7,138 research outputs found
START: Smoothed particle hydrodynamics with tree-based accelerated radiative transfer
We present a novel radiation hydrodynamics code, START, which is a smoothed
particle hydrodynamics (SPH) scheme coupled with accelerated radiative
transfer. The basic idea for the acceleration of radiative transfer is parallel
to the tree algorithm that is hitherto used to speed up the gravitational force
calculation in an N-body system. It is demonstrated that the radiative transfer
calculations can be dramatically accelerated, where the computational time is
scaled as Np log Ns for Np SPH particles and Ns radiation sources. Such
acceleration allows us to readily include not only numerous sources but also
scattering photons, even if the total number of radiation sources is comparable
to that of SPH particles. Here, a test simulation is presented for a multiple
source problem, where the results with START are compared to those with a
radiation SPH code without tree-based acceleration. We find that the results
agree well with each other if we set the tolerance parameter as < 1.0, and then
it demonstrates that START can solve radiative transfer faster without reducing
the accuracy. One of important applications with START is to solve the transfer
of diffuse ionizing photons, where each SPH particle is regarded as an emitter.
To illustrate the competence of START, we simulate the shadowing effect by
dense clumps around an ionizing source. As a result, it is found that the
erosion of shadows by diffuse recombination photons can be solved. Such an
effect is of great significance to reveal the cosmic reionization process.Comment: 14 pages, 23 figures, accepted for publication in MNRA
Secondary Star Formation in a Population III Object
We explore the possibility of subsequent star formation after a first star
forms in a Pop III object, by focusing on the radiation hydrodynamic (RHD)
feedback brought by ionizing photons as well as H2 dissociating photons. For
the purpose, we perform three-dimensional RHD simulations, where the radiative
transfer of ionizing photons and H2 dissociating photons from a first star is
self-consistently coupled with hydrodynamics based on a smoothed particle
hydrodynamics method. As a result, it is shown that density peaks above a
threshold density can keep collapsing owing to the shielding of H2 dissociating
radiation by an H2 shell formed ahead of a D-type ionization front. But, below
the threshold density, an M-type ionization front accompanied by a shock
propagates, and density peaks are radiation hydrodynamically evaporated by the
shock. The threshold density is dependent on the distance from a source star,
which is for the source distance of 30pc. Taking into
consideration that the extent of a Pop III object is pc and
density peaks within it have the density of cm, it is
concluded that the secondary star formation is allowed in the broad regions in
a Pop III object.Comment: 4pages, 2 figures, submitted to Ap
Formation Criteria and the Mass of Secondary Population III Stars
We explore the formation of secondary Population III (Pop III) stars under
radiation hydrodynamic (RHD) feedback by a preformed massive star. To properly
treat RHD feedback, we perform three-dimensional RHD simulations incorporating
the radiative transfer of ionizing photons as well as H_2 dissociating photons
from a preformed star. A collapsing gas cloud is settled at a given distance
from a 120Msun Pop III star, and the evolution of the cloud is pursued
including RHD feedback. We derive the threshold density depending on the
distance, above which the cloud can keep collapsing owing to the shielding of
H_2 dissociating radiation. We find that an H_2 shell formed ahead of an
ionizing front works effectively to shield the H_2 dissociating radiation,
leading to the positive feedback for the secondary Pop III star formation.
Also, near the threshold density, the envelope of gas cloud is stripped
significantly by a shock associated with an ionizing front. By comparing the
mass accretion timescale with the Kelvin-Helmholtz timescale, we estimate the
mass of secondary Pop III stars. It turns out that the stripping by a shock can
reduce the mass of secondary Pop III stars down to \approx 20Msum.Comment: ApJ accepted, 11 pages, 11 figure
Dissipation of Magnetic Flux in Primordial Star Formation: From Run-away Phase to Mass Accretion Phase
We investigate the dissipation of magnetic flux in primordial star-forming
clouds throughout their collapse including the run-away collapse phase as well
as the accretion phase. We solve the energy equation and the non-equilibrium
chemical reactions in the collapsing gas, in order to obtain the radial
distribution of the ionized fraction during the collapse. As a result, we find
the ionized fraction is high enough for the magnetic field to couple with the
gas throughout the evolution of the cloud. This result suggests that the jet
formation from protostars as well as the activation of magneto-rotational
instability in the accretion disk are enabled in the presence of the
cosmological seed magnetic flux proposed by Langer et al.(2003).Comment: 12 pages, 7 figures, PASJ accepte
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