2,668 research outputs found
Near-pristine gas at high redshifts: a window on early nucleosynthesis
It has now become recognised that damped Lyman alpha systems play an
important role in helping us unravel the origin of chemical elements. In this
presentation, we describe the main results of a recently completed survey of
the most metal-poor DLAs, aimed at complementing and extending studies of the
oldest stars in the Galaxy. The survey has clarified a number of lingering
issues concerning the abundances of C, N, O in the low metallicity regime, has
revealed the existence of DLA analogues to Carbon-enhanced metal-poor stars,
and is providing some of the most precise measures of the primordial abundance
of Deuterium.Comment: 11 pages, 7 Figures. Invited presentation at the XII International
Symposium on Nuclei in the Cosmos, Cairns, Australia, 5-10 August 2012. To
appear in Proceedings of Scienc
Intrinsic and emergent anomalies at deconfined critical points
It is well known that theorems of Lieb-Schultz-Mattis type prohibit the
existence of a trivial symmetric gapped ground state in certain systems
possessing a combination of internal and lattice symmetries. In the continuum
description of such systems the Lieb-Schultz-Mattis theorem is manifested in
the form of a quantum anomaly afflicting the symmetry. We demonstrate this
phenomenon in the context of the deconfined critical point between a Neel state
and a valence bond solid in an square lattice antiferromagnet, and
compare it to the case of honeycomb lattice where no anomaly is
present. We also point out that new anomalies, unrelated to the microscopic
Lieb-Schultz-Mattis theorem, can emerge prohibiting the existence of a trivial
gapped state in the immediate vicinity of critical points or phases. For
instance, no translationally invariant weak perturbation of the
gapless spin chain can open up a trivial gap even if the spin-rotation symmetry
is explicitly broken. The same result holds for the deconfined
critical point on a square lattice.Comment: 25 pages + Appendice
Incremental Learning of Nonparametric Bayesian Mixture Models
Clustering is a fundamental task in many vision applications.
To date, most clustering algorithms work in a
batch setting and training examples must be gathered in a
large group before learning can begin. Here we explore
incremental clustering, in which data can arrive continuously.
We present a novel incremental model-based clustering
algorithm based on nonparametric Bayesian methods,
which we call Memory Bounded Variational Dirichlet
Process (MB-VDP). The number of clusters are determined
flexibly by the data and the approach can be used to automatically
discover object categories. The computational requirements
required to produce model updates are bounded
and do not grow with the amount of data processed. The
technique is well suited to very large datasets, and we show
that our approach outperforms existing online alternatives
for learning nonparametric Bayesian mixture models
Implications of Spatially Variable Costs and Habitat Conversion Risk in Landscape-Scale Conservation Planning
āāStrategic habitat conservationāā refers to a process used by the U.S. Fish and Wildlife Service to develop cost-efficient strategies for conserving wildlife populations and their habitats. Strategic habitat conservation focuses on resolving uncertainties surrounding habitat conservation to meet specific wildlife population objectives (i.e., targets) and developing tools to guide where conservation actions should be focused on the landscape. Although there are examples of using optimization models to highlight where conservation should be delivered, such methods often do not explicitly account for spatial variation in the costs of conservation actions. Furthermore, many planning approaches assume that habitat protection is a preferred option, but they do not assess its value relative to other actions, such as restoration. We developed a case study to assess the implications of accounting for and ignoring spatial variation in conservation costs in optimizing conservation targets. We included assumptions about habitat loss to determine the extent to which protection or restoration would be necessary to meet an established population target. Our case study focused on optimal placement of grassland protection or restoration actions to influence bobolink Dolichonyx oryzivorus populations in the tallgrass prairie ecoregion of the north central United States. Our results show that not accounting for spatially variable costs doubled or tripled the cost of meeting the population target. Furthermore, our results suggest that one should not assume that protecting existing habitat is always a preferred option. Rather, our results show that the balance between protection and restoration can be influenced by a combination of desired targets, assumptions about habitat loss, and the relative cost of the two actions. Our analysis also points out how difficult it may be to reach targets, given the expense to meet them. We suggest that a full accounting of expected costs and benefits will help to guide development of viable management actions and meaningful conservation plans
Wind Power Study in Colorado
Renewable energy may seem like a simple source of energy, but there is a variety of detailed research required for sustainable development. We aim to find the most suitable location in Colorado for a wind farm. Using ArcGIS we preformed many experience to find the best location, resubmitting in Limon, Colorado. This research helps display the importance of renewable energy standards and development
Molecular Shattering
Recent observations suggest galaxies may ubiquitously host a molecular
component to their multiphase circumgalactic medium (CGM). However, the
structure and kinematics of the molecular CGM remains understudied
theoretically and largely unconstrained observationally. Recent work suggests
molecular gas clouds with efficient cooling survive acceleration in hot winds
similar to atomic clouds. Yet the pressure-driven fragmentation of molecular
clouds when subjected to external shocks or undergoing cooling remains
unstudied. We perform radiative, inviscid hydrodynamics simulations of clouds
perturbed out of pressure equilibrium to explore the process of hydrodynamic
fragmentation to molecular temperatures. We find molecular clouds larger than a
critical size can shatter into a mist of tiny droplets, with the critical size
deviating significantly from the atomic case. We find that cold clouds shatter
only if the sound crossing time exceeds the local maximum of the cooling time
~8000 K. Moreover, we find evidence for a universal mechanism to 'shatter' cold
clouds into a 'mist' of tiny droplets as a result of rotational fragmentation
-- a process we dub 'splintering.' Our results have implications for resolving
the molecular phase of the CGM in observations and cosmological simulations.Comment: 5 pages, 4 figures, submitted to MNRAS
- ā¦