Radii, masses, and transit-timing variations of the three-planet system orbiting the naked-eye star TOI-396
Journal article, 2025
Context. TOI-396 is an F6 V bright naked-eye star (V ≈ 6.4) orbited by three small (Rp ≈ 2 R) transiting planets discovered thanks to space-based photometry from two TESS sectors. The orbital periods of the two innermost planets, namely TOI-396 b and c, are close to the 5:3 commensurability (Pb ∼ 3.6 d and Pc ∼ 6.0 d), suggesting that the planets might be trapped in a mean motion resonance (MMR). Aims. To measure the masses of the three planets, refine their radii, and investigate whether planets b and c are in MMR, we carried out HARPS radial velocity (RV) observations of TOI-396 and retrieved archival high-precision transit photometry from four TESS sectors. Methods. We extracted the RVs via a skew-normal fit onto the HARPS cross-correlation functions and performed a Markov chain Monte Carlo joint analysis of the Doppler measurements and transit photometry, while employing the breakpoint method to remove stellar activity from the RV time series. We also performed a transit timing variation (TTV) dynamical analysis of the system and simulated the temporal evolution of the TTV amplitudes of the three planets following an N-body numerical integration. Results. Our analysis confirms that the three planets have similar sizes (Rb = 2.004−+00047045 R; Rc = 1.979−+00051054 R; Rd = 2.001−+00064063 R) and is thus in agreement with previous findings. However, our measurements are ∼ 1.4 times more precise thanks to the use of two additional TESS sectors. For the first time, we have determined the RV masses for TOI-396 b and d, finding them to be Mb = 3.55+−009496 M and Md = 7.1 ± 1.6 M, which implies bulk densities of ρb = 2.44−+006869 g cm−3 and ρd = 4.9−+1112 g cm−3, respectively. Our results suggest a quite unusual system architecture, with the outermost planet being the densest. Based on a frequency analysis of the HARPS activity indicators and TESS light curves, we find the rotation period of the star to be Prot,* = 6.7 ± 1.3 d, in agreement with the value predicted from log R′HK-based empirical relations. The Doppler reflex motion induced by TOI-396 c remains undetected in our RV time series, likely due to the proximity of the planet’s orbital period to the star’s rotation period. We also discovered that TOI-396 b and c display significant TTVs. While the TTV dynamical analysis returns a formally precise mass for TOI-396 c of Mc,dyn = 2.24−+006713 M, the result might not be accurate, owing to the poor sampling of the TTV phase. We also conclude that TOI-396 b and c are close to but out of the 5:3 MMR. Conclusions. A TTV dynamical analysis of additional transit photometry evenly covering the TTV phase and super-period is likely the most effective approach for precisely and accurately determining the mass of TOI-396 c. Our numerical simulation suggests TTV semi-amplitudes of up to five hours over a temporal baseline of ∼ 5.2 years, which should be duly taken into account when scheduling future observations of TOI-396.
techniques: photometric
stars: fundamental parameters
techniques: radial velocities
planets and satellites: fundamental parameters
Author
A. Bonfanti
Osterreichische Akademie Der Wissenschaften
I. Amateis
Uppsala University
Osterreichische Akademie Der Wissenschaften
University of Turin
D. Gandolfi
University of Turin
L. Borsato
Istituto nazionale di astrofisica (INAF)
J. A. Egger
University of Bern
P. E. Cubillos
Osterreichische Akademie Der Wissenschaften
Istituto nazionale di astrofisica (INAF)
D. J. Armstrong
The University of Warwick
I. C. Leão
Federal University of Rio Grande do Norte
Malcolm Fridlund
Chalmers, Space, Earth and Environment, Astronomy and Plasmaphysics
Leiden University
B. L.Canto Martins
Federal University of Rio Grande do Norte
Arcetri Astrophysical Observatory
S.G. Sousa
University of Porto
J. R. De Medeiros
Federal University of Rio Grande do Norte
L. Fossati
Osterreichische Akademie Der Wissenschaften
V. Adibekyan
University of Porto
A. Collier Cameron
University of St Andrews
S. Grziwa
University of Cologne
K. W.F. Lam
German Aerospace Center (DLR)
E. Goffo
Thüringer Landessternwarte Tautenburg
L. D. Nielsen
Ludwig Maximilian University of Munich (LMU)
F. Rodler
European Southern Observatory Santiago
Javier Alarcon
European Southern Observatory Santiago
J. Lillo-Box
Centro de Astrobiologia (CAB)
William Cochran
The University of Texas at Austin
College of Natural Sciences
R. Luque
University of Chicago
S. Redfield
Wesleyan University
N. C. Santos
University of Porto
S.C.C. Barros
University of Porto
D. Bayliss
The University of Warwick
X. Dumusque
University of Geneva
Marcelo Aron Fetzner Keniger
The University of Warwick
J.H. Livingston
National Institutes of Natural Sciences
National Astronomical Observatory of Japan
The Graduate University for Advanced Studies (SOKENDAI)
F. Murgas
Instituto de Astrofísica de Canarias
University of La Laguna
G. Nowak
Nicolaus Copernicus University
A. Osborn
McMaster University
H. P. Osborn
University of Bern
Swiss Federal Institute of Technology in Zürich (ETH)
Enric Palle
Instituto de Astrofísica de Canarias
University of La Laguna
Carina Persson
Chalmers, Space, Earth and Environment, Astronomy and Plasmaphysics
L. M. Serrano
University of Turin
P. A. Strøm
The University of Warwick
S. Udry
University of Geneva
P. J. Wheatley
The University of Warwick
Astronomy and Astrophysics
0004-6361 (ISSN) 1432-0746 (eISSN)
Vol. 693 A90Subject Categories (SSIF 2025)
Astronomy, Astrophysics, and Cosmology
Subatomic Physics
DOI
10.1051/0004-6361/202451300