Fingerprinting the first generation of stars

The properties of the first generation of pristine, heavy element-free stars (called Population III stars) are a mystery: little is known about when they started and stopped forming, the range of masses they span at birth (their Initial Mass Function), and the quantity of various elements they return to the interstellar and intergalactic medium. With the launch of the James Webb Telescope and a number of powerful telescopes, e.g.

HARMONI on the ELT on the horizon, the prospects of directly observing this first generation of stars have considerably improved.

However, another compelling way to track these first stars is to look for their remnants. When Pop III stars reach the end of their lifetime, depending on their mass they either explode as supernovae or collapse directly into a black hole. Mergers of these remnants, neutron stars or black holes, produce gravitational wave signals that can be picked up by space-based and ground-based detectors (LISA, Advanced LIGO).

Whether Pop III black hole binary systems contribute significantly to the gravitational wave background depends on their mass and redshift distribution, something that requires detailed modelled predictions.

This DPhil project will explore where Pop III stars and their remnants end up, to deliver predictions of optimal places to search for them in the local Universe, across a range of wavelengths (from ultra-violet to radio). The student will analyse the MEGATRON simulation (PI Katz), the first cosmological simulation with a full non-equilibrium, primordial, metal, and molecular chemistry network coupled to on-the-fly radiation hydrodynamics.

The simulation includes a state-of-the-art Pop III star formation model, with the first Pop III stars forming at redshift 28 when the simulation has ~1 pc spatial resolution, sufficient to capture the cooling of molecular hydrogen in a metal-free environment. There will also be scope for the student to design and run their own cosmological simulations as a way to refine the analysis.

Key questions to be answered include: Where do low mass Pop III stars end up versus the remnants of high mass Pop III stars? Is there a difference with one kind of Pop III star preferring the galactic halo and another the galactic bulge? What fraction of Pop III remnants form binaries and where do they end up?

What is their contribution to the gravitational wave background? Where do the low mass Pop II stars that were enriched by heavy elements produced by a single Pop III supernovae end up in the Milky Way? What are their chemical abundance signatures?