Astrophysics

Star formation and protostellar disc formation

My doctoral research (in French) is devoted to star formation, and more specifically on protostellar disc formation. These discs form around stars and are ubiquous; moreover, planets — as the Earth — are forming in such discs. To undersand how they form is therefore a crucial question of modern astrophysics. I studied through numerical simulations, an analytical study and comparisions with observations the impact of magnetic field and turbulence on protostellar disc formation processes.

The first part of my work showed how protostellar discs can form with strong magnetic fields (that is usually thought to prevent disc formation) simply by tilting the rotation axis of the protostar with regard to the magnetic field orientation. In the second part of my work, we showed first how turbulence can naturally tilt protostar rotation axis and second how turbulence is helping dissipating magnetic fields, therefore reducing its impact. These results show how protostellar discs can form with turbulence and magnetic field.

Angular momentum transport in protoplanetary discs

We used the code I developed will working at CEA to study angular momentum transport processes in protoplanetary discs (in which planets form). Angular momentum transport in accretion discs (discs that rotate around a central object that accrete matter from those discs) is a crucial issue in modern astrophysics: for the central object (as a star) to accrete efficently matter from the disc, an efficient angular momentum (associated with disc rotation) transport mechanism is required to dissipate it. Viscosity is not enough as is would dissipate momentum in cosmological times (of the order of the age of the Universe) to allow matter to accrete in the central object! Keep in mind that 90% of a forming star is coming from its accretion disc and that stars form in a timespan shorter than the age of the Universe.

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One of the mechanisms that could efficently transport angular momentum is magnetorotational instability (MRI), a magnetohydrodynamics instability that can appear with a weak magnetic field in a differentially rotating disc. The issue here is that our knowledge of the process tends to show that its efficency is decreasing with viscosity and resistivity. It is fundamental to test this process with very low viscosity and resistivity to get closer to real physical conditions in discs. We performed the largest simulation of this kind, with the highest resolution, and we showed that the MRI efficency is stable under a given threshold that could assess its efficency in real conditions.

Chemical composition of stellar atmosphere

I studied the chemical composition of the atmosphere of a specific class of stars (Ap stars that are known to have peculiar chemical composition with rare earth over-abundances) by studying the atmosphere of a probable descendant of Ap star, EK Eridani. A better understanding of the composition of these stars could help understand physical processes in the deep interior of these stars and in their atmosphere.

During this experience, I took part to an observational campaign at the Observatorio del Roque de los Muchachos, in Canarias Island, on the Mercator telescope.

Massive stars evolution

I studied through numerical simulation the evolution of super-AGB stars (massive stars, their mass being of the order of ten solar masses), taking into account mixing processes in their core. These processes can have an impact on their core composition, that can then have an impact on their death and, for the most massive ones that will blow as supernovæ, on the chemical composition of the interstellar medium.