11,524 research outputs found
Fully covariant and conformal formulation of the Z4 system in a reference-metric approach: comparison with the BSSN formulation in spherical symmetry
We adopt a reference-metric approach to generalize a covariant and conformal
version of the Z4 system of the Einstein equations. We refer to the resulting
system as ``fully covariant and conformal", or fCCZ4 for short, since it is
well suited for curvilinear as well as Cartesian coordinates. We implement this
fCCZ4 formalism in spherical polar coordinates under the assumption of
spherical symmetry using a partially-implicit Runge-Kutta (PIRK) method and
show that our code can evolve both vacuum and non-vacuum spacetimes without
encountering instabilities. Our method does not require regularization of the
equations to handle coordinate singularities, nor does it depend on
constraint-preserving outer boundary conditions, nor does it need any
modifications of the equations for evolutions of black holes. We perform
several tests and compare the performance of the fCCZ4 system, for different
choices of certain free parameters, with that of BSSN. Confirming earlier
results we find that, for an optimal choice of these parameters, and for
neutron-star spacetimes, the violations of the Hamiltonian constraint can be
between 1 and 3 orders of magnitude smaller in the fCCZ4 system than in the
BSSN formulation. For black-hole spacetimes, on the other hand, any advantages
of fCCZ4 over BSSN are less evident.Comment: 13 pages, 10 figure
Collapse of Nonlinear Gravitational Waves in Moving-Puncture Coordinates
We study numerical evolutions of nonlinear gravitational waves in
moving-puncture coordinates. We adopt two different types of initial data --
Brill and Teukolsky waves -- and evolve them with two independent codes
producing consistent results. We find that Brill data fail to produce long-term
evolutions for common choices of coordinates and parameters, unless the initial
amplitude is small, while Teukolsky wave initial data lead to stable
evolutions, at least for amplitudes sufficiently far from criticality. The
critical amplitude separates initial data whose evolutions leave behind flat
space from those that lead to a black hole. For the latter we follow the
interaction of the wave, the formation of a horizon, and the settling down into
a time-independent trumpet geometry. We explore the differences between Brill
and Teukolsky data and show that for less common choices of the parameters --
in particular negative amplitudes -- Brill data can be evolved with
moving-puncture coordinates, and behave similarly to Teukolsky waves
Population Cancer Risks Associated with Coal Mining: A Systematic Review
BACKGROUND: Coal is produced across 25 states and provides 42% of US energy. With production expected to increase 7.6% by 2035, proximate populations remain at risk of exposure to carcinogenic coal products such as silica dust and organic compounds. It is unclear if population exposure is associated with increased risk, or even which cancers have been studied in this regard.
METHODS: We performed a systematic review of English-language manuscripts published since 1980 to determine if coal mining exposure was associated with increased cancer risk (incidence and mortality).
RESULTS: Of 34 studies identified, 27 studied coal mining as an occupational exposure (coal miner cohort or as a retrospective risk factor) but only seven explored health effects in surrounding populations. Overall, risk assessments were reported for 20 cancer site categories, but their results and frequency varied considerably. Incidence and mortality risk assessments were: negative (no increase) for 12 sites; positive for 1 site; and discordant for 7 sites (e.g. lung, gastric). However, 10 sites had only a single study reporting incidence risk (4 sites had none), and 11 sites had only a single study reporting mortality risk (2 sites had none). The ecological study data were particularly meager, reporting assessments for only 9 sites. While mortality assessments were reported for each, 6 had only a single report and only 2 sites had reported incidence assessments.
CONCLUSIONS: The reported assessments are too meager, and at times contradictory, to make definitive conclusions about population cancer risk due to coal mining. However, the preponderance of this and other data support many of Hill\u27s criteria for causation. The paucity of data regarding population exposure and risk, the widespread geographical extent of coal mining activity, and the continuing importance of coal for US energy, warrant further studies of population exposure and risk
ExploreNEOs VIII: Dormant Short-Period Comets in the Near-Earth Asteroid Population
We perform a search for dormant comets, asteroidal objects of cometary
origin, in the near-Earth asteroid (NEA) population based on dynamical and
physical considerations. Our study is based on albedos derived within the
ExploreNEOs program and is extended by adding data from NEOWISE and the Akari
asteroid catalog. We use a statistical approach to identify asteroids on orbits
that resemble those of short-period near-Earth comets using the Tisserand
parameter with respect to Jupiter, the aphelion distance, and the minimum
orbital intersection distance with respect to Jupiter. From the sample of NEAs
on comet-like orbits, we select those with a geometric albedo
as dormant comet candidates, and find that only 50% of NEAs on comet-like
orbits also have comet-like albedos. We identify a total of 23 NEAs from our
sample that are likely to be dormant short-period near-Earth comets and, based
on a de-biasing procedure applied to the cryogenic NEOWISE survey, estimate
both magnitude-limited and size-limited fractions of the NEA population that
are dormant short-period comets. We find that 0.3-3.3% of the NEA population
with , and % of the population with diameters km, are dormant short-period near-Earth comets.Comment: 23 pages, 2 figures, 2 tables; accepted for publication in A
The Long Adventurous Journey of Rhombic Lip Cells in Jawed Vertebrates: A Comparative Developmental Analysis
This review summarizes vertebrate rhombic lip and early cerebellar development covering classic approaches up to modern developmental genetics which identifies the relevant differential gene expression domains and their progeny. Most of this information is derived from amniotes. However, progress in anamniotes, particularly in the zebrafish, has recently been made. The current picture suggests that rhombic lip and cerebellar development in jawed vertebrates (gnathostomes) share many characteristics. Regarding cerebellar development, these include a ptf1a expressing ventral cerebellar proliferation (VCP) giving rise to Purkinje cells and other inhibitory cerebellar cell types, and an atoh1 expressing upper rhombic lip giving rise to an external granular layer (EGL, i.e., excitatory granule cells) and an early ventral migration into the anterior rhombencephalon (cholinergic nuclei). As for the lower rhombic lip (LRL), gnathostome commonalities likely include the formation of precerebellar nuclei (mossy fiber origins) and partially primary auditory nuclei (likely convergently evolved) from the atoh1 expressing dorsal zone. The fate of the ptf1a expressing ventral LRL zone which gives rise to (excitatory cells of) the inferior olive (climbing fiber origin) and (inhibitory cells of ) cochlear nuclei in amniotes, has not been determined in anamniotes. Special for the zebrafish in comparison to amniotes is the predominant origin of anamniote excitatory deep cerebellar nuclei homologs (i.e., eurydendroid cells) from ptf1a expressing VCP cells, the sequential activity of various atoh1 paralogs and the incomplete coverage of the subpial cerebellar plate with proliferative EGL cells. Nevertheless, the conclusion that a rhombic lip and its major derivatives evolved with gnathostome vertebrates only and are thus not an ancestral craniate character complex is supported by the absence of a cerebellum (and likely absence of its afferent and efferent nuclei) in jawless fishe
The Discovery of Cometary Activity in Near-Earth Asteroid (3552) Don Quixote
The near-Earth object (NEO) population, which mainly consists of fragments
from collisions between asteroids in the main asteroid belt, is thought to
include contributions from short-period comets as well. One of the most
promising NEO candidates for a cometary origin is near-Earth asteroid (3552)
Don Quixote, which has never been reported to show activity. Here we present
the discovery of cometary activity in Don Quixote based on thermal-infrared
observations made with the Spitzer Space Telescope in its 3.6 and 4.5 {\mu}m
bands. Our observations clearly show the presence of a coma and a tail in the
4.5 {\mu}m but not in the 3.6 {\mu}m band, which is consistent with molecular
band emission from CO2. Thermal modeling of the combined photometric data on
Don Quixote reveals a diameter of 18.4 (-0.4/+0.3) km and an albedo of 0.03
(-0.01/+0.02), which confirms Don Quixote to be the third-largest known NEO. We
derive an upper limit on the dust production rate of 1.9 kg s^-1 and derive a
CO2 gas production rate of (1.1+-0.1)10^26 molecules s^-1. Spitzer IRS
spectroscopic observations indicate the presence of fine-grained silicates,
perhaps pyroxene rich, on the surface of Don Quixote. Our discovery suggests
that CO2 can be present in near-Earth space over a long time. The presence of
CO2 might also explain that Don Quixote's cometary nature remained hidden for
nearly three decades.Comment: 40 pages, 8 figures, accepted by Ap
ExploreNEOs I: Description and first results from the Warm Spitzer NEO Survey
We have begun the ExploreNEOs project in which we observe some 700 Near Earth
Objects (NEOs) at 3.6 and 4.5 microns with the Spitzer Space Telescope in its
Warm Spitzer mode. From these measurements and catalog optical photometry we
derive albedos and diameters of the observed targets. The overall goal of our
ExploreNEOs program is to study the history of near-Earth space by deriving the
physical properties of a large number of NEOs. In this paper we describe both
the scientific and technical construction of our ExploreNEOs program. We
present our observational, photometric, and thermal modeling techniques. We
present results from the first 101 targets observed in this program. We find
that the distribution of albedos in this first sample is quite broad, probably
indicating a wide range of compositions within the NEO population. Many objects
smaller than one kilometer have high albedos (>0.35), but few objects larger
than one kilometer have high albedos. This result is consistent with the idea
that these larger objects are collisionally older, and therefore possess
surfaces that are more space weathered and therefore darker, or are not subject
to other surface rejuvenating events as frequently as smaller NEOs.Comment: AJ in pres
Six simple guidelines for introducing new genera of fungi
We formulate five guidelines for introducing new genera, plus one recommendation how to publish the results of scientific research. We recommend that reviewers and editors adhere to these guidelines. We propose that the underlying research is solid, and that the results and the final solutions are properly discussed. The six criteria are: (1) all genera that are recognized should be monophyletic; (2) the coverage of the phylogenetic tree should be wide in number of species, geographic coverage, and type species of the genera under study; (3) the branching of the phylogenetic trees has to have sufficient statistical support; (4) different options for the translation of the phylogenetic tree into a formal classification should be discussed and the final decision justified; (5) the phylogenetic evidence should be based on more than one gene; and (6) all supporting evidence and background information should be included in the publication in which the new taxa are proposed, and this publication should be peer-reviewed
ExploreNEOs. II. The Accuracy of the Warm Spitzer Near-Earth Object Survey
We report on results of observations of near-Earth objects (NEOs) performed with the NASA Spitzer Space Telescope as part of our ongoing (2009-2011) Warm Spitzer NEO survey ("ExploreNEOs"), the primary aim of which is to provide sizes and albedos of some 700 NEOs. The emphasis of the work described here is an assessment of the overall accuracy of our survey results, which are based on a semi-empirical generalized model of asteroid thermal emission. The NASA Spitzer Space Telescope has been operated in the so-called Warm Spitzer mission phase since the cryogen was depleted in 2009 May, with the two shortest-wavelength channels, centered at 3.6 μm and 4.5 μm, of the Infrared Array Camera continuing to provide valuable data. The set of some 170 NEOs in our current Warm Spitzer results catalog contains 28 for which published taxonomic classifications are available, and 14 for which relatively reliable published diameters and albedos are available. A comparison of the Warm Spitzer results with previously published results ("ground truth"), complemented by a Monte Carlo error analysis, indicates that the rms Warm Spitzer diameter and albedo errors are ±20% and ±50%, respectively. Cases in which agreement with results from the literature is worse than expected are highlighted and discussed; these include the potential spacecraft target 138911 2001 AE_2. We confirm that 1.4 appears to be an appropriate overall default value for the relative reflectance between the V band and the Warm Spitzer wavelengths, for use in correction of the Warm Spitzer fluxes for reflected solar radiation
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