274 research outputs found
Precession of neutrino-cooled accretion disks in gamma-ray burst engines
Aims. We study the precession of accretion disks in the context of gamma-ray burst inner engines.
Methods. With an accretion disk model that allows for neutrino cooling, we evaluate the possible periods of disk precession and nutation due to the Lense-Thirring effect.
Results. Assuming jet ejection perpendicular to the disk midplane and a typical intrinsic time dependence for the burst, we find possible gamma-ray light curves with a temporal microstructure similar to what is observed in some subsamples.
Conclusions. We conclude that the precession and nutation of a neutrino-cooled accretion disk in the burst engine might be responsible for some events, especially those with a slow rise and a fast decay.Facultad de Ciencias Astronómicas y GeofísicasInstituto Argentino de Radioastronomí
A new rhynchocephalian from the late jurassic of Germany with a dentition that is unique amongst tetrapods.
Rhynchocephalians, the sister group of squamates (lizards and snakes), are only represented by the single genus Sphenodon today. This taxon is often considered to represent a very conservative lineage. However, rhynchocephalians were common during the late Triassic to latest Jurassic periods, but rapidly declined afterwards, which is generally attributed to their supposedly adaptive inferiority to squamates and/or Mesozoic mammals, which radiated at that time. New finds of Mesozoic rhynchocephalians can thus provide important new information on the evolutionary history of the group.
A new fossil relative of Sphenodon from the latest Jurassic of southern Germany, Oenosaurus muehlheimensis gen. et sp. nov., presents a dentition that is unique amongst tetrapods. The dentition of this taxon consists of massive, continuously growing tooth plates, probably indicating a crushing dentition, thus representing a previously unknown trophic adaptation in rhynchocephalians.
The evolution of the extraordinary dentition of Oenosaurus from the already highly specialized Zahnanlage generally present in derived rhynchocephalians demonstrates an unexpected evolutionary plasticity of these animals. Together with other lines of evidence, this seriously casts doubts on the assumption that rhynchocephalians are a conservative and adaptively inferior lineage. Furthermore, the new taxon underlines the high morphological and ecological diversity of rhynchocephalians in the latest Jurassic of Europe, just before the decline of this lineage on this continent. Thus, selection pressure by radiating squamates or Mesozoic mammals alone might not be sufficient to explain the demise of the clade in the Late Mesozoic, and climate change in the course of the fragmentation of the supercontinent of Pangaea might have played a major role
On the Origin of Asymmetries in Bilateral Supernova Remnants
AIMS: We investigate whether the morphology of bilateral supernova remnants
(BSNRs) observed in the radio band is determined mainly either by a non-uniform
interstellar medium (ISM) or by a non-uniform ambient magnetic field.
METHODS: We perform 3-D MHD simulations of a spherical SNR shock propagating
through a magnetized ISM. Two cases of shock propagation are considered: 1)
through a gradient of ambient density with a uniform ambient magnetic field; 2)
through a homogeneous medium with a gradient of ambient magnetic field
strength. From the simulations, we synthesize the synchrotron radio emission,
making different assumptions about the details of acceleration and injection of
relativistic electrons.
RESULTS: We find that asymmetric BSNRs are produced if the line-of-sight is
not aligned with the gradient of ambient plasma density or with the gradient of
ambient magnetic field strength. We derive useful parameters to quantify the
degree of asymmetry of the remnants that may provide a powerful diagnostic of
the microphysics of strong shock waves through the comparison between models
and observations.
CONCLUSIONS: BSNRs with two radio limbs of different brightness can be
explained if a gradient of ambient density or, most likely, of ambient magnetic
field strength is perpendicular to the radio limbs. BSNRs with converging
similar radio arcs can be explained if the gradient runs between the two arcs.Comment: 14 pages, 8 Figures; paper accepted for publication in A&A; the paper
with high-resolution figures can be downloaded at
http://www.astropa.unipa.it/~orlando/PAPERS/sorlando_6045.pd
Hubble Space Telescope WFPC-2 Imaging of Cassiopeia A
The young SNR Cassiopeia A was imaged with WFPC-2 through four filters
selected to capture the complete velocity range of the remnant's main shell in
several important emission lines. Primary lines detected were [O III]
4959,5007, [N II] 6583, [S II] 6716,6731 + [O II] 7319,7330 + [O I] 6300,6364,
and [S III] 9069,9532. About 3/4th of the remnant's main shell was imaged in
all four filters. Considerable detail is observed in the reverse-shocked ejecta
with typical knot scale lengths of 0.2"-0.4" (1 - 2 x 10^16 cm). Both bright
and faint emission features appear highly clumped. Large differences in [S III]
and [O III] line intensities indicating chemical abundance differences are also
seen, particularly in knots located along the bright northern limb and near the
base of the northeast jet. A line of curved overlapping filament in the
remnant's northwestern rim appears to mark the location of the remnant's
reverse shock front in this region. Finger-like ejecta structures elsewhere
suggest cases where the reverse shock front is encountering the remnant's
clumped ejecta. Narrow-band [N II] images of the remnant's circumstellar knots
("QSFs") reveal them to be 0.1"-0.6" thick knots and filaments, often with
diffuse edges facing away from the center of expansion. Three color composite
images of the whole remnant and certain sections along with individual filter
enlargements of selected regions of the bright optical shell are presented and
discussed.Comment: 26 pages, 12 figures Accepted to the Astronomical Journa
Thiamethoxam in Papaya (Carica papaya Linnaeus) Agroecosystems
Papaya (Carica papaya L.) is a profitable fruit of economic and food importance in Mexico and Central America. Veracruz is the state in Mexico with the highest cultivable area, eventhough its production presents numerous phytosanitary problems, which are being faced with the use of the pesticide thiamethoxam. The aim of this study was to make a diagnosis of the use and management of thiamethoxam in papaya agroecosystems in the municipality of Cotaxtla, Veracruz. Two surveys were applied, one to a 30% of the total number of producers organized by an association dedicated to papaya culture, and the other survey was through key informants, both surveys were designed using the snowball sampling, a non-probability sampling technique. The results indicate that 6% of papaya producers use mainly the pesticide thiamethoxam, which belongs to the chemical group of neonicotinoids. It was found out that for five years there have been records of thiamethoxam use in vertisoils. During the cycle of papaya cultivation the producers use a maximum dose of 3 L/ha and a minimum dose of 250 ml/ha per crop cycle. One hundred per cent of those who apply thiamethoxam are not aware of its use and efficient management, nor of the damage they are doing or have caused to agroecosystems
Elemental Abundances in the Possible Type Ia Supernova Remnant G344.7-0.1
Recent studies on the Galactic supernova remnant (SNR) G344.7-0.1 have
commonly claimed its origin to be a core-collapse supernova (SN) explosion,
based on its highly asymmetric morphology and/or proximity to a star forming
region. In this paper, however, we present an X-ray spectroscopic study of this
SNR using Suzaku, which is supportive of a Type Ia origin. Strong K-shell
emission from lowly ionized Fe has clearly been detected, and its origin is
determined, for the first time, to be the Fe-rich SN ejecta. The abundance
pattern is highly consistent with that expected for a somewhat-evolved Type Ia
SNR. It is suggested, therefore, that the X-ray point-like source CXOU
J170357.8-414302 located at the SNR's geometrical center is not associated with
the SNR but is likely to be a foreground object. Our result further indicates
that G344.7-0.1 is the first possible Type Ia SNR categorized as a member of
the so-called "mixed-morphology" class. In addition, we have detected emission
from He-like Al at ~1.6 keV, the first clear detection of this element in the
spectrum of an extended X-ray source. The possible enhancement of the Al/Mg
abundance ratio from the solar value suggests that the ambient interstellar
medium has a relatively high metallicity (not less than 10% of the solar
value), if this SNR has indeed a Type Ia origin. We also report marginal
detection of Cr and Mn, although the measured fluxes have large statistical and
systematic uncertainties.Comment: ApJ in pres
On the spherical-axial transition in supernova remnants
A new law of motion for supernova remnant (SNR) which introduces the quantity
of swept matter in the thin layer approximation is introduced. This new law of
motion is tested on 10 years observations of SN1993J. The introduction of an
exponential gradient in the surrounding medium allows to model an aspherical
expansion. A weakly asymmetric SNR, SN1006, and a strongly asymmetric SNR,
SN1987a, are modeled. In the case of SN1987a the three observed rings are
simulated.Comment: 19 figures and 14 pages Accepted for publication in Astrophysics &
Space Science in the year 201
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