3,947 research outputs found
A dedicated photometric system for the detection of Wolf-Rayet stars
We present here tests of a five-filter photometric system aimed at WR
classification. In addition to the well-known easy separation between the WN
and WC spectral types, these tests indicate interesting potentialities in the
discrimination of subgroups among the WN and the WC which look well related to
the classical subtypes. The proposed combinations of filters (or derived ones)
bear enough discriminating power to satisfy some evolutionary studies in
crowded fields where spectroscopic follow-up is not possible.Comment: 10 pages, 7 figures, uses l-aa.sty and psfig for figure inclusion.
Also available at http://vela.astro.ulg.ac.be/preprint/P10 Accepted for
publication in Astronomy & Astrophysics Supp
Ternary cluster decay within the liquid drop model
Longitudinal ternary and binary fission barriers of Ar, Ni and
Cf nuclei have been determined within a rotational liquid drop model
taking into account the nuclear proximity energy. For the light nuclei the
heights of the ternary fission barriers become competitive with the binary ones
at high angular momenta since the maximum lies at an outer position and has a
much higher moment of inertia.Comment: Talk presented at the 9th International Conference on Clustering
Aspects of Nuclear Structure and Dynamics (CLUSTERS'07
Rotational velocities of A-type stars II. Measurement of vsini in the northern hemisphere
This work is the second part of the set of measurements of vsini for A-type
stars, begun by Royer et al. (2002). Spectra of 249 B8 to F2-type stars
brighter than V=7 have been collected at Observatoire de Haute-Provence (OHP).
Fourier transforms of several line profiles in the range 4200--4600 A are used
to derive vsini from the frequency of the first zero. Statistical analysis of
the sample indicates that measurement error mainly depends on vsini and this
relative error of the rotational velocity is found to be about 5% on average.
The systematic shift with respect to standard values from Slettebak et al.
(1975), previously found in the first paper, is here confirmed. Comparisons
with data from the literature agree with our findings: vsini values from
Slettebak et al. are underestimated and the relation between both scales
follows a linear law: vsini(new) = 1.03 vsini(old) + 7.7. Finally, these data
are combined with those from the previous paper (Royer et al. 2002), together
with the catalogue of Abt & Morrell (1995). The resulting sample includes some
2150 stars with homogenized rotational velocities.Comment: 16 pages, includes 13 figures, accepted in A&
The perturbed sublimation rim of the dust disk around the post-AGB binary IRAS08544-4431
Context: Post-Asymptotic Giant Branch (AGB) binaries are surrounded by stable
dusty and gaseous disks similar to the ones around young stellar objects.
Whereas significant effort is spent on modeling observations of disks around
young stellar objects, the disks around post-AGB binaries receive significantly
less attention, even though they pose significant constraints on theories of
disk physics and binary evolution. Aims: We want to examine the structure of
and phenomena at play in circumbinary disks around post-AGB stars. We continue
the analysis of our near-infrared interferometric image of the inner rim of the
circumbinary disk around IRAS08544-4431. We want to understand the physics
governing this inner disk rim. Methods: We use a radiative transfer model of a
dusty disk to reproduce simultaneously the photometry as well as the
near-infrared interferometric dataset on IRAS08544-4431. The model assumes
hydrostatic equilibrium and takes dust settling self-consistently into account.
Results: The best-fit radiative transfer model shows excellent agreement with
the spectral energy distribution up to mm wavelengths as well as with the
PIONIER visibility data. It requires a rounded inner rim structure, starting at
a radius of 8.25 au. However, the model does not fully reproduce the detected
over-resolved flux nor the azimuthal flux distribution of the inner rim. While
the asymmetric inner disk rim structure is likely to be the consequence of
disk-binary interactions, the origin of the additional over-resolved flux
remains unclear. Conclusions: As in young stellar objects, the disk inner rim
of IRAS08544-4431 is ruled by dust sublimation physics. Additional observations
are needed to understand the origin of the extended flux and the azimuthal
perturbation at the inner rim of the disk.Comment: Accepted for publication in A&A, 13 figures, 13 page
Stress fractures in 2011: practical approach
Stress fractures occur when excessive loads are applied to a bone whose mechanical strength is normal. Bone insufficiency fractures, in contrast, are due to physiological loads applied to bone of inadequate mechanical strength [1]. This contradistinction is obviously an oversimplifi cation. In practice, a continuum exists between these two clearly defined situations.
The objective of the third ODISSEE meetings [2,3] held under the aegis of the GRIO was to review current knowledge on stress fractures. The pathophysiology of stress fractures is still poorly understood. When loads are applied to a bone, particularly in a repetitive manner, an elastic deformity occurs, followed by a plastic deformity and, finally, by microfractures. Bone strength varies across individuals. It depends not only on the intrinsic qualities of the bone tissue, but also on the magnitude and repetitiveness of the loads applied to the bone. Bone tissue fatigue is an inability to repair the microdamage caused by mechanical loading. The number and length of the microfractures increase, resulting in a fracture with clinical symptoms and radiographic changes [4]. Stress fractures are a common reason for physician visits among athletes and military recruits. They account for 5% to 14% of all physician visits, depending on the study population [5,6].
Although stress fractures can arise at any site, the most common locations are the tibia, particularly in runners; the metatarsals (most notably the second and third metatarsals) in hikers, runners, dancers, and military recruits; the iliopubic and ischiopubic rami of the pelvis in military recruits, gymnasts, dancers, and soccer players; and the femur in cross-country runners. The calcaneus is also a common site of involvement in all populations. Stress fractures are rare at the upper limbs, except in high- level gymnasts country runners. The calcaneus is also a common site of involvement in all populations. Stress fractures are rare at the upper limbs,except in high-level gymnasts
Droplet and cluster formation in freely falling granular streams
Particle beams are important tools for probing atomic and molecular
interactions. Here we demonstrate that particle beams also offer a unique
opportunity to investigate interactions in macroscopic systems, such as
granular media. Motivated by recent experiments on streams of grains that
exhibit liquid-like breakup into droplets, we use molecular dynamics
simulations to investigate the evolution of a dense stream of macroscopic
spheres accelerating out of an opening at the bottom of a reservoir. We show
how nanoscale details associated with energy dissipation during collisions
modify the stream's macroscopic behavior. We find that inelastic collisions
collimate the stream, while the presence of short-range attractive interactions
drives structure formation. Parameterizing the collision dynamics by the
coefficient of restitution (i.e., the ratio of relative velocities before and
after impact) and the strength of the cohesive interaction, we map out a
spectrum of behaviors that ranges from gas-like jets in which all grains drift
apart to liquid-like streams that break into large droplets containing hundreds
of grains. We also find a new, intermediate regime in which small aggregates
form by capture from the gas phase, similar to what can be observed in
molecular beams. Our results show that nearly all aspects of stream behavior
are closely related to the velocity gradient associated with vertical free
fall. Led by this observation, we propose a simple energy balance model to
explain the droplet formation process. The qualitative as well as many
quantitative features of the simulations and the model compare well with
available experimental data and provide a first quantitative measure of the
role of attractions in freely cooling granular streams
Proton radioactivity within a generalized liquid drop model
The proton radioactivity half-lives of spherical proton emitters are
investigated theoretically. The potential barriers preventing the emission of
protons are determined in the quasimolecular shape path within a generalized
liquid drop model (GLDM) including the proximity effects between nuclei in a
neck and the mass and charge asymmetry. The penetrability is calculated with
the WKB approximation. The spectroscopic factor has been taken into account in
half-life calculation, which is obtained by employing the relativistic mean
field (RMF) theory combined with the BCS method with the force NL3. The
half-lives within the GLDM are compared with the experimental data and other
theoretical values. The GLDM works quite well for spherical proton emitters
when the spectroscopic factors are considered, indicating the necessity of
introducing the spectroscopic factor and the success of the GLDM for proton
emission. Finally, we present two formulas for proton emission half-life
calculation similar to the Viola-Seaborg formulas and Royer's formulas of alpha
decay.Comment: 7 pages, 1 figur
Photon-Neutrino Interactions
We discuss the interaction of photons with neutrinos including two lepton
loops. The parity violation in the gamma-nu to gamma-nu channel due to two
lepton loops is substantially enhanced relative to the one lepton loop
contribution. However there is no corresponding enhancement in the parity
conserving amplitude in either the direct or the cross channel.Comment: 12 pages, 5 figure
Carinae's Dusty Homunculus Nebula from Near-Infrared to Submillimeter Wavelengths: Mass, Composition, and Evidence for Fading Opacity
Infrared observations of the dusty, massive Homunculus Nebula around the
luminous blue variable Carinae are crucial to characterize the mass-loss
history and help constrain the mechanisms leading to the Great Eruption. We
present the 2.4 - 670 m spectral energy distribution, constructed from
legacy ISO observations and new spectroscopy obtained with the {\em{Herschel
Space Observatory}}. Using radiative transfer modeling, we find that the two
best-fit dust models yield compositions which are consistent with CNO-processed
material, with iron, pyroxene and other metal-rich silicates, corundum, and
magnesium-iron sulfide in common. Spherical corundum grains are supported by
the good match to a narrow 20.2 m feature. Our preferred model contains
nitrides AlN and SiN in low abundances. Dust masses range from 0.25 to
0.44 but 45 in both cases due to an
expected high Fe gas-to-dust ratio. The bulk of dust is within a 5
7 central region. An additional compact feature is detected at 390 m.
We obtain = 2.96 10 , a 25\% decline from
an average of mid-IR photometric levels observed in 1971-1977. This indicates a
reduction in circumstellar extinction in conjunction with an increase in visual
brightness, allowing 25-40\% of optical and UV radiation to escape from the
central source. We also present an analysis of CO and CO through lines, showing that the abundances are consistent with
expectations for CNO-processed material. The [C~{\sc{ii}}] line is
detected in absorption, which we suspect originates in foreground material at
very low excitation temperatures.Comment: Accepted in Ap
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