FORGE'd in Fragments: Pop III star formation in a Milky Way-like progenitor halo

Abstract

We present a radiation-magnetohydrodynamic (RMHD) cosmological zoom-in simulation of Pop III star formation, drawing initial conditions from a Milky Way-like progenitor halo at redshift $z \sim 21$. We follow the collapse of primordial gas over 10 orders of magnitude, through to the formation of an accretion disk that becomes gravitationally unstable $Δt \sim -3$ kyr before the first protostar's formation. Prominent spiral arms then develop and fragment vigorously, producing $\sim 60$ sink formation events over the first $8$ kyr post-collapse evolution, with a total stellar mass reaching $> 150\,{\rm M_\odot}$ by $15$ kyr, the end of our simulation. The disk remains magnetically supercritical throughout: the mass-to-flux ratio falls from $μ_Φ\sim 10^2$-$10^3$ to $\sim 15$ by $15$ kyr, as the inflow brings in magnetic flux. The magnetic energy in the disk remains sub-dominant throughout, but grows by $\sim 3.5$ orders of magnitude after the first star forms. The disk starts sub-virial and reaches virial equilibrium within $\sim 2$ kyr. The total stellar mass of the population grows as $M_* \propto t^2$, confirming the prediction of accelerating accretion in self-gravitating turbulent collapse, at rates of $\dot{M} \sim 10^{-3}$-$10^{-2}\,{\rm M}_\odot {\rm yr}^{-1}$. The star formation efficiency reaches $\approx 90\%$ within the disk (after $\approx 11$ kyr; $\approx 80\%$ at $9.5$ kyr) and $\sim 50\%$ within a $0.1$ pc sphere. We find a top heavy IMF ($dN/dm \propto m^{-α}$) with a high-mass slope $α\approx 1$. The cosmological environment sets the boundary conditions of the disk: an envelope delivers $\sim 3\times 10^{-3}\,{\rm M}_\odot {\rm yr}^{-1}$ to the outer edge of a disk with a radius of $\sim 10^{3}$ AU; the disk spreads and, in our simulation, its fragmentation sites move outward, from $\sim 500$ to $\sim 1350$\,AU, until it has been depleted.