321 research outputs found
Constraining the Statistics of Population III Binaries
We perform a cosmological simulation in order to model the growth and
evolution of Population III (Pop III) stellar systems in a range of host
minihalo environments. A Pop III multiple system forms in each of the ten
minihaloes, and the overall mass function is top-heavy compared to the
currently observed initial mass function in the Milky Way. Using a sink
particle to represent each growing protostar, we examine the binary
characteristics of the multiple systems, resolving orbits on scales as small as
20 AU. We find a binary fraction of ~36%, with semi-major axes as large as 3000
AU. The distribution of orbital periods is slightly peaked at < 900 yr, while
the distribution of mass ratios is relatively flat. Of all sink particles
formed within the ten minihaloes, ~50% are lost to mergers with larger sinks,
and ~50% of the remaining sinks are ejected from their star-forming disks. The
large binary fraction may have important implications for Pop III evolution and
nucleosynthesis, as well as the final fate of the first stars.Comment: 16 pages, 14 figures, to appear in MNRA
Dwarf Spheroidal Satellite Formation in a Reionized Local Group
Dwarf spheroidal satellite galaxies have emerged a powerful probe of
small-scale dark matter clustering and of cosmic reionization. They exhibit
structural and chemical continuity with dwarf irregular galaxies in the field
and with spheroidal galaxies in high-density environments. By combining
empirical constraints derived for star formation at low gas column densities
and metallicities in the local universe with a model for dark matter and
baryonic mass assembly, we provide an analytical description of how the dwarf
spheroidals acquired their stellar content. Their progenitors formed stars
until the gas content, initially reduced from the cosmic average by the thermal
pressure of the reionized intergalactic medium, was finally ram pressure
stripped during the progenitors' accretion on to the host galaxy. Dwarf
spheroidal satellites of differing luminosities seem to share very similar most
massive progenitor histories that reach thresholds for gas cooling by atomic
line emission at epochs at which the Lagrangian volume of the Local Group
should have been reionized. We hypothesize that dwarf spheroidals formed the
bulk of their stars in partially rotationally supported HI disks in a reionized
universe. This model provides an explanation for the "common mass scale"
relation and reproduces the empirical luminosity-size and
luminosity-metallicity relations. Explosive feedback phenomena, such as
outflows driven by the concerted action of supernovae, need not have been
significant in the dwarf spheroidals' formation. We further speculate that the
true pre-reionization fossils should exhibit a structure distinct from that of
the dwarf spheroidals, e.g., in the form of dense isolated or nuclear star
clusters.Comment: 18 pages, 7 figures, MNRAS, in pres
The First Stars: A Low-Mass Formation Mode
We perform numerical simulations of the growth of a Population III stellar
system under photodissociating feedback. We start from cosmological initial
conditions at z = 100, self-consistently following the formation of a minihalo
at z = 15 and the subsequent collapse of its central gas to high densities. The
simulations resolve scales as small as ~ 1 AU, corresponding to gas densities
of 10^16 cm^-3. Using sink particles to represent the growing protostars, we
evolve the stellar system for the next 5000 years. We find that this emerging
stellar group accretes at an unusually low rate compared with minihalos which
form at earlier times (z = 20 - 30), or with lower baryonic angular momentum.
The stars in this unusual system will likely reach masses ranging from < 1
M_sun to 5 M_sun by the end of their main-sequence lifetimes, placing them in
the mass range for which stars will undergo an asymptotic giant branch (AGB)
phase. Based upon the simulation, we predict the rare existence of Population
III stars that have survived to the present day and have been enriched by mass
overflow from a previous AGB companion.Comment: 19 pages, 17 figures, to apper in Ap
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