8,859 research outputs found
Pop III Stellar Masses and IMF
We provide a status report on our current understanding of the mass scales
for Pop III.1 and Pop III.2 stars. Since the last review (Norman 2008),
substantial progress has been made both numerically and analytically on the
late stages of protostellar cloud core collapse, protostar formation and
accretion, and stellar evolution taking into account cloud core properties and
radiative feedback effects. Based on this, there are growing indications that
primordial stars forming from purely cosmological initial conditions (Pop
III.1) were substantially more massive than stars forming in preionized gas
(Pop III.2) where HD cooling is important. Different stellar endpoints are
predicted for these two types of Pop III stars with different chemical
enrichment signatures: the former die as pair instability supernovae or
intermediate mass black holes, whereas the latter die as iron core-collapse
supernovae, leaving behind neutron star and stellar black hole remnants. We
review recent simulations which show evidence for binary fragmentation at high
densities, and comment on the significance of these results. We then summarize
an attempt to directly calculate the Pop III.1 IMF taking into account the
latest numerical and analytical models. We conclude with suggestions for the
kind of simulations needed next to continue improving our understanding of Pop
III star formation, which is a necessary input to understanding high redshift
galaxy formation.Comment: 11 pages, 3 figures, Proceedings of "The First Stars and Galaxies:
Challenges for the Next Decade", Austin, TX, March 8-11, 201
Ionization Front Instabilities in Primordial H II Regions
Radiative cooling by metals in shocked gas mediates the formation of
ionization front instabilities in the galaxy today that are responsible for a
variety of phenomena in the interstellar medium, from the morphologies of
nebulae to triggered star formation in molecular clouds. An important question
in early reionization and chemical enrichment of the intergalactic medium is
whether such instabilities arose in the H II regions of the first stars and
primeval galaxies, which were devoid of metals. We present three-dimensional
numerical simulations that reveal both shadow and thin-shell instabilities
readily formed in primordial gas. We find that the hard UV spectra of
Population III stars broadened primordial ionization fronts, causing H2
formation capable of inciting violent thin- shell instabilities in D-type
fronts, even in the presence of intense Lyman-Werner flux. The high post- front
gas temperatures associated with He ionization sustained and exacerbated shadow
instabilities, unaided by molecular hydrogen cooling. Our models indicate that
metals eclipsed H2 cooling in I-front instabilities at modest concentrations,
from 0.001- 0.01 solar. We conclude that ionization front instabilities were
prominent in the H II regions of the first stars and galaxies, influencing the
escape of ionizing radiation and metals into the early universe.Comment: 13 pages, 11 figures, accepted by ApJ with minor revision
Historical perspective on astrophysical MHD simulations
This contribution contains the introductory remarks that I presented at IAU
Symposium 270 on ``Computational Star Formation" held in Barcelona, Spain, May
31 -- June 4, 2010. I discuss the historical development of numerical MHD
methods in astrophysics from a personal perspective. The recent advent of
robust, higher order-accurate MHD algorithms and adaptive mesh refinement
numerical simulations promises to greatly improve our understanding of the role
of magnetic fields in star formation.Comment: 11 pages, 5 figures, in "Computational Star Formation" held in
Barcelona, Spain, May 31 - June 4, 2010", Eds. J. Alves, B. G. Elmegreen, J.
M. Girart, V. Trimbl
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