Séminaires

A Mechanism for creating the hard X-rays of gamma Cas (B0.5e): Clues from Multi-wavelength Observations
Myron Smith (NOIRlab, Tucson, Arizona, USA)

Salle de réunion AGO (local -1/14), Institut d'Astrophysique et de Géophysique
Bâtiment B5c, Quartier Agora, Allée du 6 Août, 19C, B-4000 Liège 1 (Sart-Tilman)

In this talk the X-rays of gamma Cas will be examined. gamma Cas is the prototype of a subgroup of Be stars, the spectra of which exhibit hydrogen line emission, a signature of episodically ejected, flattened disks. Members of the gamma Cas subgroup also emit X-rays ~30 times more intense than the X-rays produced by winds of normal B and Be stars, and host an equally greater high plasma temperature. The mechanism responsible for this emission has been long debated. I will confine attention to one that is most consistent with observations, one positing magnetic interactions between gamma Cas and its disk. In this scenario, small-scale magnetic fields extending from the Be star entangle with a toroidal disk field. The stresses so produced release high energy particle beams that impact the star's surface explosively, resulting in observed X-rays 'flares.' This explanation is implied by correlations of X-ray, UV, and optical light curves. The narrative is also supported by features in X-ray spectra. Altogether, observations suggest an environment near gamma Cas stars unlike from those near other hot massive stars, though there are some similarities to magnetospheres of cool stars, including the Sun.

Enhancing stellar characterization through seismic inversions: selected insights and future prospects
Jérôme Bétrisey (Université de Genève)

Salle de réunion AGO (local -1/14), Institut d'Astrophysique et de Géophysique
Bâtiment B5c, Quartier Agora, Allée du 6 Août, 19C, B-4000 Liège 1 (Sart-Tilman)

With the rapid development of asteroseismology in the last two decades thanks to space-based photometry missions such as CoRoT, Kepler, and TESS, stellar characterisation has reached unprecedented levels. The field will further develop with the future PLATO mission, to be launched in 2026. The data quality of these missions enables the use of cutting-edge techniques, the so-called seismic inversions, until then restricted to helioseismology, where they were applied with tremendous success. One of the key challenges of PLATO is the precision requirements on the stellar mass, radius, and age, for which these inversion techniques will play a key role. It is therefore crucial to compare how current modelling strategies perform, and discuss the limitations and remaining challenges for PLATO, such as the so-called surface effects, the choice of the physical ingredients, and stellar activity. In this context, I will present the developments and improvements I carried out during my PhD to achieve an even more precise and accurate stellar characterization, following two guiding principles, refining the modelling strategies, and better constraining the physics in stellar models.

My scientific legacy: are the universe and cosmology inside a black hole?
Pierre Magain (AGO)

Salle S74, Amphithéâtres de l'Europe
Bâtiment B4, Quartier Agora, Boulevard du Rectorat, 3, B-4000 Liège 1 (Sart-Tilman)

Semi-annihilating Z3 scalar dark matter model and gravitational waves
Maxim Laletin (National Centre for Nuclear Research (NCBJ), Warsaw, Poland)

Salle de réunion AGO (local -1/14), Institut d'Astrophysique et de Géophysique
Bâtiment B5c, Quartier Agora, Allée du 6 Août, 19C, B-4000 Liège 1 (Sart-Tilman)

We study dark matter phenomenology with early kinetic decoupling, cosmic phase transitions and their gravitational wave signal in the semi-annihilating Z3 dark matter model, whose dark sector comprises of an inert doublet and a complex singlet. We take into account constraints from perturbativity, unitarity, vacuum stability, electroweak precision tests, direct detection and LHC measurements of the Higgs boson. We discuss the prospects for direct detection searches and gravitational wave searches.