862 research outputs found
Neutrinos from Fallback onto Newly Formed Neutron Stars
In the standard supernova picture, type Ib/c and type II supernovae are
powered by the potential energy released in the collapse of the core of a
massive star. In studying supernovae, we primarily focus on the ejecta that
makes it beyond the potential well of the collapsed core. But, as we shall show
in this paper, in most supernova explosions, a tenth of a solar mass or more of
the ejecta is decelerated enough that it does not escape the potential well of
that compact object. This material falls back onto the proto-neutron star
within the first 10-15 seconds after the launch of the explosion, releasing
more than 1e52erg of additional potential energy. Most of this energy is
emitted in the form of neutrinos and we must understand this fallback neutrino
emission if we are to use neutrino observations to study the behavior of matter
at high densities. Here we present both a 1-dimensional study of fallback using
energy-injected, supernova explosions and a first study of neutrino emission
from fallback using a suite of 2-dimensional simulations.Comment: 30 pages (including 10 figures), submitted to ApJ, comments welcom
Quantitative spectroscopic analysis of and distance to SN1999em
This work presents a detailed quantitative spectroscopic analysis of, and the
determination of the distance to, the type II supernovae (SN) SN1999em with
CMFGEN (Dessart & Hillier 2005a), based on spectrophotometric observations at
eight dates up to 40 days after discovery. We use the same iron-group metal
content for the ejecta, the same power-law density distribution (with exponent
n~10), and a Hubble-velocity law at all times. We adopt a H/He/C/N/O abundance
pattern compatible with CNO-cycle equilibrium values for a RSG/BSG progenitor,
with C/O enhanced and N depleted at later times. Based on our synthetic fits to
spectrophotometric observations of SN1999em, we obtain a distance of 11.5Mpc,
similar to that of Baron et al. (2004) and the Cepheid distance to the galaxy
host of 11.7Mpc (Leonard et al. 2003). Similarly, based on such models, the
Expanding Photosphere Method (EPM) delivers a distance of 11.6Mpc, with
negligible scatter between photometric bandpass sets; there is thus nothing
wrong with the EPM as such. Previous determinations using the tabulated
correction factors of Eastman et al. (1996) all led to 30-50% underestimates:
we find that this is caused by 1) an underestimate of the correction factors
compared to the only other study of the kind by Dessart & Hillier (2005b), 2) a
neglect of the intrinsic >20% scatter of correction factors, and 3) the use of
the EPM at late times when severe line blanketing makes the method inaccurate.
The need of detailed model computations for reliable EPM distance estimates
thus defeats the appeal and simplicity of the method. However, detailed fits to
SN optical spectra, based on tailored models for individual SN observations,
offers a promising approach to obtaining distances with 10-20% accuracy, either
through the EPM or a la Baron et al. (2004).Comment: 20 pages, 13 figures, accepted for publication in A&
Using Technology to Encourage Self-Directed Learning: The Collaborative Lecture Annotation System
The rapidly-developing 21st century world of work and knowledge calls for self-directed lifelong (SDL) learners. While higher education must embrace the types of pedagogies that foster SDL skills in graduates, the pace of change in education can be glacial. This paper describes a social annotation technology, the Collaborative Lecture Annotation System (CLAS), that can be used to leverage existing teaching and learning practices for acquisition of 21st Century SDL skills. CLAS was designed to build upon the artifacts of traditional didactic modes of teaching, create enriched opportunities for student engagement with peers and learning materials, and offer learners greater control and ownership of their individual learning strategies. Adoption of CLAS creates educational experiences that promote and foster SDL skills: motivation, self-management and self-monitoring. In addition, CLAS incorporates a suite of learning analytics for learners to evaluate their progress, and allow instructors to monitor the development of SDL skills and identify the need for learning support and guidance. CLAS stands as an example of a simple tool that can bridge the gap between traditional transmissive pedagogy and the creation of authentic and collaborative learning spaces
RAM: A Relativistic Adaptive Mesh Refinement Hydrodynamics Code
We have developed a new computer code, RAM, to solve the conservative
equations of special relativistic hydrodynamics (SRHD) using adaptive mesh
refinement (AMR) on parallel computers. We have implemented a
characteristic-wise, finite difference, weighted essentially non-oscillatory
(WENO) scheme using the full characteristic decomposition of the SRHD equations
to achieve fifth-order accuracy in space. For time integration we use the
method of lines with a third-order total variation diminishing (TVD)
Runge-Kutta scheme. We have also implemented fourth and fifth order Runge-Kutta
time integration schemes for comparison. The implementation of AMR and
parallelization is based on the FLASH code. RAM is modular and includes the
capability to easily swap hydrodynamics solvers, reconstruction methods and
physics modules. In addition to WENO we have implemented a finite volume module
with the piecewise parabolic method (PPM) for reconstruction and the modified
Marquina approximate Riemann solver to work with TVD Runge-Kutta time
integration. We examine the difficulty of accurately simulating shear flows in
numerical relativistic hydrodynamics codes. We show that under-resolved
simulations of simple test problems with transverse velocity components produce
incorrect results and demonstrate the ability of RAM to correctly solve these
problems. RAM has been tested in one, two and three dimensions and in
Cartesian, cylindrical and spherical coordinates. We have demonstrated
fifth-order accuracy for WENO in one and two dimensions and performed detailed
comparison with other schemes for which we show significantly lower convergence
rates. Extensive testing is presented demonstrating the ability of RAM to
address challenging open questions in relativistic astrophysics.Comment: ApJS in press, 21 pages including 18 figures (6 color figures
Tools for Dissecting Supernova Remnants Observed with Chandra: Methods and Application to the Galactic Remnant W49B
We introduce methods to quantify the X-ray morphologies of supernova remnants
observed with the Chandra X-ray Telescope. These include a power-ratio
technique to measure morphological asymmetries, correlation-length analysis to
probe chemical segregation and distribution, and wavelet-transform analysis to
quantify X-ray substructure. We demonstrate the utility and accuracy of these
techniques on relevant synthetic data. Additionally, we show the methods'
capabilities by applying them to the 55-ks Chandra ACIS observation of the
galactic supernova remnant W49B. We analyze the images of prominent emission
lines in W49B and use the results to discern physical properties. We find that
the iron morphology is very distinct from the other elements: it is
statistically more asymmetric, more segregated, and has 25% larger emitting
substructures than the lighter ions. Comparatively, the silicon, sulfur, argon,
and calcium are well-mixed, more isotropic, and have smaller, equally-sized
emitting substructures. Based on fits of XMM-Newton spectra in regions
identified as iron rich and iron poor, we determine that the iron in W49B must
have been anisotropically ejected. We measure the abundance ratios in many
regions, and we find that large, local variations are persistent throughout the
remnant. We compare the mean, global abundance ratios to those predicted by
spherical and bipolar core-collapse explosions; the results are consistent with
a bipolar origin from a 25 solar mass progenitor. We calculate the filling
factor of iron from the volume of its emitting substructures, enabling more
precise mass estimates than previous studies. Overall, this work is a first
step toward rigorously describing the physical properties of supernova remnants
for comparison within and between sources.Comment: 51 pages, 24 figures, accepted by ApJ. For full resolution figures,
see http://www.astro.ucsc.edu/~lopez/paper.html Fixed typo in URL; no other
change
Temporal properties of the short gamma-ray bursts
A temporal analysis has been performed on a sample of 100 bright gamma-ray
bursts (GRBs) with T90<2s from the BATSE current catalog. The GRBs were
denoised using a median filter and subjected to an automated pulse selection
algorithm as an objective way of idenitifing the effects of neighbouring
pulses. The rise times, fall times, FWHM, pulse amplitudes and areas were
measured and the frequency distributions are presented here. All are consistent
with lognormal distributions. The distribution of the time intervals between
pulses is not random but consistent with a lognormal distribution. The time
intervals between pulses and pulse amplitudes are highly correlated with each
other. These results are in excellent agreement with a similar analysis that
revealed lognormal distributions for pulse properties and correlated time
intervals between pulses in bright GRBs with T90>2s. The two sub-classes of
GRBs appear to have the same emission mechanism which is probably caused by
internal shocks. They may not have the same progenitors because of the generic
nature of the fireball model.Comment: 4 pages, 7 figure
Identification of biomarker volatile organic compounds released by three stored-grain insect pests in wheat
Monitoring and early detection of stored-grain insect infestation is essential to implement timely and effective pest management decisions to protect stored grains. We report a reliable analytical procedure based on headspace solid-phase microextraction coupled with gas chromatography–mass spectrometry (HS-SPME-GC-MS) to assess stored-grain infestation through the detection of volatile compounds emitted by insects. Four different fibre coatings were assessed; 85 µm CAR/PDMS had optimal efficiency in the extraction of analytes from wheat. The headspace profiles of volatile compounds produced by Tribolium castaneum (Herbst), Rhyzopertha dominica (Fabricius), and Sitophilus granarius (Linnaeus), either alone or with wheat, were compared with those of non-infested wheat grains. Qualitative analysis of chromatograms showed the presence of different volatile compound profiles in wheat with pest infestation compared with the wheat controls. Wheat-specific and insect-specific volatile compounds were identified, including the aggregation pheromones, dominicalure-1 and dominicalure-2, from R. dominica, and benzoquinones homologs from T. castaneum. For the first time, the presence of 3-hydroxy-2-butanone was reported from S. granarius, which might function as an alarm pheromone. These identified candidate biomarker compounds can be utilized in insect surveillance and monitoring in stored grain to safeguard our grain products in future
Global reconstruction of life-history strategies: a case study using tunas
1. Measuring the demographic parameters of exploited populations is central to predicting their vulnerability and extinction risk. However, current rates of population decline and species loss greatly outpace our ability to empirically monitor all populations that are potentially threatened.
2. The scale of this problem cannot be addressed through additional data collection alone, and therefore it is common practice to conduct population assessments based on surrogate data collected from similar species. However, this approach introduces biases and imprecisions that are difficult to quantify. Recent developments in hierarchical modelling have enabled missing values to be reconstructed based on the correlations between available life-history data, linking similar species based on phylogeny and environmental conditions.
3. However, these methods cannot resolve life-history variability among populations or species that are closely placed spatially or taxonomically. Here, theoretically motivated constraints that align with life-history theory offer a new avenue for addressing this problem. We describe a Bayesian hierarchical approach that combines fragmented, multi-species and multi-population data with established life-history theory, in order to objectively determine similarity between populations based on trait correlations (life-history trade-offs) obtained from model fitting.
4. We reconstruct 59 unobserved life-history parameters for 23 populations of tuna that sustain some of the world’s most valuable fisheries. Testing by cross-validation across different scenarios indicated that life-histories were accurately reconstructed when information was available for other populations of the same species. The reconstruction of several traits was also accurate for species represented by a single population, although credible intervals increased dramatically.
5. Synthesis and applications The described Bayesian hierarchical method provides access to life-history traits that are difficult to measure directly, and reconstructs missing life-history information useful for assessing populations and species that are directly or indirectly affected by human exploitation of natural resources. The method is particularly useful for examining populations that are spatially or taxonomically similar. The reconstructed life-history strategies described for the principal market tunas have immediate application to the world-wide management of tuna fisheries that use the steepness of the stock recruitment relationship to determine population productivity
The star-formation rate in the host of GRB 990712
We have observed the host galaxy of GRB 990712 at 1.4 GHz with the Australia
Telescope Compact Array, to obtain an estimate of its total star-formation
rate. We do not detect a source at the position of the host. The 2 sigma upper
limit of 70 microJy implies that the total star-formation rate is lower than
100 Msun/yr, using conservative values for the spectral index and cosmological
parameters. This upper limit is in stark contrast with recent reports of
radio/submillimeter-determined star-formation rates of roughly 500 Msun/yr for
two other GRB host galaxies. Our observations present the deepest
radio-determined star-formation rate limit on a GRB host galaxy yet, and show
that also from the unobscured radio point-of-view, not every GRB host galaxy is
a vigorous starburst.Comment: A&A Letters, in press, 5 pages; a high-resolution color gif version
of the paper figure is also supplie
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