Séminaires

Treatment of magnetic activity effects in an era of high-precision asteroseismology
Jérôme Bétrisey ('Université d'Uppsala.)

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

Following the success of missions like CoRoT, Kepler, and TESS, asteroseismic modelling is poised to play a pivotal role in upcoming space-based missions such as PLATO, CubeSpec, and Roman. Despite remarkable achievements, asteroseismology has also revealed significant discrepancies between observed data and theoretical stellar models, leading to non-negligible biases in stellar characterisation in our era of high-precision asteroseismology. In the past decades, magnetic activity effects were typically neglected in asteroseismic modelling of solar-type stars, assuming that these effects could be accounted for in the parametrisation of the so-called ‘surface effects’. This picture has, however, been challenged in recent years, as it was demonstrated that magnetic activity can have a significant impact on the asteroseismic characterisation using both forward and inverse methods. In this presentation, I will present these results and show that magnetic activity effects cannot be suppressed with standard methods employed to mitigate surface effects. Subsequently, I will also discuss how magnetic activity effects are averaged with longer observations and what conclusions can be drawn for future photometry missions.

Influence of a subsurface ocean on the rotation variations of large icy satellites
Alexis Coyette (UNamur)

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 use an angular momentum approach to study the Cassini states (CS) of large natural satellites such as the Galilean satellites and Titan. Unlike classical approaches where obliquity is the solution of a trigonometric equation, our approach allows us to identify not only the mean obliquity of satellites, but also their nutation in space as well as their polar motion (PM) with respect to the solid surface. Triaxiality of the satellite has a significant effect on the mean obliquities of CSI, CSII and CSIV. We assess the stability of the Cassini states over a wide range of free and forced precession frequency ratios and find that CSI and CSIII are always stable. Even if the different Galilean Moons are thought to occupy CSI, we therefore also analytically study CSIII. We here solve the dynamic equations governing CSI and CSIII up to order two in small quantities and without averaging the external torque over the mean anomaly to obtain the time-variable obliquity and polar motion (at long-period and short-period). By extending the system of equations governing CSI, including gravitational and pressure couplings between misaligned layers, we predict the orientation of the spin axes of the outer shell, internal ocean and solid interior for an ocean-bearing body.

TBA
Violeta Gamez-Rosas (STAR)

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)

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Morgan Deal (Université de Montpellier)

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Yoshiki Hatta (Graduate School of Advanced Studies, SOKENDAI, Tokyo)

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Shinnosuke Satoh (Tohoku University)

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