3,082 research outputs found
Gas expulsion in highly substructured embedded star clusters
We investigate the response of initially substructured, young, embedded star
clusters to instantaneous gas expulsion of their natal gas. We introduce
primordial substructure to the stars and the gas by simplistically modelling
the star formation process so as to obtain a variety of substructure
distributed within our modelled star forming regions. We show that, by
measuring the virial ratio of the stars alone (disregarding the gas
completely), we can estimate how much mass a star cluster will retain after gas
expulsion to within 10% accuracy, no matter how complex the background
structure of the gas is, and we present a simple analytical recipe describing
this behaviour. We show that the evolution of the star cluster while still
embedded in the natal gas, and the behavior of the gas before being expelled,
are crucial processes that affect the timescale on which the cluster can evolve
into a virialized spherical system. Embedded star clusters that have high
levels of substructure are subvirial for longer times, enabling them to survive
gas expulsion better than a virialized and spherical system. By using a more
realistic treatment for the background gas than our previous studies, we find
it very difficult to destroy the young clusters with instantaneous gas
expulsion. We conclude that gas removal may not be the main culprit for the
dissolution of young star clusters.Comment: 19 pages, 8 figures, 2 tables. Accepted for publication in MNRA
Life and death of a hero - Lessons learned from modeling the dwarf spheroidal Hercules: an incorrect orbit?
Hercules is a dwarf spheroidal satellite of the Milky Way, found at a
distance of about 138 kpc, and showing evidence of tidal disruption. It is very
elongated and exhibits a velocity gradient of 16 +/- 3 km/s/kpc. Using this
data a possible orbit of Hercules has previously been deduced in the
literature. In this study we make use of a novel approach to find a best fit
model that follows the published orbit. Instead of using trial and error, we
use a systematic approach in order to find a model that fits multiple
observables simultaneously. As such, we investigate a much wider parameter
range of initial conditions and ensure we have found the best match possible.
Using a dark matter free progenitor that undergoes tidal disruption, our
best-fit model can simultaneously match the observed luminosity, central
surface brightness, effective radius, velocity dispersion, and velocity
gradient of Hercules. However, we find it is impossible to reproduce the
observed elongation and the position angle of Hercules at the same time in our
models. This failure persists even when we vary the duration of the simulation
significantly, and consider a more cuspy density distribution for the
progenitor. We discuss how this suggests that the published orbit of Hercules
is very likely to be incorrect.Comment: accepted by MNRAS; 19 pages, 19 figures, 2 table
Ursa Major II - Reproducing the observed properties through tidal disruption
Recent deep photometry of the dwarf spheroidal Ursa Major II's morphology,
and spectroscopy of individual stars, have provided a number of new constraints
on its properties. With a velocity dispersion 6 km s, and under
the assumption that the galaxy is virialised, the mass-to-light ratio is found
to be approaching 2000 - apparently heavily dark matter dominated. Using
N-Body simulations, we demonstrate that the observed luminosity, ellipticity,
irregular morphology, velocity gradient, and the velocity dispersion can be
well reproduced through processes associated with tidal mass loss, and in the
absence of dark matter. These results highlight the considerable uncertainty
that exists in measurements of the dark matter content of Ursa Major II. The
dynamics of the inner tidal tails, and tidal stream, causes the observed
velocity dispersion of stars to be boosted to values of 5 km s (20
km s at times). This effect is responsible for raising the velocity
dispersion of our model to the observed values in UMaII. We test an iterative
rejection technique for removing unbound stars from samples of UMaII stars
whose positions on the sky, and line-of-sight velocities, are provided. We find
this technique is very effective at providing an accurate bound mass from this
information, and only fails when the galaxy has a bound mass less than 10 of
its initial mass. However when mass remains bound, mass overestimation by
3 orders of magnitude are seen. Additionally we find that mass measurements
are sensitive to measurement uncertainty in line-of-sight velocities.
Measurement uncertainties of 1-4 km s result in mass overestimates by a
factor of 1.3-5.7.Comment: 17 pages, 12 figures, accepted to MNRAS: 23rd, May, 201
Bound vortex states and exotic lattices in multi-component Bose-Einstein condensates: The role of vortex-vortex interaction
We numerically study the vortex-vortex interaction in multi-component
homogeneous Bose-Einstein condensates within the realm of the Gross-Pitaevskii
theory. We provide strong evidences that pairwise vortex interaction captures
the underlying mechanisms which determine the geometric configuration of the
vortices, such as different lattices in many-vortex states, as well as the
bound vortex states with two (dimer) or three (trimer) vortices. Specifically,
we discuss and apply our theoretical approach to investigate intra- and
inter-component vortex-vortex interactions in two- and three-component
Bose-Einstein condensates, thereby shedding light on the formation of the
exotic vortex configurations. These results correlate with current experimental
efforts in multi-component Bose-Einstein condensates, and the understanding of
the role of vortex interactions in multiband superconductors.Comment: Published in PR
Transition from single-file to two-dimensional diffusion of interacting particles in a quasi-one-dimensional channel
Diffusive properties of a monodisperse system of interacting particles
confined to a \textit{quasi}-one-dimensional (Q1D) channel are studied using
molecular dynamics (MD) simulations. We calculate numerically the mean-squared
displacement (MSD) and investigate the influence of the width of the channel
(or the strength of the confinement potential) on diffusion in finite-size
channels of different shapes (i.e., straight and circular). The transition from
single-file diffusion (SFD) to the two-dimensional diffusion regime is
investigated. This transition (regarding the calculation of the scaling
exponent () of the MSD ) as a
function of the width of the channel, is shown to change depending on the
channel's confinement profile. In particular the transition can be either
smooth (i.e., for a parabolic confinement potential) or rather sharp/stepwise
(i.e., for a hard-wall potential), as distinct from infinite channels where
this transition is abrupt. This result can be explained by qualitatively
different distributions of the particle density for the different confinement
potentials.Comment: 13 pages, 11 figure
BEC-BCS crossover in a cold and magnetized two color NJL model
The BEC-BCS crossover for a NJL model with diquark interactions is studied in
the presence of an external magnetic field. Particular attention is paid to
different regularization schemes used in the literature. A thorough comparison
of results is performed for the case of a cold and magnetized two-color NJL
model. According to our results, the critical chemical potential for the BEC
transition exhibits a clear inverse magnetic catalysis effect for magnetic
fields in the range . As for the BEC-BCS
crossover, the corresponding critical chemical potential is very weakly
sensitive to magnetic fields up to , showing a much smaller
inverse magnetic catalysis as compared to the BEC transition, and displays a
strong magnetic catalysis from this point on.Comment: 15 pages, 8 figures; v2 PRD versio
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