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: Friday November 25th, Paris (salle Danjon):
Anna GUSEVA (Leeds University)

10:00 – 10:45 Anna GUDEVA (university of Leeds):  Stellar Magnetism: a data-driven approach to nonlinear dynamos

Organized, large-scale magnetic fields are frequently encountered in the Universe, from planets and stars to accretion disks and galactic clusters. It is widely accepted that these fields are created through dynamo action, which is supported by turbulent motions of conducting fluid inside astrophysical objects and opposed by Ohmic dissipation. These dynamos can be considered as nonlinear chaotic dynamical systems, whose saturated states depend on nonlinear interactions between the flow and the magnetic field. A rigorous description of these saturation mechanisms is important for a better understanding of long-term evolution and variations in large-scale stellar and planetary flows.
A lot of insight into these nonlinearities comes from direct numerical simulations (DNS) of the dynamo flows; however, DNS are not feasible in realistic astrophysical parameter regimes, and a large amount of degrees of freedom makes the nonlinear dynamics in DNS intractable. In this work, we employ a data-driven approach to describe these nonlinearities in numerical dynamos. In essence, it enables extraction of the main dynamical components corresponding to large-scale spatial patterns of magnetic and velocity fields, and construction of a nonlinear reduced-order model from their temporal evolution. In this talk, we will demonstrate how this approach helps to understand the nonlinear behavior of the benchmark system based on Parker’s dynamo waves. We will further discuss how it can be applied to a more realistic convective dynamo flow in spherical geometry.

AIPS Seminar #3: Monday December 12th AT 14:00: EVELYNE ALECIAN & CHARLY PINÇON (salle du chateau, Meudon)

14:00-14:45 EVELYNE ALECIAN (IPAG): Magnetic fields origin and evolution during the formation of the star 

Magnetic field is one of the important ingredients at play during the formation of the stars from the collapse of molecular clouds down through the pre-main-sequence phase. Tremendous progresses have been made to characterize the properties of magnetic fields in young stars, and pre-stellar objects, but also in modelling dynamo processes in stellar interiors. Nonetheless, it is still not clear what are the origins of stellar magnetic fields, what roles play initial conditions, and how stellar magnetic fields evolve during the building of stars. I will first review our current knowledge on the stellar magnetic properties at all mass and ages, focusing on the pre-main-sequence phase. I will then discuss the different hypothesis on the origin of stellar magnetic fields, and how our new project, PROMETHEE will address the key questions regarding the origin of stellar magnetic fields.

15:15-16:00 CHARLY PINÇON (LERMA): Studying the magnetic field topology and latitudinal luminosity profile of low mass stars with global simulations.

Magnetic fields are ubiquitous in stars and are known to have a significant impact on their structure and evolution. Spectropolarimetric observations using the Zeeman-Doppler imaging technique have already revealed the large-scale topology of the surface magnetic fields of a number of low-mass stars. The impressive diversity of observed magnetic configurations questions our understanding of the dynamo process at work in the convective envelope of such stars and at the origin of these magnetic fields. This mechanism has already been widely investigated to explain the Earth core magnetism through numerical simulations of rotating  incompressible convection. In low-mass stars however, the density stratification cannot be neglected and MHD simulations within the anelastic approximation have to be  considered. Previous works based on weakly stratified models succeeded in reproducing dipolar dynamos, dominated by a large-scale axial dipole component, and multipolar dynamos, characterized by a more complex field topology with higher spatial and temporal variability. However, the impact of the density stratification on these results has not been fully grasped yet. The high-stratified regime still has to be explored and apparent contradictions also remain to be clarified, regarding for instance the stability of the dipolar branch when the stratification is increased. In this work, we address this question using a systematic parameter study based on a large set of highly-stratified 3D anelastic dynamo models. We clarify the stability of the dipolar branch distinguishing between axial and equatorial dipoles, and emphasize the link between magnetic topology and the differential rotation. As a first application, we investigate the effect of magnetic fields on the latitudinal distribution of the surface luminosity.