Radii, masses, and transit-timing variations of the three-planet system orbiting the naked-eye star TOI-396

• Type: Article in International Scientific Journal
• Date: 2025

Authors

Bonfanti, A (Author) Other; Amateis, I (Author) Other; Gandolfi, D (Author) Other; Borsato, L (Author) Other; Egger, JA (Author) Other; Cubillos, PE (Author) Other; Armstrong, D (Author) Other; Leao, IC (Author) Other; Fridlund, M (Author) Other; Martins, BLC (Author) Other; Sousa, SG (Author) Other; De Medeiros, JR (Author) Other; Fossati, L (Author) Other; Adibekyan, V (Author) Other; Cameron, AC (Author) Other; Grziwa, S (Author) Other; Lam, KWF (Author) Other; Goffo, E (Author) Other; Nielsen, LD (Author) Other; Rodler, F (Author) Other; Alarcon, J (Author) Other; Lillo-Box, J (Author) Other; Cochran, WD (Author) Other; Luque, R (Author) Other; Redfield, S (Author) Other; N.C. Santos (Author) FCUP; Barros, SCC (Author) Other; Bayliss, D (Author) Other; Dumusque, X (Author) Other; Keniger, MAF (Author) Other; Livingston, J (Author) Other; Murgas, F (Author) Other; Nowak, G (Author) Other; Osborn, A (Author) Other; Osborn, HP (Author) Other; Pallé, E (Author) Other; Persson, CM (Author) Other; Serrano, LM (Author) Other; Strom, PA (Author) Other; Udry, S (Author) Other; Wheatley, PJ (Author) Other

Journal

Title: Astronomy and Astrophysics

Vol. 693

Final page: A90

ISSN: 0004-6361

Publisher: EDP Sciences

Indexing

ISI Web of Knowledge - 0 Citations

Scopus - 0 Citations

Other information

Authenticus ID: P-017-WEZ

DOI: 10.1051/0004-6361/202451300

Abstract (EN): Context. TOI-396 is an F6 V bright naked-eye star (V approximate to 6.4) orbited by three small (R-p approximate to 2 R-circle plus) 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 (P-b similar to 3.6 d and P-c similar to 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 (R-b = 2.004(-0.047)(+0.045) R-circle plus; R-c = 1.979(-0.051)(+0.054) R-circle plus; R-d = 2.001(-0.064)(+0.063) R-circle plus) and is thus in agreement with previous findings. However, our measurements are similar to 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 M-b = 3.55(-0.96)(+0.94) M-circle plus and M-d = 7.1 +/- 1.6 M-circle plus, which implies bulk densities of rho(b) = 2.44(-0.68)(+0.69) g cm(-3) and rho(d) = 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 P-rot,P-star = 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 M-c,M-dyn = 2.24(-0.67)(+0.13) M-circle plus, 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 similar to 5.2 years, which should be duly taken into account when scheduling future observations of TOI-396.

Language: English

Type (Professor's evaluation): Scientific

No. of pages: 18