951 research outputs found
Automated microorganism Sample Collection Module
Modified Gelman Sampler obtains representative sample of microorganism population. Proposed Sample Collection Module is based on direct inoculation of selected solid growth media encased in a cartridge at all times except during inoculation. Cartridge can be handled with no danger of contamination to sample or operator
Retention of Stellar-Mass Black Holes in Globular Clusters
Globular clusters should be born with significant numbers of stellar-mass
black holes (BHs). It has been thought for two decades that very few of these
BHs could be retained through the cluster lifetime. With masses ~10 MSun, BHs
are ~20 times more massive than an average cluster star. They segregate into
the cluster core, where they may eventually decouple from the remainder of the
cluster. The small-N core then evaporates on a short timescale. This is the
so-called Spitzer instability. Here we present the results of a full dynamical
simulation of a globular cluster containing many stellar-mass BHs with a
realistic mass spectrum. Our Monte Carlo simulation code includes detailed
treatments of all relevant stellar evolution and dynamical processes. Our main
finding is that old globular clusters could still contain many BHs at present.
In our simulation, we find no evidence for the Spitzer instability. Instead,
most of the BHs remain well-mixed with the rest of the cluster, with only the
innermost few tens of BHs segregating significantly. Over the 12 Gyr evolution,
fewer than half of the BHs are dynamically ejected through strong binary
interactions in the cluster core. The presence of BHs leads to long-term
heating of the cluster, ultimately producing a core radius on the high end of
the distribution for Milky Way globular clusters (and those of other galaxies).
A crude extrapolation from our model suggests that the BH--BH merger rate from
globular clusters could be comparable to the rate in the field.Comment: 5 pages, 4 figures, 1 table, published in Astrophysical Journal
Letter
Monte Carlo Simulations of Globular Cluster Evolution. VI. The Influence of an Intermediate Mass Black Hole
We present results of a series of Monte Carlo simulations investigating the
imprint of a central intermediate-mass black hole (IMBH) on the structure of a
globular cluster. We investigate the three-dimensional and projected density
profiles, and stellar disruption rates for idealized as well as realistic
cluster models, taking into account a stellar mass spectrum and stellar
evolution, and allowing for a larger, more realistic, number of stars than was
previously possible with direct N-body methods. We compare our results to other
N-body and Fokker-Planck simulations published previously. We find, in general,
very good agreement for the overall cluster structure and dynamical evolution
between direct N-body simulations and our Monte Carlo simulations. Significant
differences exist in the number of stars that are tidally disrupted by the
IMBH, which is most likely an effect of the wandering motion of the IMBH, not
included in the Monte Carlo scheme. These differences, however, are negligible
for the final IMBH masses in realistic cluster models as the disruption rates
are generally much lower than for single-mass clusters. As a direct comparison
to observations we construct a detailed model for the cluster NGC 5694, which
is known to possess a central surface brightness cusp consistent with the
presence of an IMBH. We find that not only the inner slope but also the outer
part of the surface brightness profile agree well with observations. However,
there is only a slight preference for models harboring an IMBH compared to
models without.Comment: 37 pages, 10 figures, Accepted for publication in ApJ Supplement.
Substantial additions on modeling NGC 5694 since original versio
Understanding the Dynamical State of Globular Clusters: Core-Collapsed vs Non Core-Collapsed
We study the dynamical evolution of globular clusters using our H\'enon-type
Monte Carlo code for stellar dynamics including all relevant physics such as
two-body relaxation, single and binary stellar evolution, Galactic tidal
stripping, and strong interactions such as physical collisions and binary
mediated scattering. We compute a large database of several hundred models
starting from broad ranges of initial conditions guided by observations of
young and massive star clusters. We show that these initial conditions very
naturally lead to present day clusters with properties including the central
density, core radius, half-light radius, half-mass relaxation time, and cluster
mass, that match well with those of the old Galactic globular clusters. In
particular, we can naturally reproduce the bimodal distribution in observed
core radii separating the "core-collapsed" vs the "non core-collapsed"
clusters. We see that the core-collapsed clusters are those that have reached
or are about to reach the equilibrium "binary burning" phase. The non
core-collapsed clusters are still undergoing gravo-thermal contraction.Comment: 42 pages, 12 figures, 1 table, submitted to MNRA
The Decay of Accreting Triple Systems as Brown Dwarf Formation Scenario
We investigate the dynamical decay of non-hierarchical accreting triple
systems and its implications on the ejection model as Brown Dwarf formation
scenario. A modified chain-regularization scheme is used to integrate the
equations of motion, that also allows for mass changes over time as well as for
momentum transfer from the accreted gas mass onto the bodies. We integrate an
ensemble of triple systems within a certain volume with different accretion
rates, assuming several prescriptions of how momentum is transferred onto the
bodies. We follow their evolution until the systems have decayed. We analyze
the end states and decay times of these systems and determine the fraction of
Brown Dwarfs formed, their escape speeds as well as the semi-major axis
distribution of the formed Brown Dwarf binaries. We find that the formation
probability of Brown Dwarfs depends strongly on the assumed momentum transfer
which is related to the motion of the gas. Due to ongoing accretion and
consequent shrinkage of the systems, the median escape velocity is increased by
a factor of 2 and the binary separations are decreased by a factor of 5
compared with non-accreting systems. Furthermore, the obtained semi-major axis
distribution drops off sharply to either side of the median, which is also
supported by observations. We conclude that accretion and momentum transfer of
accreted gas during the dynamical decay of triple systems is able to produce
the observed distribution of close binary Brown Dwarfs, making the ejection
model a viable option as Brown Dwarf formation scenario.Comment: 31 pages, 8 figures, accepted for publication in Ap
Monte Carlo Simulations of Globular Cluster Evolution. V. Binary Stellar Evolution
We study the dynamical evolution of globular clusters containing primordial
binaries, including full single and binary stellar evolution using our Monte
Carlo cluster evolution code updated with an adaptation of the single and
binary stellar evolution codes SSE/BSE from Hurley et. al (2000, 2002). We
describe the modifications we have made to the code. We present several test
calculations and comparisons with existing studies to illustrate the validity
of the code. We show that our code finds very good agreement with direct N-body
simulations including primordial binaries and stellar evolution. We find
significant differences in the evolution of the global properties of the
simulated clusters using stellar evolution compared to simulations without any
stellar evolution. In particular, we find that the mass loss from stellar
evolution acts as a significant energy production channel simply by reducing
the total gravitational binding energy and can significantly prolong the
initial core contraction phase before reaching the binary-burning quasi steady
state of the cluster evolution as noticed in Paper IV. We simulate a large grid
of clusters varying the initial cluster mass, binary fraction, and
concentration and compare properties of the simulated clusters with those of
the observed Galactic globular clusters (GGCs). We find that our simulated
cluster properties agree well with the observed GGC properties. We explore in
some detail qualitatively different clusters in different phases of their
evolution, and construct synthetic Hertzprung-Russell diagrams for these
clusters.Comment: 46 preprint pages, 18 figures, 3 tables, submitted to Ap
A Spitzer IRAC Imaging Survey for T Dwarf Companions Around M, L, and T Dwarfs: Observations, Results, and Monte Carlo Population Analyses
We report observational techniques, results, and Monte Carlo population
analyses from a Spitzer Infrared Array Camera imaging survey for substellar
companions to 117 nearby M, L, and T dwarf systems (median distance of 10 pc,
mass range of 0.6 to \sim0.05 M\odot). The two-epoch survey achieves typical
detection sensitivities to substellar companions of [4.5 {\mu}m] \leq 17.2 mag
for angular separations between about 7" and 165". Based on common proper
motion analysis, we find no evidence for new substellar companions. Using Monte
Carlo orbital simulations (assuming random inclination, random eccentricity,
and random longitude of pericenter), we conclude that the observational
sensitivities translate to an ability to detect 600-1100K brown dwarf
companions at semimajor axes greater than ~35 AU, and to detect 500-600K
companions at semimajor axes greater than ~60 AU. The simulations also estimate
a 600-1100K T dwarf companion fraction of < 3.4% for 35-1200 AU separations,
and < 12.4% for the 500-600K companions, for 60-1000 AU separations.Comment: 35 pages, 6 figure
Binarity at the L/T brown dwarf transition
Current atmospheric models cannot reproduce some of the characteristics of
the transition between the L dwarfs with cloudy atmospheres and the T dwarfs
with dust-depleted photospheres. It has been proposed that a majority of the
L/T transition brown dwarfs could actually be a combinaison of a cloudy L dwarf
and a clear T dwarf. Indeed binarity seems to occur more frequently among L/T
transition brown dwarfs.
We aim to refine the statistical significance of the seemingly higher
frequency of binaries. Co-eval binaries would also be interesting test-beds for
evolutionary models. We obtained high-resolution imaging for six mid-L to
late-T dwarfs, with photometric distances between 8 and 33pc, using the
adaptive optics systems NACO at the VLT, and the Lick system, both with the
laser guide star.
We resolve none of our targets. Combining our data with published results, we
obtain a frequency of resolved L/T transition brown dwarfs of (31+21-15)%,
compared to (21+10-7)% and (14+14-7)% for mid-L and T dwarfs (90% of confidence
level). These fractions do not significantly support, nor contradict, the
hypothesis of a larger binary fraction in the L/T transition. None of our
targets has companions with effective temperatures as low as 360-1000K at
separations larger than 0.5".Comment: 6 pages, 4 figures, accepted by A&
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