21,520 research outputs found
The Environments around Long-Duration Gamma-Ray Burst Progenitors
Gamma-ray burst (GRB) afterglow observations have allowed us to significantly
constrain the engines producing these energetic explosions. Te redshift and
position information provided by these afterglows have already allowed us to
limit the progenitors of GRBs to only a few models. The afterglows may also
provide another observation that can place further constraints on the GRB
progenitor: measurements telling us about the environments surrounding GRBs.
Current analyses of GRB afterglows suggest that roughly half of long-duration
gamma-ray bursts occur in surroundings with density profiles that are uniform.
We study the constraints placed by this observation on both the classic
``collapsar'' massive star progenitor and its relative, the ``helium-merger''
progenitor. We study several aspects of wind mass-loss and find that our
modifications to the standard Wolf-Rayet mass-loss paradigm are not sufficient
to produce constant density profiles. Although this does not rule out the
standard ``collapsar'' progenitor, it does suggest a deficiency with this
model. We then focus on the He-merger models and find that such progenitors can
fit this particular constraint well. We show how detailed observations can not
only determine the correct progenitor for GRBs, but also allow us to study
binary evolution physics.Comment: 44 pages including 11 figure
Long Term Comparison of Alternative Range Livestock Management Strategies Across Extended Droughts and Cyclical Prices
Variable precipitation, more importantly drought, impacts profitability for livestock management. Financial outcomes from management decisions related to forage shortages can be exacerbated by price variability. This research examines alternative management strategies to determine the potential profitability and riskiness over a long-term horizon and across various drought event scenarios. Results indicate that late calving can be a promising strategy, but it also can result in higher variability in profits as compared to some of the other strategies analyzed. Retaining ownership of steer calves over the winter, with the option to sell if forage supplies become scarce, outperforms both partial liquidation and summer feeding, and it results in less profit variability than late calving or early weaning.Production Economics, Productivity Analysis,
Mean shear flows generated by nonlinear resonant Alfven waves
In the context of resonant absorption, nonlinearity has two different
manifestations. The first is the reduction in amplitude of perturbations around
the resonant point (wave energy absorption). The second is the generation of
mean shear flows outside the dissipative layer surrounding the resonant point.
Ruderman et al. [Phys. Plasmas 4, 75 (1997)] studied both these effects at the
slow resonance in isotropic plasmas. Clack et al. [Astron. Astrophys. 494}, 317
(2009)] investigated nonlinearity at the Alfven resonance, however, they did
not include the generation of mean shear flow. In this present paper, we
investigate the mean shear flow, analytically, and study its properties. We
find that the flow generated is parallel to the magnetic surfaces and has a
characteristic velocity proportional to , where is
the dimensionless amplitude of perturbations far away from the resonance. This
is, qualitatively, similar to the flow generated at the slow resonance. The
jumps in the derivatives of the parallel and perpendicular components of mean
shear flow across the dissipative layer are derived. We estimate the generated
mean shear flow to be of the order of in both the solar
upper chromosphere and solar corona, however, this value strongly depends on
the choice of boundary conditions. It is proposed that the generated mean shear
flow can produce a Kelvin--Helmholtz instability at the dissipative layer which
can create turbulent motions. This instability would be an additional effect,
as a Kelvin--Helmholtz instability may already exist due to the velocity field
of the resonant Alfven waves. This flow can also be superimposed onto existing
large scale motions in the solar upper atmosphere.Comment: 11 page
Sub-Alfvenic Non-Ideal MHD Turbulence Simulations with Ambipolar Diffusion: I. Turbulence Statistics
Most numerical investigations on the role of magnetic fields in turbulent
molecular clouds (MCs) are based on ideal magneto-hydrodynamics (MHD). However,
MCs are weakly ionized, so that the time scale required for the magnetic field
to diffuse through the neutral component of the plasma by ambipolar diffusion
(AD) can be comparable to the dynamical time scale. We have performed a series
of 256^3 and 512^3 simulations on supersonic but sub-Alfvenic turbulent systems
with AD using the Heavy-Ion Approximation developed in Li, McKee, & Klein
(2006). Our calculations are based on the assumption that the number of ions is
conserved, but we show that these results approximately apply to the case of
time-dependent ionization in molecular clouds as well. Convergence studies
allow us to determine the optimal value of the ionization mass fraction when
using the heavy-ion approximation for low Mach number, sub-Alfvenic turbulent
systems. We find that ambipolar diffusion steepens the velocity and magnetic
power spectra compared to the ideal MHD case. Changes in the density PDF, total
magnetic energy, and ionization fraction are determined as a function of the AD
Reynolds number. The power spectra for the neutral gas properties of a strongly
magnetized medium with a low AD Reynolds number are similar to those for a
weakly magnetized medium; in particular, the power spectrum of the neutral
velocity is close to that for Burgers turbulence.Comment: 37 pages, 11 figures, 4 table
The Radio Evolution of SN 2001gd
We present the results of observations of the radio emission from Supernova
2001gd in NGC 5033 from 2002 February 8 through 2006 September 25. The data
were obtained using the Very Large Array at wavelengths of 1.3 cm (22.4 GHz), 2
cm (14.9 GHz), 3.6 cm (8.4 GHz), 6 cm (4.9 GHz), and 20 cm (1.5 GHz), with one
upper limit at 90 cm (0.3 GHz). In addition, one detection has been provided by
the Giant Metrewave Radio Telescope at 21 cm (1.4 GHz). SN 2001gd was
discovered in the optical well past maximum light, so that it was not possible
to obtain many of the early radio "turn-on" measurements which are important
for estimating the local circumstellar medium (CSM) properties. Only at 20 cm
were turn-on data available. However, our analysis and fitting of the radio
light curves, and the assumption that the Type IIb SN 2001gd resembles the much
better studied Type IIb SN 1993J, enables us to describe the radio evolution as
being very regular through day ~550 and consistent with a nonthermal-emitting
model with a thermal absorbing CSM. The presence of synchrotron-self absorption
(SSA) at early times is implied by the data, but determination of the exact
relationship between the SSA component from the emitting region and the
free-free absorption component from the CSM is not possible as there are
insufficient early measurements to distinguish between models. After day ~550,
the radio emission exhibits a dramatically steeper decline rate which, assuming
similarity to SN 1993J, can be described as an exponential decrease with an
e-folding time of 500 days. We interpret this abrupt change in the radio flux
density decline rate as implying a transition of the shock front into a more
tenuous region of circumstellar material. A similar change in radio evolution
has been seen earlier in other SNe such as SN 1988Z, SN 1980K, and SN 1993J.Comment: 3 tables, 2 figures, To appear in the Astrophysical Journa
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Improved V I Log(gf) Values and Abundance Determinations in the Photospheres of the Sun and Metal-Poor Star HD 84937
New emission branching fraction measurements for 836 lines of the first spectrum of vanadium (V I) are determined from hollow cathode lamp spectra recorded with the National Solar Observatory 1 m Fourier transform spectrometer (FTS) and a high-resolution echelle spectrometer. The branching fractions are combined with recently published radiative lifetimes from laser-induced fluorescence measurements to determine accurate absolute atomic transition probabilities for the 836 lines. The FTS data are also used to extract new hyperfine structure A coefficients for 26 levels of neutral vanadium. These new laboratory data are applied to determine the V abundance in the Sun and metal-poor star HD 84937, yielding log epsilon(V) = 3.956 +/- 0.004 (sigma = 0.037) based on 93 V I lines and log epsilon(V) = 1.89 +/- 0.03 (sigma = 0.07) based on nine Vi lines, respectively, using the Holweger-Muller 1D model. These new V I abundance values for the Sun and HD 84937 agree well with our earlier determinations based upon V II.NASA NNX10AN93GNSF AST-1211055, AST-1211585Astronom
Radio sources in the Chandra Galactic Bulge Survey
We discuss radio sources in the Chandra Galactic Bulge Survey region. By cross-matching the X-ray sources in this field with the NRAO VLA Sky Survey archival data, we find 12 candidate matches. We present a classification scheme for radio/X-ray matches in surveys taken in or near the Galactic plane, taking into account other multiwavelength data. We show that none of the matches found here is likely to be due to coronal activity from normal stars because the radio to X-ray flux ratios are systematically too high. We show that one of the source could be a radio pulsar, and that one could be a planetary nebula, but that the bulk of the sources are likely to be background active galactic nuclei (AGN), with many confirmed through a variety of approaches. Several of the AGN are bright enough in the near-infrared (and presumably in the optical) to use as probes of the interstellar medium in the inner Galaxy
Dynamical mechanism of atrial fibrillation: a topological approach
While spiral wave breakup has been implicated in the emergence of atrial
fibrillation, its role in maintaining this complex type of cardiac arrhythmia
is less clear. We used the Karma model of cardiac excitation to investigate the
dynamical mechanisms that sustain atrial fibrillation once it has been
established. The results of our numerical study show that spatiotemporally
chaotic dynamics in this regime can be described as a dynamical equilibrium
between topologically distinct types of transitions that increase or decrease
the number of wavelets, in general agreement with the multiple wavelets
hypothesis. Surprisingly, we found that the process of continuous excitation
waves breaking up into discontinuous pieces plays no role whatsoever in
maintaining spatiotemporal complexity. Instead this complexity is maintained as
a dynamical balance between wave coalescence -- a unique, previously
unidentified, topological process that increases the number of wavelets -- and
wave collapse -- a different topological process that decreases their number.Comment: 15 pages, 14 figure
A Radiation Scalar for Numerical Relativity
This letter describes a scalar curvature invariant for general relativity
with a certain, distinctive feature. While many such invariants exist, this one
vanishes in regions of space-time which can be said unambiguously to contain no
gravitational radiation. In more general regions which incontrovertibly support
non-trivial radiation fields, it can be used to extract local,
coordinate-independent information partially characterizing that radiation.
While a clear, physical interpretation is possible only in such radiation
zones, a simple algorithm can be given to extend the definition smoothly to
generic regions of space-time.Comment: 4 pages, 1 EPS figur
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