194 research outputs found
Stellar and brown dwarf properties from numerical simulations
We review the statistical properties of stars and brown dwarfs obtained from
the first hydrodynamical simulation of star cluster formation to produce more
than a thousand stars and brown dwarfs while simultaneously resolving the
lowest mass brown dwarfs (those with masses set by the opacity limit for
fragmentation), binaries with separations down to 1 AU, and discs with radii
greater than 10 AU. In particular, we present the eccentricity distribution of
the calculation's very-low-mass and brown dwarf binaries which has not been
previously published.Comment: To be published in Highlights of Astronomy, Vol 15 (CUP) from Special
Session 7 of IAU XXVII. 2 pages, 1 table, 1 figure
Non-convergence of the critical cooling timescale for fragmentation of self-gravitating discs
We carry out a resolution study on the fragmentation boundary of
self-gravitating discs. We perform three-dimensional Smoothed Particle
Hydrodynamics simulations of discs to determine whether the critical value of
the cooling timescale in units of the orbital timescale, beta_{crit}, converges
with increasing resolution. Using particle numbers ranging from 31,250 to 16
million (the highest resolution simulations to date) we do not find
convergence. Instead, fragmentation occurs for longer cooling timescales as the
resolution is increased. These results suggest that at the very least, the
critical value of the cooling timescale is longer than previously thought.
However, the absence of convergence also raises the question of whether or not
a critical value exists. In light of these results, we caution against using
cooling timescale or gravitational stress arguments to deduce whether
gravitational instability may or may not have been the formation mechanism for
observed planetary systems.Comment: Accepted for publication by MNRAS Letters. 6 pages, 3 figure
The formation of close binary systems
A viable solution to the origin of close binary systems, unaccounted for in
recent theories, is presented. Fragmentation, occurring at the end of the
secondary collapse phase (during which molecular hydrogen is dissociating), can
form binary systems with separations less than 1 au. Two fragmentation modes
are found to occur after the collapse is halted. The first consists of the
fragmentation of a protostellar disc due to rotational instabilities in a
protostellar core, involving both an and an mode. This
fragmentation mechanism is found to be insensitive to the initial density
distribution: it can occur in both centrally condensed and uniform initial
conditions. The second fragmentation mode involves the formation of a rapidly
rotating core at the end of the collapse phase which is unstable to the
axisymmetric perturbations. This core bounces into a ring which quickly
fragments into several components. The binary systems thus formed contain less
than 1 per cent of a solar mass and therefore will need to accrete most of
their final mass if they are to form a binary star system. Their orbital
properties will thus be determined by the properties of the accreted matter.Comment: 6 pages, uuencoded compressed postscript file (containing 2 figures
Two fluid dust and gas mixtures in SPH: A semi-implicit approach
A method to avoid the explicit time integration of small dust grains in the
two fluid gas/dust smoothed particle hydrodynamics (SPH) approach is proposed.
By assuming a very simple exponential decay model for the relative velocity
between the gas and dust components, all the effective characteristics of the
drag force can be reproduced. A series of tests has been performed to compare
the accuracy of the method with analytical and explicit integration results. We
find that the method performs well on a wide range of tests, and can provide
large speed ups over explicit integration when the dust stopping time is small.
We have also found that the method is much less dissipative than conventional
explicit or implicit two-fluid SPH approaches when modelling dusty shocks.Comment: 20 pages, 14 figures. Accepted for publication in MNRA
Two-fluid dust and gas mixtures in smoothed particle hydrodynamics II: an improved semi-implicit approach
We present an improved version of the Loren-Aguilar & Bate (2014) method to
integrate the two-fluid dust/gas equations that correctly captures the limiting
velocity of small grains in the presence of net differences (excluding the drag
force) between the accelerations of the dust and the gas. A series of
accelerated DUSTYBOX tests and a simulation of dust-settling in a
protoplanetary disc are performed comparing the performance of the new and old
methods. The modified method can accurately capture the correct limiting
velocity while preserving all the conservation properties of the original
method.Comment: Accepted for publication in MNRA
The Origin of the Initial Mass Function and Its Dependence on the Mean Jeans Mass in Molecular Clouds
We investigate the dependence of stellar properties on the mean thermal Jeans
mass in molecular clouds. We compare the results from the two largest
hydrodynamical simulations of star formation to resolve the fragmentation
process down to the opacity limit, the first of which was reported by Bate,
Bonnell & Bromm. The initial conditions of the two calculations are identical
except for the radii of the clouds, which are chosen so that the mean densities
and mean thermal Jeans masses of the clouds differ by factors of nine and
three, respectively. We find that the denser cloud, with the lower mean thermal
Jeans mass, produces a higher proportion of brown dwarfs and has a lower
characteristic (median) mass of the stars and brown dwarfs. This dependence of
the initial mass function (IMF) on the density of the cloud may explain the
observation that the Taurus star-forming region appears to be deficient in
brown dwarfs when compared with the Orion Trapezium cluster. The new
calculation also produces wide binaries (separations >20 AU), one of which is a
wide binary brown dwarf system. Based on the hydrodynamical calculations, we
develop a simple accretion/ejection model for the origin of the IMF. In the
model, all stars and brown dwarfs begin with the same mass (set by the opacity
limit for fragmentation) and grow in mass until their accretion is terminated
stochastically by their ejection from the cloud through dynamically
interactions. The model predicts that the main variation of the IMF in
different star-forming environments should be in the location of the peak (due
to variations in the mean thermal Jeans mass of the cloud) and in the
substellar regime. However, the slope of the IMF at high-masses may depend on
the dispersion in the accretion rates of protostars.Comment: 22 pages, 14 figures, accepted for publication in MNRAS. Paper with
high-resolution figures and animations available from
http://www.astro.ex.ac.uk/people/mbate/ Replacement removes inconsistent
definitions of base 10 logarithm
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