1,513 research outputs found
The thermodynamics of collapsing molecular cloud cores using smoothed particle hydrodynamics with radiative transfer
We present the results of a series of calculations studying the collapse of
molecular cloud cores performed using a three-dimensional smoothed particle
hydr odynamics code with radiative transfer in the flux-limited diffusion
approximation. The opacities and specific heat capacities are identical for
each calculation. However, we find that the temperature evolution during the
simulations varies significantly when starting from different initial
conditions. Even spherically-symmetric clouds with different initial densities
show markedly different development. We conclude that simple barotropic
equations of state like those used in some previous calculations provide at
best a crude approximation to the thermal behaviour of the gas. Radiative
transfer is necessary to obtain accurate temperatures.Comment: 8 pages, 9 figures, accepted for publication in MNRA
The effect of magnetic fields on star cluster formation
We examine the effect of magnetic fields on star cluster formation by
performing simulations following the self-gravitating collapse of a turbulent
molecular cloud to form stars in ideal MHD. The collapse of the cloud is
computed for global mass-to-flux ratios of infinity, 20, 10, 5 and 3, that is
using both weak and strong magnetic fields. Whilst even at very low strengths
the magnetic field is able to significantly influence the star formation
process, for magnetic fields with plasma beta < 1 the results are substantially
different to the hydrodynamic case. In these cases we find large-scale
magnetically-supported voids imprinted in the cloud structure; anisotropic
turbulent motions and column density structure aligned with the magnetic field
lines, both of which have recently been observed in the Taurus molecular cloud.
We also find strongly suppressed accretion in the magnetised runs, leading to
up to a 75% reduction in the amount of mass converted into stars over the
course of the calculations and a more quiescent mode of star formation. There
is also some indication that the relative formation efficiency of brown dwarfs
is lower in the strongly magnetised runs due to the reduction in the importance
of protostellar ejections.Comment: 16 pages, 9 figures, 8 very pretty movies, MNRAS, accepted. Version
with high-res figures + movies available from
http://www.astro.ex.ac.uk/people/dprice/pubs/mcluster/index.htm
Stellar Encounters with Massive Star-Disk Systems
The dense, clustered environment in which massive stars form can lead to
interactions with neighboring stars. It has been hypothesized that collisions
and mergers may contribute to the growth of the most massive stars. In this
paper we extend the study of star-disk interactions to explore encounters
between a massive protostar and a less massive cluster sibling using the
publicly available SPH code GADGET-2. Collisions do not occur in the parameter
space studied, but the end state of many encounters is an eccentric binary with
a semi-major axis ~ 100 AU. Disk material is sometimes captured by the
impactor. Most encounters result in disruption and destruction of the initial
disk, and periodic torquing of the remnant disk. We consider the effect of the
changing orientation of the disk on an accretion driven jet, and the evolution
of the systems in the presence of on-going accretion from the parent core.Comment: 11 pages, 10 figures, accepted to Ap
TreeCol: a novel approach to estimating column densities in astrophysical simulations
We present TreeCol, a new and efficient tree-based scheme to calculate column
densities in numerical simulations. Knowing the column density in any direction
at any location in space is a prerequisite for modelling the propagation of
radiation through the computational domain. TreeCol therefore forms the basis
for a fast, approximate method for modelling the attenuation of radiation
within large numerical simulations. It constructs a HEALPix sphere at any
desired location and accumulates the column density by walking the tree and by
adding up the contributions from all tree nodes whose line of sight contributes
to the pixel under consideration. In particular when combined with widely-used
tree-based gravity solvers the new scheme requires little additional
computational cost. In a simulation with resolution elements, the
computational cost of TreeCol scales as , instead of the
scaling of most other radiative transfer schemes. TreeCol is naturally
adaptable to arbitrary density distributions and is easy to implement and to
parallelize. We discuss its accuracy and performance characteristics for the
examples of a spherical protostellar core and for the turbulent interstellar
medium. We find that the column density estimates provided by TreeCol are on
average accurate to better than 10 percent. In another application, we compute
the dust temperatures for solar neighborhood conditions and compare with the
result of a full-fledged Monte Carlo radiation-transfer calculation. We find
that both methods give very similar answers. We conclude that TreeCol provides
a fast, easy to use, and sufficiently accurate method of calculating column
densities that comes with little additional computational cost when combined
with an existing tree-based gravity solver.Comment: 11 pages, 10 figures, submitted to MNRA
Antisymmetric magnetoresistance in magnetic multilayers with perpendicular anisotropy
While magnetoresistance (MR) has generally been found to be symmetric in
applied field in non-magnetic or magnetic metals, we have observed
antisymmetric MR in Co/Pt multilayers. Simultaneous domain imaging and
transport measurements show that the antisymmetric MR is due to the appearance
of domain walls that run perpendicular to both the magnetization and the
current, a geometry existing only in materials with perpendicular magnetic
anisotropy. As a result, the extraordinary Hall effect (EHE) gives rise to
circulating currents in the vicinity of the domain walls that contributes to
the MR. The antisymmetric MR and EHE have been quantitatively accounted for by
a theoretical model.Comment: 17 pages, 4 figure
Gravitational Collapse in Turbulent Molecular Clouds. I. Gasdynamical Turbulence
Observed molecular clouds often appear to have very low star formation
efficiencies and lifetimes an order of magnitude longer than their free-fall
times. Their support is attributed to the random supersonic motions observed in
them. We study the support of molecular clouds against gravitational collapse
by supersonic, gas dynamical turbulence using direct numerical simulation.
Computations with two different algorithms are compared: a particle-based,
Lagrangian method (SPH), and a grid-based, Eulerian, second-order method
(ZEUS). The effects of both algorithm and resolution can be studied with this
method. We find that, under typical molecular cloud conditions, global collapse
can indeed be prevented, but density enhancements caused by strong shocks
nevertheless become gravitationally unstable and collapse into dense cores and,
presumably, stars. The occurance and efficiency of local collapse decreases as
the driving wave length decreases and the driving strength increases. It
appears that local collapse can only be prevented entirely with unrealistically
short wave length driving, but observed core formation rates can be reproduced
with more realistic driving. At high collapse rates, cores are formed on short
time scales in coherent structures with high efficiency, while at low collapse
rates they are scattered randomly throughout the region and exhibit
considerable age spread. We suggest that this naturally explains the observed
distinction between isolated and clustered star formation.Comment: Minor revisions in response to referee, thirteen figures, accepted to
Astrophys.
On the relative motions of dense cores and envelopes in star-forming molecular clouds
Hydrodynamical simulations of star formation indicate that the motions of
protostars through their natal molecular clouds may be crucial in determining
the properties of stars through competitive accretion and dynamical
interactions. Walsh, Myers & Burton recently investigated whether such motions
might be observable in the earliest stages of star formation by measuring the
relative shifts of line-centre velocities of low- and high-density tracers of
low-mass star-forming cores. They found very small (~0.1 km/s) relative
motions. In this paper, we analyse the hydrodynamical simulation of Bate,
Bonnell & Bromm and find that it also gives small relative velocities between
high-density cores and low-density envelopes, despite the fact that competitive
accretion and dynamical interactions occur between protostars in the
simulation. Thus, the simulation is consistent with the observations in this
respect. However, we also find some differences between the simulation and the
observations. Overall, we find that the high-density gas has a higher velocity
dispersion than that observed by Walsh et al. We explore this by examining the
dependence of the gas velocity dispersion on density and its evolution with
time during the simulation. We find that early in the simulation the gas
velocity dispersion decreases monotonically with increasing density, while
later in the simulation, when the dense cores have formed multiple objects, the
velocity dispersion of the high-density gas increases. Thus, the simulation is
in best agreement with the observations early on, before many objects have
formed in each dense core.Comment: 8 pages, 7 figures. Accepted for publication in MNRA
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