Issue |
A&A
Volume 693, January 2025
|
|
---|---|---|
Article Number | A90 | |
Number of page(s) | 18 | |
Section | Planets, planetary systems, and small bodies | |
DOI | https://doi.org/10.1051/0004-6361/202451300 | |
Published online | 07 January 2025 |
Radii, masses, and transit-timing variations of the three-planet system orbiting the naked-eye star TOI-396★
1
Austrian Academy of Sciences,
Schmiedlstrasse 6,
8042
Graz,
Austria
2
Dipartimento di Fisica, Università degli Studi di Torino,
via Pietro Giuria 1,
10125
Torino,
Italy
3
Department of Physics and Astronomy, Uppsala University,
Box 516,
Uppsala
75120,
Sweden
4
INAF, Osservatorio Astronomico di Padova,
Vicolo dell’Osservatorio 5,
35122
Padova,
Italy
5
Weltraumforschung und Planetologie, Physikalisches Institut, University of Bern,
Gesellschaftsstrasse 6,
3012
Bern,
Switzerland
6
INAF, Osservatorio Astrofisico di Torino,
Via Osservatorio, 20,
10025
Pino Torinese To,
Italy
7
Department of Physics, University of Warwick,
Coventry
CV4 7AL,
UK
8
Centre for Exoplanets and Habitability, University of Warwick,
Gibbet Hill Road,
Coventry
CV4 7AL,
UK
9
Departamento de Física Teórica e Experimental, Universidade Federal do Rio Grande do Norte, Campus Universitário,
Natal,
RN 59072-970,
Brazil
10
Leiden Observatory, University of Leiden,
PO Box 9513,
2300 RA
Leiden,
The Netherlands
11
Chalmers University of Technology, Department of Space, Earth and Environment, Onsala Space Observatory,
SE-439 92
Onsala,
Sweden
12
INAF – Osservatorio Astrofisico di Arcetri,
Largo E. Fermi 5
Florence,
Italy
13
Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas,
4150-762
Porto,
Portugal
14
School of Physics and Astronomy,
Physical Science Building, North Haugh,
St Andrews,
UK
15
Rhenish Institute for Environmental Research, Dep. of Planetary Research, University of Cologne,
Aachener Str. 209,
50931
Cologne,
Germany
16
Institute of Planetary Research, German Aerospace Center (DLR),
Rutherfordstrasse 2,
12489
Berlin,
Germany
17
Thüringer Landessternwarte Tautenburg,
Sternwarte 5,
07778
Tautenburg,
Germany
18
University Observatory Munich, Ludwig-Maximilians-Universität,
Scheinerstr. 1,
81679
Munich,
Germany
19
European Southern Observatory,
Alonso de Cordova 3107, Vitacura, Casilla,
19001
Santiago,
Chile
20
European Southern Observatory,
Alonso de Cordova 3107, Vitacura,
Santiago de Chile,
Chile
21
Centro de Astrobiología (CAB), CSIC-INTA,
Camino Bajo del Castillo s/n,
28692
Villanueva de la Cañada (Madrid),
Spain
22
McDonald Observatory, The University of Texas,
Austin,
TX,
USA
23
Center for Planetary Systems Habitability, The University of Texas,
Austin,
TX,
USA
24
Department of Astronomy & Astrophysics, University of Chicago,
Chicago,
IL
60637,
USA
25
Astronomy Department and Van Vleck Observatory, Wesleyan University,
Middletown,
CT
06459,
USA
26
Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre,
4169-007
Porto,
Portugal
27
Department of Astronomy of the University of Geneva,
chemin Pegasi 51,
1290
Versoix,
Switzerland
28
Astrobiology Center, NINS,
2-21-1 Osawa, Mitaka,
Tokyo
181-8588,
Japan
29
National Astronomical Observatory of Japan, NINS,
2-21-1 Osawa, Mitaka,
Tokyo
181-8588,
Japan
30
Astronomical Science Program, Graduate University for Advanced Studies, SOKENDAI,
2-21-1, Osawa, Mitaka,
Tokyo,
181-8588,
Japan
31
Instituto de Astrofísica de Canarias (IAC),
calle Vía Láctea s/n,
38205
La Laguna,
Tenerife,
Spain
32
Departamento de Astrofísica, Universidad de La Laguna (ULL),
38206
La Laguna,
Tenerife,
Spain
33
Institute of Astronomy, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University,
Grudziądzka 5,
87-100
Toruń,
Poland
34
Department of Physics and Astronomy, McMaster University,
1280 Main St W,
Hamilton,
ON
L8S 4L8,
Canada
35
Department of Physics, ETH Zurich,
Wolfgang-Pauli-Strasse 2,
8093
Zurich,
Switzerland
★★ Corresponding author; andrea.bonfanti@oeaw.ac.at
Received:
28
June
2024
Accepted:
20
November
2024
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 = 2.004−0.047+0.045 R⊕ ; Rc = 1.979−0.051+0.054 R⊕; Rd = 2.001−0.064+0.063 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 = 3.55−0.96+0.94 M⊕ and Md = 7.1 ± 1.6 M⊕, which implies bulk densities of ρb = 2.44−0.68+0.69 g cm−3 and ρd = 4.9−1.1+1.2 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 = 2.24−0.67+0.13 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.
Key words: techniques: photometric / techniques: radial velocities / planets and satellites: fundamental parameters / stars: fundamental parameters
© The Authors 2025
Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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