Gravitational Collapse and Neutrino Emission of Population III Massive Stars

Abstract

Pop III stars are the first stars in the universe. They do not contain metals and their formation and evolution may be different from that of stars of later generations. In fact, according to the theory of star formation, Pop III stars might have very massive components ($\sim 100 - 10000M_\odot$). In this paper, we compute the spherically symmetric gravitational collapse of these Pop III massive stars. We solve the general relativistic hydrodynamics and neutrino transfer equations simultaneously, treating neutrino reactions in detail. Unlike supermassive stars ($\gtrsim 10^5 M_\odot$), the stars of concern in this paper become opaque to neutrinos. The collapse is simulated until after an apparent horizon is formed. We confirm that the neutrino transfer plays a crucial role in the dynamics of gravitational collapse, and find also that the $β$-equilibration leads to a somewhat unfamiliar evolution of electron fraction. Contrary to the naive expectation, the neutrino spectrum does not become harder for more massive stars. This is mainly because the neutrino cooling is more efficient and the outer core is more massive as the stellar mass increases. Here the outer core is the outer part of the iron core falling supersonically. We also evaluate the flux of relic neutrino from Pop III massive stars. As expected, the detection of these neutrinos is difficult for the currently operating detectors. However, if ever observed, the spectrum will enable us to obtain the information on the formation history of Pop III stars. We investigate 18 models covering the mass range of $300 - 10^4 M_\odot$, making this study the most detailed numerical exploration of spherical gravitational collapse of Pop III massive stars. This will also serve as an important foundation for multi-dimensional investigations.