1,426 research outputs found
Toward Five-dimensional Core-collapse Supernova Simulations
The computational difficulty of six-dimensional neutrino radiation
hydrodynamics has spawned a variety of approximations, provoking a long history
of uncertainty in the core-collapse supernova explosion mechanism. Under the
auspices of the Terascale Supernova Initiative, we are honoring the physical
complexity of supernovae by meeting the computational challenge head-on,
undertaking the development of a new adaptive mesh refinement code for
self-gravitating, six-dimensional neutrino radiation magnetohydrodynamics. This
code--called {\em GenASiS,} for {\em Gen}eral {\em A}strophysical {\em
Si}mulation {\em S}ystem--is designed for modularity and extensibility of the
physics. Presently in use or under development are capabilities for Newtonian
self-gravity, Newtonian and special relativistic magnetohydrodynamics (with
`realistic' equation of state), and special relativistic energy- and
angle-dependent neutrino transport--including full treatment of the energy and
angle dependence of scattering and pair interactions.Comment: 5 pages. Proceedings of SciDAC 2005, Scientific Discovery through
Advanced Computing, San Francisco, CA, 26-30 June 200
Analysis of the Type IIn Supernova 1998S: Effects of Circumstellar Interaction on Observed Spectra
We present spectral analysis of early observations of the Type IIn supernova
1998S using the general non-local thermodynamic equilibrium atmosphere code \tt
PHOENIX}. We model both the underlying supernova spectrum and the overlying
circumstellar interaction region and produce spectra in good agreement with
observations. The early spectra are well fit by lines produced primarily in the
circumstellar region itself, and later spectra are due primarily to the
supernova ejecta. Intermediate spectra are affected by both regions. A
mass-loss rate of order \msol yr is inferred
for a wind speed of 100-1000 \kmps. We discuss how future self-consistent
models will better clarify the underlying progenitor structure.Comment: to appear in ApJ, 2001, 54
Observations and a model of net calcification declines in Palau's largest coral reef lagoon between 1992 and 2015
Author Posting. © American Geophysical Union, 2020. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 125(8), (2020):
e2020JC016147, doi:10.1029/2020JC016147.Net ecosystem calcification (NEC) rates of Palau's largest lagoon and barrier reef system between 1992 and 2015 are estimated from sparse total alkalinity (TA) and salinity measurements and a tidal exchange model in which surface lagoon water transported offshore on the ebb tide is replaced by saltier (denser) ocean water that sinks to the bottom after entering the lagoon on the flood tide. Observed lagoon salinities are accurately reproduced by the model with no adjustable parameters. To accurately reproduce observed lagoon TA, NEC for the lagoonâbarrier reef system was 70 mmols mâ2 dayâ1 from 1992 to 1998, 35 mmols mâ2 dayâ1 from 1999 to 2012, and 25 mmols mâ2 dayâ1 from 2013 to 2015. This indicates that Palau's largest lagoon and barrier reef system has not recovered, as of 2015, from the 50% decline in NEC in 1998 caused by the loss of coral cover following a severe bleaching event. The cause of the further decline in NEC in 2012â2013 is unclear. Lagoon residence times vary from 8 days during spring tides to 14 days during neap tides and drive substantial springâneap variations in lagoon TA (~25% of the mean salinityânormalized oceanâlagoon TA difference). Sparse measurements that do not resolve these springâneap variations can exhibit apparent longâterm variations in alkalinity that are not due to changes in NEC.This work was partially supported by NSF award 1220529 to A.L.C., S.J.L., and K.E.F.S and NSF award 1737311 to A.L.C. and the Oceanography Department, Texas A&M University K.E.F.S.2021-01-0
Design and calibration of percolation samplers for measuring polyacrylamide-amended furrow-irrigation effects on drainage water quality
Amending irrigation furrow inflows with polyacrylamide (PAM) at low
concentrations (10 mg L -1 ) reduces irrigation-induced erosion by 94% and increases
infiltration by 15%, relative to untreated furrows. We hypothesized that PAM erosion-control
technology would allow irrigation managers to increase furrow inflows to speed
furrow-stream advance, produce a more uniform water distribution down field, and
reduce the leaching hazard at the upper end (due to reduced infiltration opportunity time
and/or shorter sets). We developed, tested, and installed instruments in a furrow
irrigated Portneuf silt loam (Co-Si, mixed, mesic, Durixerollic Calciorthids with 1.6%
slope) to investigate this premise. Soils were instrumented with repeating pulse
multivibrator (CS-615) soil water sensors, thermocouples, tensiometers, and percolation
soil water samplers at upper and lower ends of the furrows. Percolation samplers
consisted of a 23-cm-deep, 20-cm-dia. stainless-steel beaker with a 1 7-cm-long, 4-cm-dia.,
0.5 bar air-entry ceramic cup imbedded in a 5-cm-deep silica flour layer, slurried into
the beaker bottom. Water was collected under suction (~ 1.4x ambient) via teflon tubes.
Percolation sampler design and testing, field installation, and study experimental design
are discussed
Quantitative analysis of cell types during growth and morphogenesis in Hydra
Tissue maceration was used to determine the absolute number and the distribution of cell types in Hydra. It was shown that the total number of cells per animal as well as the distribution of cells vary depending on temperature, feeding conditions, and state of growth. During head and foot regeneration and during budding the first detectable change in the cell distribution is an increase in the number of nerve cells at the site of morphogenesis. These results and the finding that nerve cells are most concentrated in the head region, diminishing in density down the body column, are discussed in relation to tissue polarity
Gravitational Waves from Core Collapse Supernovae
We present the gravitational wave signatures for a suite of axisymmetric core
collapse supernova models with progenitors masses between 12 and 25 solar
masses. These models are distinguished by the fact they explode and contain
essential physics (in particular, multi-frequency neutrino transport and
general relativity) needed for a more realistic description. Thus, we are able
to compute complete waveforms (i.e., through explosion) based on
non-parameterized, first-principles models. This is essential if the waveform
amplitudes and time scales are to be computed more precisely. Fourier
decomposition shows that the gravitational wave signals we predict should be
observable by AdvLIGO across the range of progenitors considered here. The
fundamental limitation of these models is in their imposition of axisymmetry.
Further progress will require counterpart three-dimensional models.Comment: 10 pages, 5 figure
Evidence for Asphericity in the Type IIn Supernova 1998S
We present optical spectropolarimetry obtained at the Keck-II 10-m telescope
on 1998 March 7 UT along with total flux spectra spanning the first 494 days
after discovery (1998 March 2 UT) of the peculiar type IIn supernova (SN)
1998S. The SN is found to exhibit a high degree of linear polarization,
implying significant asphericity for its continuum-scattering environment.
Prior to removal of the interstellar polarization, the polarization spectrum is
characterized by a flat continuum (at p ~ 2%) with distinct changes in
polarization associated with both the broad (FWZI >= 20,000 km/s) and narrow
(unresolved, FWHM < 300 km/s) line emission seen in the total flux spectrum.
When analyzed in terms of a polarized continuum with unpolarized broad-line
recombination emission, an intrinsic continuum polarization of p ~ 3% results
(the highest yet found for a SN), suggesting a global asphericity of >= 45%
from the oblate, electron-scattering dominated models of Hoflich (1991). The
smooth, blue continuum evident at early times is shown to be inconsistent with
a reddened, single-temperature blackbody, instead having a color temperature
that increases with decreasing wavelength. Broad emission-line profiles with
distinct blue and red peaks are seen in the total flux spectra at later times,
perhaps suggesting a disk-like or ring-like morphology for the dense (n_e ~
10^7 cm^{-3}) circumstellar medium. Implications of the circumstellar
scattering environment for the spectropolarimetry are discussed, as are the
effects of uncertain removal of interstellar polarization.Comment: 25 pages + 2 tables + 14 figures, Submitted to The Astrophysical
Journa
Probing two-electron multiplets in bilayer graphene quantum dots
Understanding how the electron spin is coupled to orbital degrees of freedom,
such as a valley degree of freedom in solid-state systems is central to
applications in spin-based electronics and quantum computation. Recent
developments in the preparation of electrostatically-confined quantum dots in
gapped bilayer graphene (BLG) enables to study the low-energy single-electron
spectra in BLG quantum dots, which is crucial for potential spin and
spin-valley qubit operations. Here, we present the observation of the
spin-valley coupling in a bilayer graphene quantum dot in the single-electron
regime. By making use of a highly-tunable double quantum dot device we achieve
an energy resolution allowing us to resolve the lifting of the fourfold spin
and valley degeneracy by a Kane-Mele type spin-orbit coupling of eV. Also, we find an upper limit of a potentially disorder-induced
mixing of the and states below eV.Comment: 5 Pages 5 Figure
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