2,270 research outputs found
Satellite decay in flattened dark matter haloes
We carry out a set of self-consistent N-body calculations to compare the
decay rates of satellite dwarf galaxies orbiting a disc galaxy embedded in a
dark matter halo (DMH). We consider both spherical and oblate axisymmetric DMHs
of aspect ratio q_h=0.6. The satellites are given different initial orbital
inclinations, orbital periods and mass. The live flattened DMHs with embedded
discs and bulges are set-up using a new fast algorithm, MaGalie (Boily, Kroupa
and Pe\~{n}arrubia 2001).
We find that the range of survival times of satellites within a flattened DMH
becomes of the order of 100% larger than the same satellites within a spherical
DMH. In the oblate DMH, satellites on polar orbits have the longest survival
time, whereas satellites on coplanar prograde orbits are destroyed most
rapidly. The orbital plane of a satellite tilts as a result of anisotropic
dynamical friction, causing the satellite's orbit to align with the plane of
symmetry of the DMH. Polar orbits are not subjected to alignment. Therefore the
decay of a satellites in an axisymmetric DMH may provide a natural explanation
for the observed lack of satellites within (0-30) degrees of their host
galaxy's disc (Holmberg 1969; Zaritsky and Gonz\'alez 1999).
The computations furthermore indicate that the evolution of the orbital
eccentricity is highly dependent of its initial value e(t=0) and the DMH's
shape. We also discuss some implications of flattened DMHs for satellite debris
streams.Comment: 13 pages, 9 figures. Accepted by MNRA
Post-Newtonian SPH calculations of binary neutron star coalescence. II. Binary mass ratio, equation of state, and spin dependence
Using our new Post-Newtonian SPH (smoothed particle hydrodynamics) code, we
study the final coalescence and merging of neutron star (NS) binaries. We vary
the stiffness of the equation of state (EOS) as well as the initial binary mass
ratio and stellar spins. Results are compared to those of Newtonian
calculations, with and without the inclusion of the gravitational radiation
reaction. We find a much steeper decrease in the gravity wave peak strain and
luminosity with decreasing mass ratio than would be predicted by simple
point-mass formulae. For NS with softer EOS (which we model as simple
polytropes) we find a stronger gravity wave emission, with a
different morphology than for stiffer EOS (modeled as polytropes as
in our previous work). We also calculate the coalescence of NS binaries with an
irrotational initial condition, and find that the gravity wave signal is
relatively suppressed compared to the synchronized case, but shows a very
significant second peak of emission. Mass shedding is also greatly reduced, and
occurs via a different mechanism than in the synchronized case. We discuss the
implications of our results for gravity wave astronomy with laser
interferometers such as LIGO, and for theoretical models of gamma-ray bursts
(GRBs) based on NS mergers.Comment: RevTeX, 38 pages, 24 figures, Minor Corrections, to appear in Phys.
Rev.
Magnetically Regulated Star Formation in 3D: The Case of Taurus Molecular Cloud Complex
We carry out three-dimensional MHD simulations of star formation in
turbulent, magnetized clouds, including ambipolar diffusion and feedback from
protostellar outflows. The calculations focus on relatively diffuse clouds
threaded by a strong magnetic field capable of resisting severe tangling by
turbulent motions and retarding global gravitational contraction in the
cross-field direction. They are motivated by observations of the Taurus
molecular cloud complex (and, to a lesser extent, Pipe Nebula), which shows an
ordered large-scale magnetic field, as well as elongated condensations that are
generally perpendicular to the large-scale field. We find that stars form in
earnest in such clouds when enough material has settled gravitationally along
the field lines that the mass-to-flux ratios of the condensations approach the
critical value. Only a small fraction (of order 1% or less) of the nearly
magnetically-critical, condensed material is turned into stars per local
free-fall time, however. The slow star formation takes place in condensations
that are moderately supersonic; it is regulated primarily by magnetic fields,
rather than turbulence. The quiescent condensations are surrounded by diffuse
halos that are much more turbulent, as observed in the Taurus complex. Strong
support for magnetic regulation of star formation in this complex comes from
the extremely slow conversion of the already condensed, relatively quiescent
CO gas into stars, at a rate two orders of magnitude below the maximum,
free-fall value. We analyze the properties of dense cores, including their mass
spectrum, which resembles the stellar initial mass function.Comment: submitted to Ap
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