36 research outputs found
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
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
The Mutual Interaction Between Population III Stars and Self-Annihilating Dark Matter
We use cosmological simulations of high-redshift minihalos to investigate the
effect of dark matter annihilation (DMA) on the collapse of primordial gas. We
numerically investigate the evolution of the gas as it assembles in a
Population III stellar disk. We find that when DMA effects are neglected, the
disk undergoes multiple fragmentation events beginning at ~ 500 yr after the
appearance of the first protostar. On the other hand, DMA heating and
ionization of the gas speeds the initial collapse of gas to protostellar
densities and also affects the stability of the developing disk against
fragmentation, depending on the DM distribution. We compare the evolution when
we model the DM density with an analytical DM profile which remains centrally
peaked, and when we simulate the DM profile using N-body particles (the 'live'
DM halo). When utilizing the analytical DM profile, DMA suppresses disk
fragmentation for ~ 3500 yr after the first protostar forms, in agreement with
earlier work. However, when using a 'live' DM halo, the central DM density peak
is gradually flattened due to the mutual interaction between the DM and the
rotating gaseous disk, reducing the effects of DMA on the gas, and enabling
secondary protostars of mass ~ 1 M_sol to be formed within ~ 900 yr. These
simulations demonstrate that DMA is ineffective in suppressing gas collapse and
subsequent fragmentation, rendering the formation of long-lived dark stars
unlikely. However, DMA effects may still be significant in the early collapse
and disk formation phase of primordial gas evolution.Comment: 17 pages, 11 figures, to appear in MNRA
The First Stars: formation under X-ray feedback
We investigate the impact of a cosmic X-ray background (CXB) on Population
III stars forming in a minihalo at . Using the smoothed particle
hydrodynamics code GADGET-2, we attain sufficient numerical resolution to
follow gas collapsing into the centre of the minihalo from cosmological initial
conditions up to densities of , at which point we form
sink particles. This allows us to study how the presence of a CXB affects the
formation of H and HD in the gas prior to becoming fully molecular. Using a
suite of simulations for a range of possible CXB models, we follow each
simulation for 5000\yr after the first sink particle forms. The CXB provides
two competing effects, with X-rays both heating the gas and increasing the free
electron fraction, allowing more H to form. X-ray heating dominates below
, while the additional H cooling becomes more
important above . The gas becomes optically thick to
X-rays as it exits the quasi-hydrostatic `loitering phase,' such that the
primary impact of the CXB is to cool the gas at intermediate densities,
resulting in an earlier onset of baryonic collapse into the dark matter halo.
At the highest densities, self-shielding results in similar thermodynamic
behaviour across a wide range of CXB strengths. Consequently, we find that star
formation is relatively insensitive to the presence of a CXB; both the number
and the characteristic mass of the stars formed remains quite similar even as
the strength of the CXB varies by several orders of magnitude.Comment: Accepted for publication in MNRAS. Includes improved treatment of
X-ray optical depth. 13 pages, 12 figure
The First Stars: Mass Growth Under Protostellar Feedback
We perform three-dimensional cosmological simulations to examine the growth
of metal-free, Population III (Pop III) stars under radiative feedback. We
begin our simulation at z=100 and trace the evolution of gas and dark matter
until the formation of the first minihalo. We then follow the collapse of the
gas within the minihalo up to densities of n = 10^12 cm^-3, at which point we
replace the high-density particles with a sink particle to represent the
growing protostar. We model the effect of Lyman-Werner (LW) radiation emitted
by the protostar, and employ a ray-tracing scheme to follow the growth of the
surrounding H II region over the next 5000 yr. We find that a disk assembles
around the first protostar, and that radiative feedback will not prevent
further fragmentation of the disk to form multiple Pop III stars. Ionization of
neutral hydrogen and photodissociation of H_2 by LW radiation leads to heating
of the dense gas to several thousand Kelvin, and this warm region expands
outward at the gas sound speed. Once the extent of this warm region becomes
equivalent to the size of the disk, the disk mass declines while the accretion
rate onto the protostars is reduced by an order of magnitude. This occurs when
the largest sink has grown to ~ 20 M_sol while the second sink has grown to 7
M_sol, and we estimate the main sink will approach an asymptotic value of ~ 30
M_sol by the time it reaches the main sequence. Our simulation thus indicates
that the most likely outcome is a massive Pop III binary. However, we simulate
only one minihalo, and the statistical variation between minihaloes may be
substantial. If Pop III stars were typically unable to grow to more than a few
tens of solar masses, this would have important consequences for the occurence
of pair-instability supernovae in the early Universe as well as the Pop III
chemical signature in the oldest stars observable today.Comment: 21 pages, 11 figures, to appear in MNRA
Effect of Population III Multiplicity on Dark Star Formation
We numerically study the mutual interaction between dark matter (DM) and
Population III (Pop III) stellar systems in order to explore the possibility of
Pop III dark stars within this physical scenario. We perform a cosmological
simulation, initialized at z ~ 100, which follows the evolution of gas and DM.
We analyze the formation of the first minihalo at z ~ 20 and the subsequent
collapse of the gas to densities of 10^12 cm^-3. We then use this simulation to
initialize a set of smaller-scale `cut-out' simulations in which we further
refine the DM to have spatial resolution similar to that of the gas. We test
multiple DM density profiles, and we employ the sink particle method to
represent the accreting star-forming region. We find that, for a range of DM
configurations, the motion of the Pop III star-disk system serves to separate
the positions of the protostars with respect to the DM density peak, such that
there is insufficient DM to influence the formation and evolution of the
protostars for more than ~ 5000 years. In addition, the star-disk system causes
gravitational scattering of the central DM to lower densities, further
decreasing the influence of DM over time. Any DM-powered phase of Pop III stars
will thus be very short-lived for the typical multiple system, and DM will not
serve to significantly prolong the life of Pop III stars.Comment: 16 pages, 11 figures, to appear in MNRA