237 research outputs found
On Random Walks with a General Moving Barrier
Random walks with a general, nonlinear barrier have found recent applications
ranging from reionization topology to refinements in the excursion set theory
of halos. Here, we derive the first-crossing distribution of random walks with
a moving barrier of an arbitrary shape. Such a distribution is shown to satisfy
an integral equation that can be solved by a simple matrix inversion, without
the need for Monte Carlo simulations, making this useful for exploring a large
parameter space. We discuss examples in which common analytic approximations
fail, a failure which can be remedied using the method described here.Comment: 6 pages, 7 figures, submitted to Ap
A dynamical interpretation of the radio jet in 3C 31
The radio source 3C 31, which is positionally identified with the galaxy NGC 383, has recently been shown to contain two curved radio jets emanating from the galaxy's nucleus. In this paper, we demonstrate that the projected shape of these jets may be produced by a dynamical interaction between NGC 383 and its neighbour NGC 382. On the basis of this hypothesis, the orbital elements of the NGC383/NGC382 system are estimated and a mean V magnitude mass to light ratio of about 11 solar units is derived for the system. The inferred value for the jet velocity is approximately 500km/s. Some consequences for theoretical models of other extragalactic radio sources are briefly outlined
3D Radiative Transfer in Carinae: Application of the SimpleX Algorithm to 3D SPH Simulations of Binary Colliding Winds
Eta Carinae is an ideal astrophysical laboratory for studying massive binary
interactions and evolution, and stellar wind-wind collisions. Recent
three-dimensional (3D) simulations set the stage for understanding the highly
complex 3D flows in Car. Observations of different broad high- and
low-ionization forbidden emission lines provide an excellent tool to constrain
the orientation of the system, the primary's mass-loss rate, and the ionizing
flux of the hot secondary. In this work we present the first steps towards
generating synthetic observations to compare with available and future HST/STIS
data. We present initial results from full 3D radiative transfer simulations of
the interacting winds in Car. We use the SimpleX algorithm to
post-process the output from 3D SPH simulations and obtain the ionization
fractions of hydrogen and helium assuming three different mass-loss rates for
the primary star. The resultant ionization maps of both species constrain the
regions where the observed forbidden emission lines can form. Including
collisional ionization is necessary to achieve a better description of the
ionization states, especially in the areas shielded from the secondary's
radiation. We find that reducing the primary's mass-loss rate increases the
volume of ionized gas, creating larger areas where the forbidden emission lines
can form. We conclude that post processing 3D SPH data with SimpleX is a viable
tool to create ionization maps for Car.Comment: 18 pages, 11 figures, accepted for publication in MNRA
Hydrodynamical Models of Outflow Collimation in YSOs
We explore the physics of time-dependent hydrodynamic collimation of jets
from Young Stellar Objects (YSOs). Using parameters appropriate to YSOs we have
carried out high resolution hydrodynamic simulations modeling the interaction
of a central wind with an environment characterized by a moderate opening angle
toroidal density distribution. The results show that the the wind/environment
interaction produces strongly collimated supersonic jets. The jet is composed
of shocked wind gas. Using analytical models of wind blown bubble evolution we
show that the scenario studied here should be applicable to YSOs and can, in
principle, initiate collimation on the correct scales (R ~ 100 AU). The
simulations reveal a number of time-dependent non-linear features not
anticipated in previous analytical studies including: a prolate wind shock; a
chimney of cold swept-up ambient material dragged into the bubble cavity; a
plug of dense material between the jet and bow shocks. We find that the
collimation of the jet occurs through both de Laval nozzles and focusing of the
wind via the prolate wind shock. Using an analytical model for shock focusing
we demonstrate that a prolate wind shock can, by itself, produce highly
collimated supersonic jets.Comment: Accepted by ApJ, 31 pages with 12 figures (3 JPEG's) now included,
using aasms.sty, Also available in postscript via a gzipped tar file at
ftp://s1.msi.umn.edu/pub/afrank/SFIC1/SFIC.tar.g
Solar-Like Cycle in Asymptotic Giant Branch Stars
I propose that the mechanism behind the formation of concentric semi-periodic
shells found in several planetary nebulae (PNs) and proto-PNs, and around one
asymptotic giant branch (AGB) star, is a solar-like magnetic activity cycle in
the progenitor AGB stars. The time intervals between consecutive ejection
events is about 200-1,000 years, which is assumed to be the cycle period (the
full magnetic cycle can be twice as long, as is the 22-year period in the sun).
The magnetic field has no dynamical effects; it regulates the mass loss rate by
the formation of magnetic cool spots. The enhanced magnetic activity at the
cycle maximum results in more magnetic cool spots, which facilitate the
formation of dust, hence increasing the mass loss rate. The strong magnetic
activity implies that the AGB star is spun up by a companion, via a tidal or
common envelope interaction. The strong interaction with a stellar companion
explains the observations that the concentric semi-periodic shells are found
mainly in bipolar PNs.Comment: 10 pages, submitted to Ap
ZOBOV: a parameter-free void-finding algorithm
ZOBOV (ZOnes Bordering On Voidness) is an algorithm that finds density
depressions in a set of points, without any free parameters, or assumptions
about shape. It uses the Voronoi tessellation to estimate densities, which it
uses to find both voids and subvoids. It also measures probabilities that each
void or subvoid arises from Poisson fluctuations. This paper describes the
ZOBOV algorithm, and the results from its application to the dark-matter
particles in a region of the Millennium Simulation. Additionally, the paper
points out an interesting high-density peak in the probability distribution of
dark-matter particle densities.Comment: 10 pages, 8 figures, MNRAS, accepted. Added explanatory figures, and
better edge-detection methods. ZOBOV code available at
http://www.ifa.hawaii.edu/~neyrinck/vobo
Periodic bursts of Star Formation in Irregular Galaxies
We present N-body/SPH simulations of the evolution of an isolated dwarf
galaxy including a detailed model for the ISM, star formation and stellar
feedback. Depending on the strength of the feedback, the modelled dwarf galaxy
shows periodic or quasi-periodic bursts of star formation of moderate strength.
The period of the variations is related to the dynamical timescale, of the
order of yr. We show that the results of these simulations are in
good agreement with recent detailed observations of dwarf irregulars (dIrr) and
that the peculiar kinematic and morphological properties of these objects,as
revealed by high resolution HI studies, are fully reproduced. We discuss these
results in the context of recent surveys of dwarf galaxies and point out that
if the star formation pattern of our model galaxy is typical for dwarf
irregulars this could explain the scatter of observed properties of dwarf
galaxies. Specifically, we show that the time sampled distribution of the ratio
between the instanteneous star formation rate (SFR) and the mean SFR is similar
to that distribution in observed sample of dwarf galaxies.Comment: 11 pages, 6 figures, accepted for A&
Unequal-mass galaxy mergers and the creation of cluster S0 galaxies
It is a longstanding and remarkable problem when and how red S0 galaxies were
formed in clusters of galaxies.
We here propose that the major mechanism for the S0 creation is galaxy
merging between two spirals with unequal mass.
Our numerical simulations demonstrate that galaxy merging exhausts a large
amount of interstellar medium of two gas-rich spirals owing to the moderately
enhanced star formation, and subsequently transforms the two into one gas-poor
S0 galaxy with structure and kinematics strikingly similar to the observed
ones.
This secondary S0 formation with enhanced star formation explains a smaller
fraction of S0 population recently observed in some distant clusters of
galaxies.
Unequal-mass galaxy mergers thus provide an evolutionary link between a
larger number of blue spirals observed in intermediate redshift clusters and
red S0s prevalent in the present-day ones.Comment: 14 pages 3 figures (ps file), ApJL in pres
A G1-like globular cluster in NGC 1023
The structure of a very bright (MV = -10.9) globular cluster in NGC 1023 is
analyzed on two sets of images taken with the Hubble Space Telescope. From
careful modeling of King profile fits to the cluster image, a core radius of
0.55+/-0.1 pc, effective radius 3.7+/-0.3 pc and a central V-band surface
brightness of 12.9+/-0.5 mag / square arcsec are derived. This makes the
cluster much more compact than Omega Cen, but very similar to the brightest
globular cluster in M31, G1 = Mayall II. The cluster in NGC 1023 appears to be
very highly flattened with an ellipticity of about 0.37, even higher than for
Omega Cen and G1, and similar to the most flattened clusters in the Large
Magellanic Cloud.Comment: 14 pages, 3 figures, 1 table. Accepted for AJ, Oct 200
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