2,076 research outputs found
Reconstructing the Accretion History of the Galactic Stellar Halo from Chemical Abundance Ratio Distributions
Observational studies of halo stars during the last two decades have placed
some limits on the quantity and nature of accreted dwarf galaxy contributions
to the Milky Way stellar halo by typically utilizing stellar phase-space
information to identify the most recent halo accretion events. In this study we
tested the prospects of using 2-D chemical abundance ratio distributions
(CARDs) found in stars of the stellar halo to determine its formation history.
First, we used simulated data from eleven "MW-like" halos to generate satellite
template sets of 2-D CARDs of accreted dwarf satellites which are comprised of
accreted dwarfs from various mass regimes and epochs of accretion. Next, we
randomly drew samples of mock observations of stellar chemical
abundance ratios ([/Fe], [Fe/H]) from those eleven halos to generate
samples of the underlying densities for our CARDs to be compared to our
templates in our analysis. Finally, we used the expectation-maximization
algorithm to derive accretion histories in relation to the satellite template
set (STS) used and the sample size. For certain STS used we typically can
identify the relative mass contributions of all accreted satellites to within a
factor of 2. We also find that this method is particularly sensitive to older
accretion events involving low-luminous dwarfs e.g. ultra-faint dwarfs -
precisely those events that are too ancient to be seen by phase-space studies
of stars and too faint to be seen by high-z studies of the early Universe.
Since our results only exploit two chemical dimensions and near-future surveys
promise to provide dimensions, we conclude that these new
high-resolution spectroscopic surveys of the stellar halo will allow us to
recover its accretion history - and the luminosity function of infalling dwarf
galaxies - across cosmic time.Comment: Article contains 18 pages total (16 pages of main text + 2 pages of
Appendix) with 12 figures; accepted for publication in Ap
Variational Integrators for Almost-Integrable Systems
We construct several variational integrators--integrators based on a discrete
variational principle--for systems with Lagrangians of the form L = L_A +
epsilon L_B, with epsilon << 1, where L_A describes an integrable system. These
integrators exploit that epsilon << 1 to increase their accuracy by
constructing discrete Lagrangians based on the assumption that the integrator
trajectory is close to that of the integrable system. Several of the
integrators we present are equivalent to well-known symplectic integrators for
the equivalent perturbed Hamiltonian systems, but their construction and error
analysis is significantly simpler in the variational framework. One novel
method we present, involving a weighted time-averaging of the perturbing terms,
removes all errors from the integration at O(epsilon). This last method is
implicit, and involves evaluating a potentially expensive time-integral, but
for some systems and some error tolerances it can significantly outperform
traditional simulation methods.Comment: 14 pages, 4 figures. Version 2: added informative example; as
accepted by Celestial Mechanics and Dynamical Astronom
Multiply Folded Graphene
The folding of paper, hide, and woven fabric has been used for millennia to
achieve enhanced articulation, curvature, and visual appeal for intrinsically
flat, two-dimensional materials. For graphene, an ideal two-dimensional
material, folding may transform it to complex shapes with new and distinct
properties. Here, we present experimental results that folded structures in
graphene, termed grafold, exist, and their formations can be controlled by
introducing anisotropic surface curvature during graphene synthesis or transfer
processes. Using pseudopotential-density functional theory calculations, we
also show that double folding modifies the electronic band structure of
graphene. Furthermore, we demonstrate the intercalation of C60 into the
grafolds. Intercalation or functionalization of the chemically reactive folds
further expands grafold's mechanical, chemical, optical, and electronic
diversity.Comment: 29 pages, 10 figures (accepted in Phys. Rev. B
Selection of chromosomal DNA libraries using a multiplex CRISPR system.
The directed evolution of biomolecules to improve or change their activity is central to many engineering and synthetic biology efforts. However, selecting improved variants from gene libraries in living cells requires plasmid expression systems that suffer from variable copy number effects, or the use of complex marker-dependent chromosomal integration strategies. We developed quantitative gene assembly and DNA library insertion into the Saccharomyces cerevisiae genome by optimizing an efficient single-step and marker-free genome editing system using CRISPR-Cas9. With this Multiplex CRISPR (CRISPRm) system, we selected an improved cellobiose utilization pathway in diploid yeast in a single round of mutagenesis and selection, which increased cellobiose fermentation rates by over 10-fold. Mutations recovered in the best cellodextrin transporters reveal synergy between substrate binding and transporter dynamics, and demonstrate the power of CRISPRm to accelerate selection experiments and discoveries of the molecular determinants that enhance biomolecule function
Indigenous sharing, collaboration and synchronous learning
Online learning is progressively accepted in Indigenous communities with the realized potential for sharing, collaboration and learning for adults living in remote and isolated communities. This study used a design-based research approach that provided opportunity to integrate the current literature, literacy practitioners\u27 views and community members\u27 self identified literacy needs to generate ten draft guiding principles which guided this study. A collaborative community engagement project was created by the community members in consideration of these principles and presented in three iterations in a synchronous environment which will lead to design-based principles for working with technology and Indigenous communities. This paper examines the framework and approach for this study, provides a short literature review and presents the draft guiding principles drawn from data collected from the stakeholders and from which the project was created
Gravitomagnetism in Metric Theories: Analysis of Earth Satellites Results, and its Coupling with Spin
Employing the PPN formalism the gravitomagnetic field in different metric
theories is considered in the analysis of the LAGEOS results. It will be shown
that there are several models that predict exactly the same effect that general
relativity comprises. In other words, these Earth satellites results can be
taken as experimental evidence that the orbital angular momentum of a body does
indeed generate space--time geometry, notwithstanding they do not endow general
relativity with an outstanding status among metric theories. Additionally the
coupling spin--gravitomagnetic field is analyzed with the introduction of the
Rabi transitions that this field produces on a quantum system with spin 1/2.
Afterwards, a continuous measurement of the energy of this system is
introduced, and the consequences upon the corresponding probabilities of the
involved gravitomagnetic field will be obtained. Finally, it will be proved
that these proposals allows us, not only to confront against future experiments
the usual assumption of the coupling spin--gravotimagnetism, but also to
measure some PPN parameters and to obtain functional dependences among them.Comment: 10 page
Detection strategies for scalar gravitational waves with interferometers and resonant spheres
We compute the response and the angular pattern function of an interferometer
for a scalar component of gravitational radiation in Brans-Dicke theory. We
examine the problem of detecting a stochastic background of scalar GWs and
compute the scalar overlap reduction function in the correlation between an
interferometer and the monopole mode of a resonant sphere. While the
correlation between two interferometers is maximized taking them as close as
possible, the interferometer-sphere correlation is maximized at a finite value
of f*d, where `f' is the resonance frequency of the sphere and `d' the distance
between the detectors. This defines an optimal resonance frequency of the
sphere as a function of the distance. For the correlation between the Virgo
interferometer located near Pisa and a sphere located in Frascati, near Rome,
we find an optimal resonance frequency f=590 Hz. We also briefly discuss the
difficulties in applying this analysis to the dilaton and moduli fields
predicted by string theory.Comment: 26 pages, Latex, 4 Postscript figures. Various minor improvements,
misprint in eqs. 42, 127, 138 corrected, references adde
Quantification and Evidence for Mechanically Metered Release of Pygidial Secretions in Formic Acid-Producing Carabid Beetles
This study is the first to measure the quantity of pygidial gland secretions released defensively by carabid beetles (Coleoptera: Carabidae) and to accurately measure the relative quantity of formic acid contained in their pygidial gland reservoirs and spray emissions. Individuals of three typical formic acid producing species were induced to repeatedly spray, ultimately exhausting their chemical compound reserves. Beetles were subjected to faux attacks using forceps and weighed before and after each ejection of chemicals. Platynus brunneomarginatus (Mannerheim) (Platynini), P. ovipennis (Mannerheim) (Platynini) and Calathus ruficollis Dejean (Sphodrini), sprayed average quantities with standard error of 0.313 ± 0.172 mg, 0.337 ± 0.230 mg, and 0.197 ± 0.117 mg per spray event, respectively. The quantity an individual beetle released when induced to spray tended to decrease with each subsequent spray event. The quantity emitted in a single spray was correlated to the quantity held in the reservoirs at the time of spraying for beetles whose reserves are greater than the average amount emitted in a spray event. For beetles with a quantity less than the average amount sprayed in reserve there was no significant correlation. For beetles comparable in terms of size, physiological condition and gland reservoir fullness, the shape of the gland reservoirs and musculature determined that a similar effort at each spray event would mechanically meter out the release so that a greater amount was emitted when more was available in the reservoir. The average percentage of formic acid was established for these species as 34.2%, 73.5% and 34.1% for for P. brunneomarginatus, P. ovipennis and C. ruficollis, respectively. The average quantities of formic acid released by individuals of these species was less than two-thirds the amount shown to be lethal to ants in previously published experiments. However, the total quantity from multiple spray events from a single individual could aggregate to quantities at or above the lethal level, and lesser quantities are known to act as ant alarm pheromones. Using a model, one directed spray of the formic acid and hydrocarbon mix could spread to an area of 5–8 cm diameter and persisted for 9–22 seconds at a threshold level known to induce alarm behaviors in ants. These results show that carabid defensive secretions may act as a potent and relatively prolonged defense against ants or similar predators even at a sub-lethal dose
The Distinguishability of Interacting Dark Energy from Modified Gravity
We study the observational viability of coupled quintessence models with
their expansion and growth histories matched to modified gravity cosmologies.
We find that for a Dvali-Gabadadze-Porrati model which has been fitted to
observations, the matched interacting dark energy models are observationally
disfavoured. We also study the distinguishability of interacting dark energy
models matched to scalar-tensor theory cosmologies and show that it is not
always possible to find a physical interacting dark energy model which shares
their expansion and growth histories.Comment: 8 pages, 5 figure
Dynamical evolution of boson stars in Brans-Dicke theory
We study the dynamics of a self-gravitating scalar field solitonic object
(boson star) in the Jordan-Brans-Dicke (BD) theory of gravity. We show
dynamical processes of this system such as (i) black hole formation of
perturbed equilibrium configuration on an unstable branch; (ii) migration of
perturbed equilibrium configuration from the unstable branch to stable branch;
(iii) transition from excited state to a ground state. We find that the
dynamical behavior of boson stars in BD theory is quite similar to that in
general relativity (GR), with comparable scalar wave emission. We also
demonstrate the formation of a stable boson star from a Gaussian scalar field
packet with flat gravitational scalar field initial data. This suggests that
boson stars can be formed in the BD theory in much the same way as in GR.Comment: 13 pages by RevTeX, epsf.sty, 16 figures, comments added, refs
updated, to appear in Phys. Rev.
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