143 research outputs found
Simulation of the Gravitational Collapse and Fragmentation of Rotating Molecular Clouds
In this paper we study the process of the subsequent (runaway) fragmentation
of the rotating isothermal Giant Molecular Cloud (GMC) complex. Our own
developed Smoothed Particle Hydrodynamics (SPH) gas-dynamical model
successfully reproduce the observed Cloud Mass-distribution Function (CMF) in
our Galaxy (even the differences between the inner and outer parts of our
Galaxy). The steady state CMF is established during the collapse within a
free-fall timescale of the GMC. We show that one of the key parameters, which
defines the observed slope of the present day CMF, is the initial ratio of the
rotational (turbulent) and gravitational energy inside the fragmented GMC.Comment: 8 pages, 9 EPS figures, special forma.cls class file is use
Physical Processes in Star-Gas Systems
First we present a recently developed 3D chemodynamical code for galaxy
evolution from the K**2 collaboration. It follows the evolution of all
components of a galaxy such as dark matter, stars, molecular clouds and diffuse
interstellar matter (ISM). Dark matter and stars are treated as collisionless
N-body systems. The ISM is numerically described by a smoothed particle
hydrodynamics (SPH) approach for the diffuse (hot) gas and a sticky particle
scheme for the (cool) molecular clouds. Physical processs such as star
formation, stellar death or condensation and evaporation processes of clouds
interacting with the ISM are described locally. An example application of the
model to a star forming dwarf galaxy will be shown for comparison with other
codes. Secondly we will discuss new kinds of exotic chemodynamical processes,
as they occur in dense gas-star systems in galactic nuclei, such as
non-standard ``drag''-force interactions, destructive and gas producing stellar
collisions. Their implementation in 1D dynamical models of galactic nuclei is
presented. Future prospects to generalize these to 3D are work in progress and
will be discussed.Comment: 4 pages, 4 figures, "The 5th Workshop on Galactic Chemodynamics" -
Swinburne University (9-11 July 2003). To be published in the Publications of
the Astronomical Society of Australia in 2004 (B.K. Gibson and D. Kawata,
eds.). Accepted version, minor changes relative to origina
The star cluster survivability after gas expulsion is independent of the impact of the Galactic tidal field
We study the impact of the tidal field on the survivability of star clusters
following instantaneous gas expulsion. Our model clusters are formed with a
centrally-peaked star-formation efficiency profile as a result of
star-formation taking place with a constant efficiency per free-fall time. We
define the impact of the tidal field as the ratio of the cluster half-mass
radius to its Jacobi radius immediately after gas expulsion, . We vary by varying either the Galactocentric distance,
or the size (hence volume density) of star clusters.
We propose a new method to measure the violent relaxation duration, in which
we compare the total mass-loss rate of star clusters with their stellar
evolutionary mass-loss rate. That way, we can robustly estimate the bound mass
fraction of our model clusters at the end of violent relaxation. The duration
of violent relaxation correlates linearly with the Jacobi radius, when
considering identical clusters at different Galactocentric distances. In
contrast, it is nearly constant for the solar neighbourhood clusters, slightly
decreasing with . The violent relaxation does not last longer than 50
Myr in our simulations.
Identical model clusters placed at different Galactocentric distances have
the same final bound fraction, despite experiencing different impacts of the
tidal field. The solar neighbourhood clusters with different densities
experience only limited variations of their final bound fraction.
In general, we conclude that the cluster survivability after instantaneous
gas expulsion, as measured by their bound mass fraction at the end of violent
relaxation, , is independent of the impact of the tidal field,
.Comment: accepted for publication in MNRAS, 8 pages, 5 figures,3 table
Effects of galaxy--satellite interactions on bar formation
Aims. We aim to show how encounters with low-mass satellite galaxies may
alter the bar formation in a Milky Way-like disc galaxy. Methods. We use
high-resolution N-body simulations of a disc galaxy prone to mild bar
instability. For realistic initial conditions of satellites, we take advantage
of cosmological simulations of Milky Way-like dark matter haloes. Results. The
satellites may have a significant impact on the time of bar formation. Some
runs with satellites demonstrate a delay, while others show an advancement in
bar formation compared to the isolated run, with such time differences reaching
1 Gyr. Meanwhile, the final bar configuration, including its very
appearance and the bar characteristics such as the pattern speed and the
exponential growth rate of its amplitude are independent of the number of
encounters and their orbits. The contribution of satellites with masses below
is insignificant, unless their pericentre distances are small.
We suggest that the encounters act indirectly via inducing perturbations across
the disc that evolve to delayed waves in the central part and interfere with an
emerging seed bar. The predicted effect for the present-day host galaxy is
expected to be even more significant at redshifts .Comment: 16 pages, 14 figures and 4 table
Orbital evolution of the Carina dwarf galaxy and self-consistent star formation history determination
We present a new study of the evolution of the Carina dwarf galaxy that
includes a simultaneous derivation of its orbit and star formation history. The
structure of the galaxy is constrained through orbital parameters derived from
the observed distance, proper motions, radial velocity and star formation
history. The different orbits admitted by the large proper motion errors are
investigated in relation to the tidal force exerted by an external potential
representing the Milky Way (MW). Our analysis is performed with the aid of
fully consistent N-body simulations that are able to follow the dynamics and
the stellar evolution of the dwarf system in order to determine
self-consistently the star formation history of Carina. We find a star
formation history characterized by several bursts, partially matching the
observational expectation. We find also compatible results between dynamical
projected quantities and the observational constraints. The possibility of a
past interaction between Carina and the Magellanic Clouds is also separately
considered and deemed unlikely.Comment: Accepted in A&
Mitigating potentially hazardous asteroid impacts revisited
Context: Potentially hazardous asteroids (PHA) in Earth-crossing orbits pose
a constant threat to life on Earth. Several mitigation methods have been
proposed, and the most feasible technique appears to be the disintegration of
the impactor and the generation of a fragment cloud by explosive penetrators at
interception. Mitigation analyses, however neglect the effect of orbital
dynamics on fragments trajectory.
Aims: We aim at studying the effect of orbital dynamics of the impactor's
cloud on the number of fragments that hit the Earth assuming different
interception dates. The effect of self-gravitational cohesion and the axial
rotation of the impactor are also investigated.
Methods: The orbits of 10^5 fragments are computed with a high-precision
direct N-body integrator of the 8th order, running on GPUs. We consider orbital
perturbations from all large bodies in the Solar System and the self-gravity of
the cloud fragments.
Results: Using a series of numerical experiments, we show that orbital shear
causes the fragment cloud to adopt the shape of a triaxial ellipsoid. The shape
and alignment of the triaxial ellipsoid are strongly modulated by the cloud's
orbital trajectory, and hence the impact cross-section of the cloud with
respect to the Earth. Therefore, the number of fragments hitting the Earth is
strongly influenced by the orbit of the impactor and the time of interception.
A minimum number of impacts occurs for a well-defined orientation of the
impactor rotational axis, depending on the date of interception.
Conclusions: To minimise the lethal consequences of an PHA's impact, a
well-constrained interception timing is necessary. Too early interception may
not be ideal for PHAs in the Apollo or Aten groups. The best time to intercept
PHA is when it is at the pericentre of its orbit.Comment: Accepted for publication in A&A Letter
Galactic Halo Stars in Phase Space :A Hint of Satellite Accretion?
The present day chemical and dynamical properties of the Milky Way bear the
imprint of the Galaxy's formation and evolutionary history. One of the most
enduring and critical debates surrounding Galactic evolution is that regarding
the competition between ``satellite accretion'' and ``monolithic collapse'';
the apparent strong correlation between orbital eccentricity and metallicity of
halo stars was originally used as supporting evidence for the latter. While
modern-day unbiased samples no longer support the claims for a significant
correlation, recent evidence has been presented by Chiba & Beers
(2000,AJ,119,2843) for the existence of a minor population of high-eccentricity
metal-deficient halo stars. It has been suggested that these stars represent
the signature of a rapid (if minor) collapse phase in the Galaxy's history.
Employing velocity- and integrals of motion-phase space projections of these
stars, coupled with a series of N-body/Smoothed Particle Hydrodynamic (SPH)
chemodynamical simulations, we suggest an alternative mechanism for creating
such stars may be the recent accretion of a polar orbit dwarf galaxy.Comment: 12 pages(incl. figures). Accepted for publication in ApJ letters
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