Seminars/Lectures

This page is dedicated to all the actions tailored to fuel scientific interaction between the members of the AIPS and beyond. In addition to workshops (up to 2 per year), we organise more regular actions such as seminars that will allow the actors to meet and interact as often as possible.

AIPS Seminars #1: Monday September 26th at 14:00 Meudon (build. 17):
SEBASTIEN DEHEUVELS & LUDOVIC PETITDEMANGE

14:00-14:45 Sébastien Deheuvels (IRAP): Strong magnetic fields discovered in red giant cores using seismology

Magnetic fields affect stars at all stages of their evolution. In particular, they are expected to play a central role in the redistribution of angular momentum inside stars, and thus in the transport of chemical elements. While surface magnetic fields have been detected in stars across the HR diagram, internal magnetic fields have remained inaccessible to direct observations. In red giant stars, the detection of mixed modes – that is, oscillation modes that behave as gravity modes in the core and as pressure modes in the envelope – has shown that the cores of red giant stars are rotating slowly. This yielded evidence that angular momentum is redistributed much more efficiently than if only purely hydrodynamical processes were at work. Magnetic fields could produce the additional transport that is needed.
Here we report the first direct detection of magnetic fields in the cores of red giant stars. Magnetic fields induce shifts in the frequencies of oscillation modes and break the symmetry of dipole mixed mode multiplets. Also, strong fields can significantly modify the pattern of gravity modes in a characteristic way. We detect such features in a dozen red giant stars observed with the Kepler satellite and we find that they closely follow the predictions of magnetic perturbations to oscillation modes. We measure field strengths ranging from a few tens to a few hundreds of kilogauss in the vicinity of the hydrogen-burning shell, and we place constraints on the field topology. We compare the measured field intensities to the critical field strength above which magneto-gravity waves can no longer propagate, and we discuss the potential link with the suppression of dipole mixed modes that is observed in certain red giant stars.

15:15-16:00 Ludovic Petitdemange (LERMA): Hidden Dynamo spins down radiative stars

The life and death of a star are controlled by its internal rotation dynamics through subtle transport and mixing mechanisms, which so far remain poorly understood. While magnetic fields must play a crucial role in transporting angular momentum and chemical species, the very origin of magnetism in radiative stellar layers and its influence on spinning dynamics are yet to be unraveled. Using global numerical modeling, we report the existence of a dynamo sharing many characteristics with the (never observed) Tayler-Spruit model, which can generate strong magnetic fields and significantly enhance transport in radiative zones. The resulting toroidal fields are screened by the quasi-insulating interstellar medium, allowing for the existence of intense magnetism in radiative stars where no magnetic fields could be directly observed so far.

AIPS Seminar #2: Tuesday October 18th, Paris (salle du conseil, TBC):
ANDREA MIGLIO & PHILIPPE ZARKA

10:00-10:45 Andrea Miglio (university of Bologna): Assembly History and Evolution of the Milky Way Through the Lens of Asteroseismic Ages 

Our understanding of the formation and evolution of the Milky Way is often blurred and biased by the lack of precise and accurate stellar ages. In this contribution I will present the ongoing efforts and recent results of the asterochronometry project (https://asterochronometry.eu/), which aims both at testing and improving our knowledge of stellar physics, and at determining precise and accurate ages of stars (to 10-15%) in the regions of the Galaxy sampled by Kepler, K2, CoRoT, and TESS. Examples of recent and ongoing work will include age-dating stars using data from the TESS mission and inferences on the ages of both Gaia-Enceladus and in-situ stars observed by Kepler. Finally, I will discuss the prospects for extending these studies to larger samples, and briefly present the science case for a future mission dedicated to asteroseismology of crowded fields.

11:15-12:00 Philippe Zarka (LESIA): Low-frequencies radioastronomy and stellar physics

At low radio frequencies (below ~1 GHz), stellar observations have been long known to reveal the stars’ activity, likely of magnetic origin.Following theoretical work in the past few years, they are also expected to reveal star-planet plasma interactions, that may be especially strong for close-in exoplanets. I will briefly review the status of the field and focus on LOFAR observations (in the range 110-250 MHz), that start revealing time-frequency variable stellar activity. Prospects at even lower frequencies (<100 MHz) with NenuFAR will be addressed. In the coming years, SKA will bring in an increase of >1 order of magnitude in sensitivity above 50 MHz.