Hot B Subdwarf Pulsators Observed with TESS and Asteroseismology of TIC 441725813

Soutenance de thèse de Wenchao Su (Salle de Conférence)

Résumé de la thèse :

Hot subdwarf B (sdB) stars are compact, core-helium-burning stars that occupy a critical stage in stellar evolution. They play a pivotal role in understanding binary evolution, late stages of stellar evolution, and the formation of type Ia supernovae and gravitational wave sources. Despite their importance, the detailed formation channels and internal structures of sdB stars remain a subject of active research, with several open questions regarding their rotational properties, envelope compositions, and binary interactions.

This thesis presents a comprehensive study of sdB stars, with a particular focus on the hybrid pulsator TIC 441725813. As part of this work, we conducted a systematic survey of sdB candidates using photometric data from the Transiting Exoplanet Survey Satellite (TESS). Through rigorous selection and classification procedures, we identified a significant sample of targets that exhibit at least two out of three key features: g-mode pulsations, p-mode pulsations, and signs of binarity. This extensive catalog enriches the current understanding of the pulsation properties of sdB stars and provides an invaluable foundation for subsequent seismic modelling.

For TIC 441725813, we performed an in-depth analysis of its TESS light curves, spanning over 670 days of continuous observations. The resulting frequency spectrum reveals a rich and complex pattern of g-mode and p-mode pulsations, along with clear signatures of rotational splitting and hints of binarity. From the observed splitting, we estimate the core rotation period to be approximately 85 days, with the envelope rotating about 4.7 times faster, suggesting differential rotation and possibly tidal interactions with a companion.

Building on the observed pulsation frequencies, we carried out detailed asteroseismic modelling of TIC 441725813 using fifth-generation (5G) forward models. The modelling was performed with a real-coded genetic algorithm that explored a 13-dimensional parameter space and evaluated nearly one million stellar models. The best-fit model successfully reproduces all 37 observed pulsation modes — including 35 g-modes and 2 p-modes — with a mean relative period deviation of only 0.14%. This marks the first seismic solution for a hybrid sdB pulsator that simultaneously fits both g- and p-mode frequencies at a comparable level of precision.

The resulting stellar parameters are tightly constrained: a total mass of 0.481 ± 0.006 solar masses, a radius of 0.219 ± 0.006 solar radii, and an effective temperature of 28150 ± 525 K, fully consistent with independent estimates from spectroscopy and spectral energy distribution (SED) fitting. Internally, the star is characterized by a double-layered hydrogen/helium envelope and a relatively large convective core, with a helium mass fraction of 0.29 ± 0.05, and extending beyond the core boundary predicted by standard one-dimensional models. This provides observational evidence for significant convective core overshoot and efficient semi-convective mixing, in line with predictions from recent three-dimensional hydrodynamic simulations.

Composition du jury de thèse :

• Orlagh CREEVEY, Rapporteure, Observatoire de la Côte d’Azur – LAGRANGE
• Zhanwen HAN, Rapporteur, Yunnan Observatories, Chinese academy of sciences
• Michel RIEUTORD, Examinateur, IRAP-CNRS/Université de Toulouse
• Valérie VAN GROOTEL, Examinatrice, Université de Liège
• Stéphane CHARPINET, Directeur de thèse, IRAP-CNRS