The Formation of the First Stars II. Radiative Feedback Processes and Implications for the Initial Mass Function

TL;DR

This paper investigates radiative feedback mechanisms during the formation of the first stars, analyzing how processes like H_2 photodissociation, Ly-alpha radiation, and HII region expansion influence the maximum stellar mass and the initial mass function.

Contribution

It provides a detailed evaluation of how various radiative feedback processes set the upper mass limits for Population III stars, integrating multiple effects into a comprehensive model.

Findings

H_2 photodissociation occurs early but does not prevent subsequent accretion.

Ly-alpha radiation pressure can reverse infall at ~20-30Msun in polar directions.

HII region expansion halts accretion at ~50-100Msun, with continued accretion possible from the disk.

Abstract

We consider the radiative feedback processes that operate during the formation of the first stars, including the photodissociation of H_2, Ly-alpha radiation pressure, formation and expansion of an HII region, and disk photoevaporation. These processes may inhibit continued accretion once the stellar mass has reached a critical value, and we evaluate this mass separately for each process. Photodissociation of H_2 in the local dark matter minihalo occurs relatively early in the growth of the protostar, but we argue this does not affect subsequent accretion since by this time the depth of the potential is large enough for accretion to be mediated by atomic cooling. However, neighboring starless minihalos can be affected. Ionization creates an HII region in the infalling envelope above and below the accretion disk. Ly-alpha radiation pressure acting at the boundary of the HII region is effective at reversing infall from narrow polar directions when the star reaches ~20-30Msun, but cannot prevent infall from other directions. Expansion of the HII region beyond the gravitational escape radius for ionized gas occurs at masses ~50-100Msun, depending on the accretion rate and angular momentum of the inflow. However, again, accretion from the equatorial regions can continue since the neutral accretion disk has a finite thickness and shields a substantial fraction of the accretion envelope from direct ionizing flux. At higher stellar masses, ~140Msun in the fiducial case, the combination of declining accretion rates and increasing photoevaporation-driven mass loss from the disk act to effectively halt the increase in the protostellar mass. We identify this process as the mechanism that terminates the growth of Population III stars... (abridged)