My research interests focus on accretion discs physics, turbulence, dynamo action and planet formation. You'll find in these pages some informations about these researches plus some flash movies (just click on the snapshots to see the videos). The links below will direct you to a specific topic.
Dynamics and evolution of protoplanetary discs
Cataclysmic variables

Dynamics and evolution of protoplanetary discs

It is often assumed that the magnetorotational instability (MRI), a magnetic instability, is responsible for the generation of turbulence in accretion discs. However, in planet-forming discs, also known as protoplanetary discs, the temperature is so low that the gas is not a good electrical conductor. It is therefore important to take into account the finite conductivity of the plasma, which is modelled as non-ideal magneto-hydrodynamical (MHD) effects. These effects have a dramatic impact on the dynamics of the disc: the MRI disappears and one is left with a mostly laminar disc.

How can one explain that these discs are still accreting then? That's the question I have been adressing with Matt Kunz (Princeton) and my PhD student William Bethune. For us, the answer lies in the presence of large scale magnetic winds, which are sufficient to drive accretion at the disc surface. We have demonstrated this process by performing the first fully global non-ideal MHD simulation of a protoplanetary disc in 2017 (see movie below).

In addition to accretion, we have been looking for hints of large scale structures, such as rings and gaps, as seen in recent ALMA and VLT observation. To our surprise, we found that magnetic effects in the non-ideal regime were driving self-organisation in the disc, forming structures that are very similar to the ones we see in our telescopes! There is still a long way to demonstrate that what our simulations are predicting is actually what we are seeing, but we've never been so close...

Above: 3D simulation of a protoplanetary disc in the region 10-100 au including all of the non-ideal MHD effects. This proof of concept demonstrates the presence of large scale winds driving accretion and of self-organisation.

Cataclysmic variables

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Cataclysmic variables are binary systems made of a classical star and a white dwarf orbiting around each other. The white dwarf is so close to the companion star that it deforms it and slowly accretes its material (see artist view on the left). The gas slowly falls on the white dwarf by forming an accretion disc.

There are several kind of Cataclysmic variables. I'm mostly interested in dwarf novae, which are known to undergo episodic burst of luminosity which last for a few days. It has been shown that these bursts were due to change in the accretion efficiency of the disc surrounding the white dwarf. Bursts of luminosity are due to a highly accreting and hot disc, while the quiescent state corredonds to a cold and weakly accreting disc.

I'm trying to understand how and why the disc has different accretion states, using local and global numerical simulations. As in protoplanetary discs (see above), the quiescent state in dwarf novae is so called that the ionisation fraction is likely to be too low to sustain the MRI. Still, there are good indications that these objects are still accreting in quiessence (like X-ray emission due to gas falling on the surface of the white dwarf). How are these cold disc accreting? It could be due to the companion which excites giant spiral density waves in the disc, or to global winds, like in the dead zone of protoplanetary discs. We're investigating these different possibilities with Guillaume Dubus and Nicolas Scepi.

Above: 2D simulation of a cataclysmic variable. The simulation is computed in the frame rotating with the system. The companion star is therefore on the right and stellar material starts to fall on the white dwarf and forms an accretion disc.