PhD position: "Formation of the first stars: collapse and fragmentation of primordial gas clouds" - CRAL (UMR 5574)

Short description

The PhD position is proposed for a 3-year period (36 months). The legal gross salary is €1768 per month (plus social benefits). An annual €2 000 package for travels and equipment will be allotted. The candidate is expected to submit a thesis manuscript to the university of Lyon for a formal presentation in front of a jury before the end of the 3-yr period.

Starting date of the contract: October the 1st, 2021

Research project

Context
Pop III stars are the first stars that form in the Universe, out of gas of pristine composition. Their intense radiation ionises their environments, and their explosions releases the first heavy elements in the Universe. This sets the initial conditions for the formation of the next generations of stars (Pop II) and the first galaxies. While the properties of the present day stars are well-established by observations and increasingly understood by theory, little is known about the Pop III stars since their observation remains hypothetical. In particular, the mass distribution of the first stars is currently highly debated. In the early 2000s, with the apparition of the first 3D simulations, it was thought that the Pop III stars were all massive and short-lived (e.g. Abel et al. 2002). It has been recently shown that both accurate primordial cooling modelling and magnetic fields greatly impact the fragmentation of primordial gas clouds and possibly allows low-mass Pop III to form which could be still observable today (Sharda et al. 2020). These results can dramatically change the impact of Pop III stars on their environments and thus on the birth sites of the first galaxies. On top of this, current large scale cosmological reionization models assume a present day Initial Mass Function (IMF) to calibrate star formation and feedback whereas first stars could still be at play (e.g. Rosdahl et al. 2018).
It is thus of prime importance to constrain the Pop III star IMF to understand the formation of the large scale structures in the Universe. We propose to take over these pioneering studies, using an updated chemical network for primordial chemistry, based on theoretical chemistry works, needed to accurately estimate the cooling by H2 and hydrogen deuteride HD (e.g., Galli & Palla 2013, Bossion et al. 2018), coupled to a state-of-the-art 3D radiation-magneto-hydrodynamics (RMHD) numerical framework built upon the expertise developed at CRAL in the past years.

Proposed research plan
The first part of the PhD work will be devoted to the implementation of a state-of-the-art chemical network for primordial chemistry in the RMHD code RAMSES (Teyssier 2002) which is intensively used and developed at CRAL. The numerical development will be applied to the collapse of isolated massive primordial clouds which a focus on the fragmentation and disk/outflow formation in order to determine the star formation efficiency. In a second part, the numerical framework will be adapted in a prototype code such as “mini-ramses”, designed for exascale computing. We aim to provide proof-of-concept large numerical models, starting from a cosmological environment with a deep AMR hierarchy and designed to study the IMF of Pop III stars.