415 research outputs found
Improving early warning of drought in Australia
This invited review outlines a selection of recent technical and communication advances, in certain areas of climate and weather science that could improve the capability and utility of drought early warning systems in Australia. First, a selection of current operational outputs and their significance for drought early warning is reviewed, then a selection of advancements in the Research and Development (R&D) pipeline are considered, which have potential to help enable better decision-making by stakeholders subject to drought risk. The next generation of drought early warning systems should have a focus on index-and impact-based prediction models that go beyond basic weather and climate parameters, at seasonal through to multi-year timescales. Convergence and integration of emerging research, science and technology is called for across the fields of cli-mate, agronomy, environment, economics and social science, to improve early warning information. The enablement of more predictively based drought policy, should facilitate more proactive re-sponses by stakeholders throughout the agricultural value chain, and should make stakeholders more drought resilient
Antimatter spectra from a time-dependent modeling of supernova remnants
We calculate the energy spectra of cosmic rays (CR) and their secondaries
produced in a supernova remnant (SNR), taking into account the time-dependence
of the SNR shock. We model the trajectories of charged particles as a random
walk with a prescribed diffusioncoefficient, accelerating the particles at each
shock crossing. Secondary production by CRs colliding with gas is included as a
Monte Carlo process. We find that SNRs produce less antimatter than suggested
previously: The positron/electron ratio and the antiproton/proton ratio are a
few percent and few , respectively. Both ratios do not rise
with energy.Comment: 4 pages, 4 eps figures; v2: results for time-dependent magnetic field
adde
Co-Occurrence Patterns of Common and Rare Leaf-Litter Frogs, Epiphytic Ferns and Dung Beetles across a Gradient of Human Disturbance
Indicator taxa are commonly used to identify priority areas for conservation or to measure biological responses to environmental change. Despite their widespread use, there is no general consensus about the ability of indicator taxa to predict wider trends in biodiversity. Many studies have focused on large-scale patterns of species co-occurrence to identify areas of high biodiversity, threat or endemism, but there is much less information about patterns of species co-occurrence at local scales. In this study, we assess fine-scale co-occurrence patterns of three indicator taxa (epiphytic ferns, leaf litter frogs and dung beetles) across a remotely sensed gradient of human disturbance in the Ecuadorian Amazon. We measure the relative contribution of rare and common species to patterns of total richness in each taxon and determine the ability of common and rare species to act as surrogate measures of human disturbance and each other. We find that the species richness of indicator taxa changed across the human disturbance gradient but that the response differed among taxa, and between rare and common species. Although we find several patterns of co-occurrence, these patterns differed between common and rare species. Despite showing complex patterns of species co-occurrence, our results suggest that species or taxa can act as reliable indicators of each other but that this relationship must be established and not assumed
Formation Process of the Circumstellar Disk: Long-term Simulations in the Main Accretion Phase of Star Formation
The formation and evolution of the circumstellar disk in unmagnetized
molecular clouds is investigated using three-dimensional hydrodynamic
simulations from the prestellar core until the end of the main accretion phase.
In collapsing clouds, the first (adiabatic) core with a size of ~10AU forms
prior to the formation of the protostar. At its formation, the first core has a
thick disk-like structure, and is mainly supported by the thermal pressure.
After the protostar formation, it decreases the thickness gradually, and
becomes supported by the centrifugal force. We found that the first core is a
precursor of the circumstellar disk. This indicates that the circumstellar disk
is formed before the protostar formation with a size of ~10AU, which means that
no protoplanetary disk smaller than <10AU exists. Reflecting the thermodynamics
of the collapsing gas, at the protostar formation epoch, the circumstellar disk
has a mass of ~0.01-0.1 solar mass, while the protostar has a mass of ~10^-3
solar mass. Thus, just after the protostar formation, the circumstellar disk is
about 10-100 times more massive than the protostar. Even in the main accretion
phase that lasts for ~10^5yr, the circumstellar disk mass dominates the
protostellar mass. Such a massive disk is unstable to gravitational
instability, and tends to show fragmentation. Our calculations indicate that
the planet or brown-dwarf mass object may form in the circumstellar disk in the
main accretion phase. In addition, the mass accretion rate onto the protostar
shows strong time variability that is caused by the perturbation of
proto-planets and/or the spiral arms in the circumstellar disk. Such
variability provides a useful signature for detecting the planet-sized
companion in the circumstellar disk around very young protostars.Comment: 32 pages, 11 figures, Submitted to ApJ. For high resolution figures
see
http://www2-tap.scphys.kyoto-u.ac.jp/~machidam/astro-ph/CircumstellarDisk.pd
Secondary Cosmic Ray Nuclei from Supernova Remnants and Constraints to the Propagation Parameters
The secondary-to-primary B/C ratio is widely used to study the cosmic ray
(CR) propagation processes in the Galaxy. It is usually assumed that secondary
nuclei such as Li-Be-B are entirely generated by collisions of heavier CR
nuclei with the interstellar medium (ISM). We study the CR propagation under a
scenario where secondary nuclei can also be produced or accelerated from
galactic sources. We consider the processes of hadronic interactions inside
supernova remnants (SNRs) and re-acceleration of background CRs in strong
shocks. Thus, we investigate their impact in the propagation parameter
determination within present and future data. The spectra of Li-Be-B nuclei
emitted from SNRs are harder than those due to CR collisions with the ISM. The
secondary-to-primary ratios flatten significantly at ~TeV/n energies, both from
spallation and re-acceleration in the sources. The two mechanisms are
complementary to each other and depend on the properties of the local ISM
around the expanding remnants. The secondary production in SNRs is significant
for dense background media, n ~1 cm^-3, while the amount of re-accelerated CRs
is relevant for SNRs expanding into rarefied media, n ~0.1 cm-3. Due to these
effects, the the diffusion parameter 'delta' may be misunderstood by a factor
of ~5-15%. Our estimations indicate that an experiment of the AMS-02 caliber
can constrain the key propagation parameters while breaking the
source-transport degeneracy, for a wide class of B/C-consistent models. Given
the precision of the data expected from on-going experiments, the SNR
production/acceleration of secondary nuclei should be considered, if any, to
prevent a possible mis-determination of the CR transport parameters.Comment: 13 pages, 9 figures; matches the published versio
The Outer Shock of the Oxygen-Rich Supernova Remnant G292.0+1.8: Evidence for the Interaction with the Stellar Winds from its Massive Progenitor
We study the outer-shock structure of the oxygen-rich supernova remnant
G292.0+1.8, using a deep observation with the Chandra X-ray Observatory. We
measure radial variations of the electron temperature and emission measure that
we identify as the outer shock propagating into a medium with a radially
decreasing density profile. The inferred ambient density structure is
consistent with models for the circumstellar wind of a massive progenitor star
rather than for a uniform interstellar medium. The estimated wind density n_H =
0.1 ~ 0.3 cm^-3) at the current outer radius (~7.7 pc) of the remnant is
consistent with a slow wind from a red supergiant (RSG) star. The total mass of
the wind is estimated to be ~ 15 - 40 solar mass (depending on the estimated
density range), assuming that the wind extended down to near the surface of the
progenitor. The overall kinematics of G292.0+1.8 are consistent with the
remnant expanding through the RSG wind.Comment: 9 pages (2-column), 5 figures, accepted for Ap
Probing the formation of intermediate- to high-mass stars in protoclusters II. Comparison between millimeter interferometric observations of NGC 2264-C and SPH simulations of a collapsing clump
The earliest phases of massive star formation in clusters are still poorly
understood. Here, we test the hypothesis for high-mass star formation proposed
in our earlier paper (Peretto et al. 2006). In order to confirm the physical
validity of this hypothesis, we carried out IRAM Plateau de Bure interferometer
observations of NGC 2264-C and performed SPH numerical simulations of the
collapse of a Jeans-unstable, prolate dense clump. Our Plateau de Bure
observations reveal the presence of a new compact source (C-MM13) located only
\~ 10000 AU away, but separated by ~ 1.1 km/s in (projected) velocity, from the
most massive Class 0 object (C-MM3) lying at the very center of NGC 2264-C.
Detailed comparison with our numerical SPH simulations supports the view that
NGC 2264-C is an elongated cluster-forming clump in the process of collapsing
and fragmenting along its long axis, leading to a strong dynamical interaction
and possible protostar merger in the central region of the clump. The present
study also sets several quantitative constraints on the initial conditions of
large-scale collapse in NGC 2264-C. Our hydrodynamic simulations indicate that
the observed velocity pattern characterizes an early phase of protocluster
collapse which survives for an only short period of time (i.e., < 10^5 yr). To
provide a good match to the observations the simulations require an initial
ratio of turbulent to gravitational energy of only ~ 5 %, which strongly
suggests that the NGC 2264-C clump is structured primarily by gravity rather
than turbulence. The required "cold'' initial conditions may result from rapid
compression by an external trigger.Comment: 15 pages, 8 figures, accepted for publication in A&
Recurrent Planet Formation and Intermittent Protostellar Outflows Induced by Episodic Mass Accretion
The formation and evolution of a circumstellar disk in magnetized cloud cores
is investigated from prestellar core stage until sim 10^4 yr after protostar
formation. In the circumstellar disk, fragmentation first occurs due to
gravitational instability in a magnetically inactive region, and
substellar-mass objects appear. The substellar-mass objects lose their orbital
angular momenta by gravitational interaction with the massive circumstellar
disk and finally fall onto the protostar. After this fall, the circumstellar
disk increases its mass by mass accretion and again induces fragmentation. The
formation and falling of substellar-mass objects are repeated in the
circumstellar disk until the end of the main accretion phase. In this process,
the mass of fragments remain small, because the circumstellar disk loses its
mass by fragmentation and subsequent falling of fragments before it becomes
very massive. In addition, when fragments orbit near the protostar, they
disturb the inner disk region and promote mass accretion onto the protostar.
The orbital motion of substellar-mass objects clearly synchronizes with the
time variation of the accretion luminosity of the protostar. Moreover, as the
objects fall, the protostar shows a strong brightening for a short duration.
The intermittent protostellar outflows are also driven by the circumstellar
disk whose magnetic field lines are highly tangled owing to the orbital motion
of fragments. The time-variable protostellar luminosity and intermittent
outflows may be a clue for detecting planetary-mass objects in the
circumstellar disk.Comment: 48 pages, 16 figures, accepted for publication in Ap
Evaluating tools for the spatial management of fisheries
1. The ability to define the spatial dynamics of fish stocks is critical to fisheries man- agement. Combating illegal, unreported and unregulated fishing and the regula- tion of area-based management through physical patrols and port side controls are growing areas of management attention. Augmenting the existing approaches to fisheries management with forensic techniques has the potential to increase compliance and enforcement success rates. 2. We tested the accuracy of three techniques (genotyping, otolith microchemistry and morphometrics) that can be used to identify geographic origin. We used fish caught from three fishing grounds, separated by a minimum of 5 km and a maxi- mum of 60 km, to test the accuracy of these approaches at relatively small spatial scales. 3. Using nearest-neighbour analyses, morphometric analysis was the most accurate (79.5%) in assigning individual fish to their fishing ground of origin. Neither otolith microchemistry (54.0%) or genetic analyses (52.4%) had sufficient accuracy at the spatial scales we examined. 4. Synthesis and applications. The combination of accuracy and minimal resource re- quirements make morphometric analysis a promising tool for assessing compli- ance with area-based fishing restrictions at the scale of kilometres. Furthermore, this approach has promising application, in small-scale fisheries through to com- munity-based management approaches where technical and financial resources are limited
Turbulent Mixing in the Interstellar Medium -- an application for Lagrangian Tracer Particles
We use 3-dimensional numerical simulations of self-gravitating compressible
turbulent gas in combination with Lagrangian tracer particles to investigate
the mixing process of molecular hydrogen (H2) in interstellar clouds. Tracer
particles are used to represent shock-compressed dense gas, which is associated
with H2. We deposit tracer particles in regions of density contrast in excess
of ten times the mean density. Following their trajectories and using
probability distribution functions, we find an upper limit for the mixing
timescale of H2, which is of order 0.3 Myr. This is significantly smaller than
the lifetime of molecular clouds, which demonstrates the importance of the
turbulent mixing of H2 as a preliminary stage to star formation.Comment: 10 pages, 5 figures, conference proceedings "Turbulent Mixing and
Beyond 2007
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