárczy, D. Barrado Navascues, S. C. C. Barros, W. Baumjohann, T. Beck, W. Benz, N. Billot, X. Bonfils, L. Borsato, C. Broeg, J. Cabrera, S. Charnoz, D. M. Conti, S. Csizmadia, P. E. Cubillos, M. B. Davies, M. Deleuil, L. Delrez, O. D. S. Demangeon, B.-O. Demory, D. Dragomir, C. D. Dressing, D. Ehrenreich, A. Erikson, Z. Essack, J. Farinato, L. Fossati, M. Fridlund, E. Furlan, H. Gill, M. Gillon, C. L. Gnilka, E. J. Gonzales, M. Güdel, M. N. Günther, S. Hoyer, K. G Isaak, J. M. Jenkins, L. L. Kiss, J. Laskar, D. W. Latham, N. M. Law, A. Lecavelier Des Etangs, G. Lo Curto, C. Lovis, R. Luque, D. Magrin, A. W. Mann, P. F. L. Maxted, M. Mayor, S. McDermott, M. Mecina, C. Mordasini, A. M. J. Mortier, V. Nascimbeni, R. Ottensamer, I. Pagano, E. Pallé, G. Peter, G. Piotto, D. Pollacco, T. Pritchard, R. Ragazzoni, N. Rando, F. Ratti, H. Rauer, I. Ribas, G. R. Ricker, M. Rieder, N. C. Santos, A. B. Savel, G. Scandariato, R. P. Schwarz, S. Seager, D. Ségransan, A. Shporer, A. E. Simon, A. M. S. Smith, M. Steller, C. Stockdale, Gy. M. Szabó, N. L. Thomas, G. Torres, R. Tronsgaard, S. Udry, B. Ulmer, V. Van Grootel, R. K. Vanderspek, J. Venturini, N. A. Walton, J. N. Winn, B. Wohler

We report the discovery of two warm sub-Neptunes transiting the bright (G = 9.5 mag) K-dwarf HD 15906 (TOI 461, TIC 4646810). This star was observed by the Transiting Exoplanet Survey Satellite (TESS) in sectors 4 and 31, revealing two small transiting planets. The inner planet, HD 15906 b, was detected with an unambiguous period but the outer planet, HD 15906 c, showed only two transits separated by ∼ 734 d, leading to 36 possible values of its period. We performed follow-up observations with the CHaracterising ExOPlanet Satellite (CHEOPS) to confirm the true period of HD 15906 c and improve the radius precision of the two planets. From TESS, CHEOPS, and additional ground-based photometry, we find that HD 15906 b has a radius of 2.24 ± 0.08 R and a period of 10.924709 ± 0.000032 d, whilst HD 15906 c has a radius of 2.93-0.06+0.07 R and a period of 21.583298-0.000055+0.000052 d. Assuming zero bond albedo and full day-night heat redistribution, the inner and outer planet have equilibrium temperatures of 668 ± 13 K and 532 ± 10 K, respectively. The HD 15906 system has become one of only six multiplanet systems with two warm (≲ 700 K) sub-Neptune sized planets transiting a bright star (G ≤ 10 mag). It is an excellent target for detailed characterization studies to constrain the composition of sub-Neptune planets and test theories of planet formation and evolution.

techniques: photometric, planets and satellites: detection, planets and satellites: fundamental parameters, stars: fundamental parameters, stars: individual: HD 15906 (TOI 461, TIC 4646810)

Monthly Notices of the Royal Astronomical Society
Volume 523, Issue 2, Page 28
2023 August

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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