3,102 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
Collapse of a Molecular Cloud Core to Stellar Densities: The First Three-Dimensional Calculations
We present results from the first three-dimensional calculations ever to
follow the collapse of a molecular cloud core (~ 10^{-18} g cm^{-3}) to stellar
densities (> 0.01 g cm^{-3}). The calculations resolve structures over 7 orders
of magnitude in spatial extent (~ 5000 AU - 0.1 R_\odot), and over 17 orders of
magnitude in density contrast. With these calculations, we consider whether
fragmentation to form a close binary stellar system can occur during the second
collapse phase. We find that, if the quasistatic core that forms before the
second collapse phase is dynamically unstable to the growth of non-axisymmetric
perturbations, the angular momentum extracted from the central regions of the
core, via gravitational torques, is sufficient to prevent fragmentation and the
formation of a close binary during the subsequent second collapse.Comment: ApJ Letters, in press (will appear in Nov 20 issue; available from
the ApJ Rapid Release web page). 7 pages, incl. 5 figures. Also available at
http://www.mpia-hd.mpg.de/theory/bat
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 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
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
- …