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

Tuson, A; Queloz, D; Osborn, H P; Wilson, T G; Hooton, M J; Beck, M; Lendl, M; Olofsson, G; Fortier, A; Bonfanti, A; Brandeker, A; Buchhave, L A; Collier Cameron, A; Ciardi, D R; Collins, K A; Gandolfi, D; Garai, Z; Giacalone, S; Gomes da Silva, J; Howell, S B; Patel, J A; Persson, C M; Serrano, L M; Sousa, S G; Ulmer-Moll, S; Vanderburg, A; Ziegler, C; Alibert, Y; Alonso, R; Anglada, G; Bárczy, T; Barrado Navascues, D; Barros, S C C; Baumjohann, W; Beck, T; Benz, W; Billot, N; Bonfils, X; Borsato, L; Broeg, C; Cabrera, J; Charnoz, S; Conti, D M; Csizmadia, Sz; Cubillos, P E; Davies, M B; Deleuil, M; Delrez, L; Demangeon, O D S; Demory, B-O; Dragomir, D; Dressing, C D; Ehrenreich, D; Erikson, A; Essack, Z; Farinato, J; Fossati, L; Fridlund, M; Furlan, E; Gill, H; Gillon, M; Gnilka, C L; Gonzales, E; Güdel, M; Günther, M N; Hoyer, S; Isaak, K G; Jenkins, J M; Kiss, L L; Laskar, J; Latham, D W; Law, N; Lecavelier des Etangs, A; Curto, G Lo; Lovis, C; Luque, R; Magrin, D; Mann, A W; Maxte...

Authors

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

Sz 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 Gonzales

M Güdel

M N Günther

S Hoyer

K G Isaak

J M Jenkins

L L Kiss

J Laskar

D W Latham

N Law

A Lecavelier des Etangs

G Lo Curto

C Lovis

R Luque

D Magrin

A W Mann

M Mayor

S McDermott

M Mecina

C Mordasini

A 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 Thomas

G Torres

R Tronsgaard

S Udry

B Ulmer

V Van Grootel

R Vanderspek

J Venturini

N A Walton

J N Winn

B Wohler



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 ∼ 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$^{+0.07}_{-0.06}$ R⊕ and a period of 21.583298$^{+0.000052}_{-0.000055}$ 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.

Journal Article Type Article
Acceptance Date Jun 8, 2023
Online Publication Date Jun 8, 2023
Publication Date May 30, 2023
Deposit Date Jun 29, 2023
Journal Monthly Notices of the Royal Astronomical Society
Print ISSN 0035-8711
Electronic ISSN 1365-2966
Publisher Oxford University Press
Peer Reviewed Peer Reviewed
Volume 523
Issue 2
Pages 3090-3118
DOI https://doi.org/10.1093/mnras/stad1369
Keywords Space and Planetary Science, Astronomy and Astrophysics