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arXiv · 2306.04511

TESS and CHEOPS Discover Two Warm Sub-Neptunes Transiting the Bright K-dwarf HD 15906

Amy Tuson·Didier Queloz·Hugh P. Osborn·Thomas G. Wilson·Matthew J. Hooton·Mathias Beck·Monika Lendl·Göran Olofsson·Andrea Fortier·Andrea Bonfanti·Alexis Brandeker·Lars A. Buchhave·Andrew Collier Cameron·David R. Ciardi·Karen A. Collins·Davide Gandolfi·Zoltan Garai·Steven Giacalone·João Gomes da Silva·Steve B. Howell·Jayshil A. Patel·Carina M. Persson·Luisa M. Serrano·Sérgio G. Sousa·Solène Ulmer-Moll·Andrew Vanderburg·Carl Ziegler·Yann Alibert·Roi Alonso·Guillem Anglada·Tamas Bárczy·David Barrado Navascues·Susana C. C. Barros·Wolfgang Baumjohann·Thomas Beck·Willy Benz·Nicolas Billot·Xavier Bonfils·Luca Borsato·Christopher Broeg·Juan Cabrera·Sébastien Charnoz·Dennis M. Conti·Szilard Csizmadia·Patricio E. Cubillos·Melvyn B. Davies·Magali Deleuil·Laetitia Delrez·Olivier D. S. Demangeon·Brice-Olivier Demory·Diana Dragomir·Courtney D. Dressing·David Ehrenreich·Anders Erikson·Zahra Essack·Jacopo Farinato·Luca Fossati·Malcolm Fridlund·Elise Furlan·Holden Gill·Michaël Gillon·Crystal L. Gnilka·Erica Gonzales·Manuel Güdel·Maximilian N. Günther·Sergio Hoyer·Kate G. Isaak·Jon M. Jenkins·Laszlo L. Kiss·Jacques Laskar·David W. Latham·Nicholas Law·Alain Lecavelier des Etangs·Gaspare Lo Curto·Christophe Lovis·Rafael Luque·Demetrio Magrin·Andrew W. Mann·Pierre F. L. Maxted·Michel Mayor·Scott McDermott·Marko Mecina·Christoph Mordasini·Annelies Mortier·Valerio Nascimbeni·Roland Ottensamer·Isabella Pagano·Enric Pallé·Gisbert Peter·Giampaolo Piotto·Don Pollacco·Tyler Pritchard·Roberto Ragazzoni·Nicola Rando·Francesco Ratti·Heike Rauer·Ignasi Ribas·George R. Ricker·Martin Rieder·Nuno C. Santos
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Abstract

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 $\sim$ 734 days, 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 $\pm$ 0.08 R$_\oplus$ and a period of 10.924709 $\pm$ 0.000032 days, whilst HD 15906 c has a radius of 2.93$^{+0.07}_{-0.06}$ R$_\oplus$ and a period of 21.583298$^{+0.000052}_{-0.000055}$ days. Assuming zero bond albedo and full day-night heat redistribution, the inner and outer planet have equilibrium temperatures of 668 $\pm$ 13 K and 532 $\pm$ 10 K, respectively. The HD 15906 system has become one of only six multiplanet systems with two warm ($\lesssim$ 700 K) sub-Neptune sized planets transiting a bright star (G $\leq$ 10 mag). It is an excellent target for detailed characterisation studies to constrain the composition of sub-Neptune planets and test theories of planet formation and evolution.

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Amy Tuson, Didier Queloz, Hugh P. Osborn, Thomas G. Wilson, Matthew J. Hooton, Mathias Beck, Monika Lendl, Göran Olofsson, Andrea Fortier, Andrea Bonfanti, Alexis Brandeker, Lars A. Buchhave, Andrew Collier Cameron, David R. Ciardi, Karen A. Collins, Davide Gandolfi, Zoltan Garai, Steven Giacalone, João Gomes da Silva, Steve B. Howell, Jayshil A. Patel, Carina M. Persson, Luisa M. Serrano, Sérgio G. Sousa, Solène Ulmer-Moll, Andrew Vanderburg, Carl Ziegler, Yann Alibert, Roi Alonso, Guillem Anglada, Tamas Bárczy, David Barrado Navascues, Susana C. C. Barros, Wolfgang Baumjohann, Thomas Beck, Willy Benz, Nicolas Billot, Xavier Bonfils, Luca Borsato, Christopher Broeg, Juan Cabrera, Sébastien Charnoz, Dennis M. Conti, Szilard Csizmadia, Patricio E. Cubillos, Melvyn B. Davies, Magali Deleuil, Laetitia Delrez, Olivier D. S. Demangeon, Brice-Olivier Demory, Diana Dragomir, Courtney D. Dressing, David Ehrenreich, Anders Erikson, Zahra Essack, Jacopo Farinato, Luca Fossati, Malcolm Fridlund, Elise Furlan, Holden Gill, Michaël Gillon, Crystal L. Gnilka, Erica Gonzales, Manuel Güdel, Maximilian N. Günther, Sergio Hoyer, Kate G. Isaak, Jon M. Jenkins, Laszlo L. Kiss, Jacques Laskar, David W. Latham, Nicholas Law, Alain Lecavelier des Etangs, Gaspare Lo Curto, Christophe Lovis, Rafael Luque, Demetrio Magrin, Andrew W. Mann, Pierre F. L. Maxted, Michel Mayor, Scott McDermott, Marko Mecina, Christoph Mordasini, Annelies Mortier, Valerio Nascimbeni, Roland Ottensamer, Isabella Pagano, Enric Pallé, Gisbert Peter, Giampaolo Piotto, Don Pollacco, Tyler Pritchard, Roberto Ragazzoni, Nicola Rando, Francesco Ratti, Heike Rauer, Ignasi Ribas, George R. Ricker, Martin Rieder, Nuno C. Santos. 2023-06-07. TESS and CHEOPS Discover Two Warm Sub-Neptunes Transiting the Bright K-dwarf HD 15906. https://doi.org/10.1093/mnras%2Fstad1369

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