266 research outputs found
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
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
The onset of star formation in primordial haloes
Star formation remains an unsolved problem in astrophysics. Numerical studies
of large-scale structure simulations cannot resolve the whole process and their
approach usually assumes that only gas denser than a typical threshold can host
and form stars. We investigate the onset of cosmological star formation and
compare several very-high-resolution, three-dimensional, N-body/SPH simulations
that include non-equilibrium, atomic and molecular chemistry, star formation
prescriptions, and feedback effects. We study how primordial star formation
depends on gas density thresholds, cosmological parameters and initial set-ups.
For mean-density initial conditions, we find that standard low-density
star-formation threshold (0.2 h^2/cm3) models predict the onset of star
formation at z~25-31, depending on the adopted cosmology. In these models stars
are formed regardless of the time between the moment when the threshold is
reached and the effective runaway collapse. At high redshift, this time
interval represents a significant fraction of the Hubble time and thus this
assumption can induce large artificial off-sets to the onset of star formation.
Choosing higher density thresholds (135 h^2/cm3) allows the entire cooling
process to be followed, and the onset of star formation is then estimated to be
at redshift z~12-16. When isolated, rare, high-density peaks are considered,
the chemical evolution is much faster and the first star formation episodes
occur at z > 40, almost regardless of the choice for the density threshold.
These results could have implications for the formation redshift of the first
cosmological objects, as inferred from direct numerical simulations of
mean-density environments, and on the studies of the reionization history of
the universe.Comment: 10 pages, 1 table, 5 figures; in press. Minor changes don
Star Forming Dense Cloud Cores in the TeV {\gamma}-ray SNR RX J1713.7-3946
RX J1713.7-3946 is one of the TeV {\gamma}-ray supernova remnants (SNRs)
emitting synchrotron X rays. The SNR is associated with molecular gas located
at ~1 kpc. We made new molecular observations toward the dense cloud cores,
peaks A, C and D, in the SNR in the 12CO(J=2-1) and 13CO(J=2-1) transitions at
angular resolution of 90". The most intense core in 13CO, peak C, was also
mapped in the 12CO(J=4-3) transition at angular resolution of 38". Peak C shows
strong signs of active star formation including bipolar outflow and a
far-infrared protostellar source and has a steep gradient with a
r^{-2.20.4} variation in the average density within radius r. Peak C and
the other dense cloud cores are rim-brightened in synchrotron X rays,
suggesting that the dense cloud cores are embedded within or on the outer
boundary of the SNR shell. This confirms the earlier suggestion that the X rays
are physically associated with the molecular gas (Fukui et al. 2003). We
present a scenario where the densest molecular core, peak C, survived against
the blast wave and is now embedded within the SNR. Numerical simulations of the
shock-cloud interaction indicate that a dense clump can indeed survive shock
erosion, since shock propagation speed is stalled in the dense clump.
Additionally, the shock-cloud interaction induces turbulence and magnetic field
amplification around the dense clump that may facilitate particle acceleration
in the lower-density inter-clump space leading to the enhanced synchrotron X
rays around dense cores.Comment: 22 pages, 7 figures, to accepted in The Astrophysical Journal. A full
color version with higher resolution figures is available at
http://www.a.phys.nagoya-u.ac.jp/~sano/ApJ10/ms_sano.pd
MHD models of Pulsar Wind Nebulae
Pulsar Wind Nebulae (PWNe) are bubbles or relativistic plasma that form when
the pulsar wind is confined by the SNR or the ISM. Recent observations have
shown a richness of emission features that has driven a renewed interest in the
theoretical modeling of these objects. In recent years a MHD paradigm has been
developed, capable of reproducing almost all of the observed properties of
PWNe, shedding new light on many old issues. Given that PWNe are perhaps the
nearest systems where processes related to relativistic dynamics can be
investigated with high accuracy, a reliable model of their behavior is
paramount for a correct understanding of high energy astrophysics in general. I
will review the present status of MHD models: what are the key ingredients,
their successes, and open questions that still need further investigation.Comment: 18 pages, 5 figures, Invited Review, Proceedings of the "ICREA
Workshop on The High-Energy Emission from Pulsars and their Systems", Sant
Cugat, Spain, April 12-16, 201
Astrophysical turbulence modeling
The role of turbulence in various astrophysical settings is reviewed. Among
the differences to laboratory and atmospheric turbulence we highlight the
ubiquitous presence of magnetic fields that are generally produced and
maintained by dynamo action. The extreme temperature and density contrasts and
stratifications are emphasized in connection with turbulence in the
interstellar medium and in stars with outer convection zones, respectively. In
many cases turbulence plays an essential role in facilitating enhanced
transport of mass, momentum, energy, and magnetic fields in terms of the
corresponding coarse-grained mean fields. Those transport properties are
usually strongly modified by anisotropies and often completely new effects
emerge in such a description that have no correspondence in terms of the
original (non coarse-grained) fields.Comment: 88 pages, 26 figures, published in Reports on Progress in Physic
Comparison of allocation strategies of convalescent plasma to reduce excess infections and mortality from SARS-CoV-2 in a US-like population
Background: While the use of convalescent plasma (CP) in the ongoing COVID-19 pandemic has been inconsistent, CP has the potential to reduce excess morbidity and mortality in future pandemics. Given constraints on CP supply, decisions surrounding the allocation of CP must be made. Study Design and Methods: Using a discrete-time stochastic compartmental model, we simulated implementation of four potential allocation strategies: administering CP to individuals in early hospitalization with COVID-19; administering CP to individuals in outpatient settings; administering CP to hospitalized individuals and administering any remaining CP to outpatient individuals and administering CP in both settings while prioritizing outpatient individuals. We examined the final size of SARS-CoV-2 infections, peak and cumulative hospitalizations, and cumulative deaths under each of the allocation scenarios over a 180-day period. We compared the cost per weighted health benefit under each strategy. Results: Prioritizing administration to patients in early hospitalization, with remaining plasma administered in outpatient settings, resulted in the highest reduction in mortality, averting on average 15% more COVID-19 deaths than administering to hospitalized individuals alone (95% CI [11%–18%]). Prioritizing administration to outpatients, with remaining plasma administered to hospitalized individuals, had the highest percentage of hospitalizations averted (22% [21%–23%] higher than administering to hospitalized individuals alone). Discussion: Convalescent plasma allocation strategy should be determined by the relative priority of averting deaths, infections, or hospitalizations. Under conditions considered, mixed allocation strategies (allocating CP to both outpatient and hospitalized individuals) resulted in a larger percentage of infections and deaths averted than administering CP in a single setting
Six Years of Chandra Observations of Supernova Remnants
We present a review of the first six years of Chandra X-ray Observatory
observations of supernova remnants. From the official "first-light" observation
of Cassiopeia A that revealed for the first time the compact remnant of the
explosion, to the recent million-second spectrally-resolved observation that
revealed new details of the stellar composition and dynamics of the original
explosion, Chandra observations have provided new insights into the supernova
phenomenon. We present an admittedly biased overview of six years of these
observations, highlighting new discoveries made possible by Chandra's unique
capabilities.Comment: 82 pages, 28 figures, for the book Astrophysics Update
Control of star formation by supersonic turbulence
Understanding the formation of stars in galaxies is central to much of modern
astrophysics. For several decades it has been thought that stellar birth is
primarily controlled by the interplay between gravity and magnetostatic
support, modulated by ambipolar diffusion. Recently, however, both
observational and numerical work has begun to suggest that support by
supersonic turbulence rather than magnetic fields controls star formation. In
this review we outline a new theory of star formation relying on the control by
turbulence. We demonstrate that although supersonic turbulence can provide
global support, it nevertheless produces density enhancements that allow local
collapse. Inefficient, isolated star formation is a hallmark of turbulent
support, while efficient, clustered star formation occurs in its absence. The
consequences of this theory are then explored for both local star formation and
galactic scale star formation. (ABSTRACT ABBREVIATED)Comment: Invited review for "Reviews of Modern Physics", 87 pages including 28
figures, in pres
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