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Università degli Studi di Milano - State University of Milan (Italy)
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Papers by Bob Van Veelen
Proceedings of the International Astronomical Union, 2008
ABSTRACT
Numerical simulations suggest that as single primordial stars consecutively formed in small dark ... more Numerical simulations suggest that as single primordial stars consecutively formed in small dark matter halos at high redshift, gravitational mergers congregated the halos into the first primitive galaxies. In this picture, protogalaxies were assembled a few stars at a time and were only gradually contaminated by the first heavy elements because primordial supernovae preferentially expelled them into low density voids, away from the sites of mergers. However, the large computational boxes required to form the halos from cosmological initial conditions in these models prevent them from resolving the true fate of the supernova remnants. We present one-dimensional calculations of Population III supernovae in both neutral halos and primordial H II regions that resolve the flows over all relevant spatial scales. Our models indicate that the cycle of stellar birth, H II region formation, and second star formation within a halo may have been punctuated by prompt star formation in the supernova remnant. If so, low-mass stars may have been swept up into the first galaxies at earlier times and in much greater numbers than are now supposed. The chemical signatures and luminosity functions of such galaxies would be very different from those in the current paradigm, and observations by JWST and ALMA may soon discriminate between these two formation pathways. We also find that differential mixing of metals in primordial explosions may account for the skewed C and O to Fe ratios observed in extremely metal poor (EMP) and ultra metal poor (UMP) stars in the galactic halo, which may be remnants of the first few generations of stars in the universe.
The Astrophysical Journal, 2008
We present numerical simulations of primordial supernovae in cosmological minihalos at z ∼ 20. We... more We present numerical simulations of primordial supernovae in cosmological minihalos at z ∼ 20. We consider Type II supernovae, hypernovae, and pair instability supernovae (PISN) in halos from 6.9 × 10 5 -1.2 × 10 7 M ⊙ , those in which Population III stars are expected to form via H 2 cooling. Our simulations are novel in that they are the first to follow the evolution of the blast from a free expansion on spatial scales of 10 −4 pc until its approach to pressure equilibrium in the relic H II region of the progenitor, ∼ 1000 pc. Our models include nine-species primordial chemistry together with all atomic H and He cooling processes, inverse Compton cooling and free-free emission. The supernovae evolve along two evolutionary paths according to whether they explode in H II regions or neutral halos. Those in H II regions first expand adiabatically and then radiate strongly upon collision with baryons ejected from the halo during its photoevaporation by the progenitor. Explosions in neutral halos promptly emit most of their kinetic energy as x-rays, but retain enough momentum to seriously disrupt the halo. We find that the least energetic of the supernovae are capable of destroying halos 10 7 M ⊙ , while a single PISN can destroy even more massive halos. Blasts in H II regions disperse heavy elements into the IGM, but neutral halos confine the explosion and its metals. Primordial supernova remnants develop dynamical instabilities at early times capable of enriching up to 10 6 M ⊙ of baryons with metals to levels 0.01 Z ⊙ , well above that required for low-mass star formation. In H II regions, a prompt second generation of stars may form in the remnant at radii of 100 -200 pc in the halo. Explosions confined by large halos instead recollapse, with infall rates in excess of 10 −2 M ⊙ yr −1 that heavily contaminate their interior. This fallback may either fuel massive black hole growth at very high redshifts or create the first globular cluster with a radius of 10 -20 pc at the center of the halo. Our findings allow the possibility that the first primitive galaxies formed sooner, with greater numbers of stars and distinct chemical abundance patterns, than in current models.
Monthly Notices of the Royal Astronomical Society, 2010
Recent observations of luminous Type IIn supernovae (SNe) provide compelling evidence that massiv... more Recent observations of luminous Type IIn supernovae (SNe) provide compelling evidence that massive circumstellar shells surround their progenitors.
Proceedings of the International Astronomical Union, 2008
ABSTRACT
Numerical simulations suggest that as single primordial stars consecutively formed in small dark ... more Numerical simulations suggest that as single primordial stars consecutively formed in small dark matter halos at high redshift, gravitational mergers congregated the halos into the first primitive galaxies. In this picture, protogalaxies were assembled a few stars at a time and were only gradually contaminated by the first heavy elements because primordial supernovae preferentially expelled them into low density voids, away from the sites of mergers. However, the large computational boxes required to form the halos from cosmological initial conditions in these models prevent them from resolving the true fate of the supernova remnants. We present one-dimensional calculations of Population III supernovae in both neutral halos and primordial H II regions that resolve the flows over all relevant spatial scales. Our models indicate that the cycle of stellar birth, H II region formation, and second star formation within a halo may have been punctuated by prompt star formation in the supernova remnant. If so, low-mass stars may have been swept up into the first galaxies at earlier times and in much greater numbers than are now supposed. The chemical signatures and luminosity functions of such galaxies would be very different from those in the current paradigm, and observations by JWST and ALMA may soon discriminate between these two formation pathways. We also find that differential mixing of metals in primordial explosions may account for the skewed C and O to Fe ratios observed in extremely metal poor (EMP) and ultra metal poor (UMP) stars in the galactic halo, which may be remnants of the first few generations of stars in the universe.
The Astrophysical Journal, 2008
We present numerical simulations of primordial supernovae in cosmological minihalos at z ∼ 20. We... more We present numerical simulations of primordial supernovae in cosmological minihalos at z ∼ 20. We consider Type II supernovae, hypernovae, and pair instability supernovae (PISN) in halos from 6.9 × 10 5 -1.2 × 10 7 M ⊙ , those in which Population III stars are expected to form via H 2 cooling. Our simulations are novel in that they are the first to follow the evolution of the blast from a free expansion on spatial scales of 10 −4 pc until its approach to pressure equilibrium in the relic H II region of the progenitor, ∼ 1000 pc. Our models include nine-species primordial chemistry together with all atomic H and He cooling processes, inverse Compton cooling and free-free emission. The supernovae evolve along two evolutionary paths according to whether they explode in H II regions or neutral halos. Those in H II regions first expand adiabatically and then radiate strongly upon collision with baryons ejected from the halo during its photoevaporation by the progenitor. Explosions in neutral halos promptly emit most of their kinetic energy as x-rays, but retain enough momentum to seriously disrupt the halo. We find that the least energetic of the supernovae are capable of destroying halos 10 7 M ⊙ , while a single PISN can destroy even more massive halos. Blasts in H II regions disperse heavy elements into the IGM, but neutral halos confine the explosion and its metals. Primordial supernova remnants develop dynamical instabilities at early times capable of enriching up to 10 6 M ⊙ of baryons with metals to levels 0.01 Z ⊙ , well above that required for low-mass star formation. In H II regions, a prompt second generation of stars may form in the remnant at radii of 100 -200 pc in the halo. Explosions confined by large halos instead recollapse, with infall rates in excess of 10 −2 M ⊙ yr −1 that heavily contaminate their interior. This fallback may either fuel massive black hole growth at very high redshifts or create the first globular cluster with a radius of 10 -20 pc at the center of the halo. Our findings allow the possibility that the first primitive galaxies formed sooner, with greater numbers of stars and distinct chemical abundance patterns, than in current models.
Monthly Notices of the Royal Astronomical Society, 2010
Recent observations of luminous Type IIn supernovae (SNe) provide compelling evidence that massiv... more Recent observations of luminous Type IIn supernovae (SNe) provide compelling evidence that massive circumstellar shells surround their progenitors.