9,770 research outputs found
A Possible Divot in the Size Distribution of the Kuiper Belt's Scattering Objects
Via joint analysis of a calibrated telescopic survey, which found scattering
Kuiper Belt objects, and models of their expected orbital distribution, we
measure the form of the scattering object's size distribution. Ruling out a
single power-law at greater than 99% confidence, we constrain the form of the
size distribution and find that, surprisingly, our analysis favours a very
sudden decrease (a divot) in the number distribution as diameters decrease
below 100 km, with the number of smaller objects then rising again as expected
via collisional equilibrium. Extrapolating at this collisional equilibrium
slope produced enough kilometer-scale scattering objects to supply the nearby
Jupiter-Family comets. Our interpretation is that this divot feature is a
preserved relic of the size distribution made by planetesimal formation, now
"frozen in" to portions of the Kuiper Belt sharing a "hot" orbital inclination
distribution, explaining several puzzles in Kuiper Belt science. Additionally,
we show that to match today's scattering-object inclination distribution, the
supply source that was scattered outward must have already been vertically
heated to of order 10 degrees.Comment: accepted 2013 January 8; published 2013 January 22 21 pages, 4
figure
Artificially induced positronium oscillations in a two-sheeted spacetime: consequences on the observed decay processes
Following recent theoretical results, it is suggested that positronium (Ps)
might undergo spontaneous oscillations between two 4D spacetime sheets whenever
subjected to constant irrotational magnetic vector potentials. We show that
these oscillations that would come together with o-Ps/p-Ps oscillations should
have important consequences on Ps decay rates. Experimental setup and
conditions are also suggested for demonstrating in non accelerator experiments
this new invisible decay mode.Comment: 9 pages, 2 figures. Minor form correction. Accepted for publication
in Int. J. of Modern Physics
Matter localization and resonant deconfinement in a two-sheeted spacetime
In recent papers, a model of a two-sheeted spacetime M4XZ2 was introduced and
the quantum dynamics of massive fermions was studied in this framework. In the
present study, we show that the physical predictions of the model are perfectly
consistent with observations and most important, it can solve the puzzling
problem of the four-dimensional localization of the fermion species in
multidimensional spacetimes. It is demonstrated that fermion localization on
the sheets arises from the combination of the discrete bulk structure and
environmental interactions. The mechanism described in this paper can be seen
as an alternative to the domain wall localization arising in continuous five
dimensional spacetimes. Although tightly constrained, motions between the
sheets are, however, not completely prohibited. As an illustration, a resonant
mechanism through which fermion oscillations between the sheets might occur is
described.Comment: 9 pages, 1 figure. Published version. Accepted for publication in
Int. J. of Modern Physics
The different origins of magnetic fields and activity in the Hertzsprung gap stars, OU Andromedae and 31 Comae
Context: When crossing the Hertzsprung gap, intermediate-mass stars develop a
convective envelope. Fast rotators on the main sequence, or Ap star
descendants, are expected to become magnetic active subgiants during this
evolutionary phase. Aims: We compare the surface magnetic fields and activity
indicators of two active, fast rotating red giants with similar masses and
spectral class but diferent rotation rates - OU And (Prot=24.2 d) and 31 Com
(Prot=6.8 d) - to address the question of the origin of their magnetism and
high activity.
Methods: Observations were carried out with the Narval spectropolarimeter in
2008 and 2013.We used the least squares deconvolution technique to extract
Stokes V and I profiles to detect Zeeman signatures of the magnetic field of
the stars. We provide Zeeman-Doppler imaging, activity indicator monitoring,
and a precise estimation of stellar parameters. We use stellar evolutionary
models to infer the evolutionary status and the initial rotation velocity on
the main sequence.
Results: The detected magnetic field of OU And is a strong one. Its
longitudinal component Bl reaches 40 G and presents an about sinusoidal
variation with reversal of the polarity. The magnetic topology of OU And is
dominated by large scale elements and is mainly poloidal with an important
dipole component, and a significant toroidal component. The detected magnetic
field of 31 Com is weaker, with a magnetic map showing a more complex field
geometry, and poloidal and toroidal components of equal contributions. The
evolutionary models show that the progenitors of OU And and 31 Com must have
been rotat
Conclusions: OU And appears to be the probable descendant of a magnetic Ap
star, and 31 Com the descendant of a relatively fast rotator on the main
sequence.Comment: 16 pages, 12 figure
Search for surface magnetic fields in Mira stars. First detection in chi Cyg
In order to complete the knowledge of the magnetic field and of its influence
during the transition from Asymptotic Giant Branch to Planetary Nebulae stages,
we have undertaken a search for magnetic fields at the surface of Mira stars.
We used spectropolarimetric observations, collected with the Narval instrument
at TBL, in order to detect - with Least Squares Deconvolution method - a Zeeman
signature in the visible part of the spectrum. We present the first
spectropolarimetric observations of the S-type Mira star chi Cyg, performed
around its maximum light. We have detected a polarimetric signal in the Stokes
V spectra and we have established its Zeeman origin. We claim that it is likely
to be related to a weak magnetic field present at the photospheric level and in
the lower part of the stellar atmosphere. We have estimated the strength of its
longitudinal component to about 2-3 Gauss. This result favors a 1/r law for the
variation of the magnetic field strength across the circumstellar envelope of
chi Cyg. This is the first detection of a weak magnetic field at the stellar
surface of a Mira star and we discuss its origin in the framework of shock
waves periodically propagating throughout the atmosphere of these radially
pulsating stars. At the date of our observations of chi Cyg, the shock wave
reaches its maximum intensity, and it is likely that the shock amplifies a weak
stellar magnetic field during its passage through the atmosphere. Without such
an amplification by the shock, the magnetic field strength would have been too
low to be detected. For the first time, we also report strong Stokes Q and U
signatures (linear polarization) centered onto the zero velocity (i.e., at the
shock front position). They seem to indicate that the radial direction would be
favored by the shock during its propagation throughout the atmosphere.Comment: 9 pages, 4 figures accepted by Astronomy and Astrophysics (21
November 2013
Influence of the C/O ratio on titanium and vanadium oxides in protoplanetary disks
Context. The observation of carbon-rich disks have motivated several studies
questioning the influence of the C/O ratio on their gas phase composition in
order to establish the connection between the metallicity of hot-Jupiters and
that of their parent stars.
Aims. We to propose a method that allows the characterization of the adopted
C/O ratio in protoplanetary disks independently from the determination of the
host star composition. Titanium and vanadium chemistries are investigated
because they are strong optical absorbers and also because their oxides are
known to be sensitive to the C/O ratio in some exoplanet atmospheres.
Methods. We use a commercial package based on the Gibbs energy minimization
technique to compute the titanium and vanadium equilibrium chemistries in
protoplanetary disks for C/O ratios ranging from 0.05 to 10. Our calculations
are performed for pressures ranging from 1e-6 to 1e-2 bar, and for temperatures
ranging from 50 to 2000 K.
Results. We find that the vanadium nitride/vanadium oxide and titanium
hydride/titanium oxide gas phase ratios strongly depend on the C/O ratio in the
hot parts of disks (T > 1000 K). Our calculations suggest that, in these
regions, these ratios can be used as tracers of the C/O value in protoplanetary
disks.Comment: Accepted for publication in A&
Chronic low back pain, chronic disability at work, chronic management issues.
Low-back pain (LBP) stands out as the leading musculoskeletal disorder because it is both highly prevalent and the disability with which people live for the greatest number of years (1, 2). Reaching a peak between the ages of 30–50 years, LBP affects a population at a time of career advancement (3, 4). Back pain is the most expensive disease in terms of indirect costs due to sickness absence and work disability. Indirect (or productivity) costs contribute 93% to total costs, illustrating the importance of the consequences of the disease for work performance (5, 6). On a personal level, low self-motivation and self-confidence make it harder to initiate the return-to-work (RTW) process, especially when problems at work are related to the reason for sick leave (7, 8). At the workplace level, colleagues take over the tasks of the worker on sick leave, work piles up, or another worker is hired to take over the tasks. Timely RTW is thus of great benefit for both injured workers and their employers. [...
Colloquium: Comparison of Astrophysical and Terrestrial Frequency Standards
We have re-analyzed the stability of pulse arrival times from pulsars and
white dwarfs using several analysis tools for measuring the noise
characteristics of sampled time and frequency data. We show that the best
terrestrial artificial clocks substantially exceed the performance of
astronomical sources as time-keepers in terms of accuracy (as defined by cesium
primary frequency standards) and stability. This superiority in stability can
be directly demonstrated over time periods up to two years, where there is high
quality data for both. Beyond 2 years there is a deficiency of data for
clock/clock comparisons and both terrestrial and astronomical clocks show equal
performance being equally limited by the quality of the reference timescales
used to make the comparisons. Nonetheless, we show that detailed accuracy
evaluations of modern terrestrial clocks imply that these new clocks are likely
to have a stability better than any astronomical source up to comparison times
of at least hundreds of years. This article is intended to provide a correct
appreciation of the relative merits of natural and artificial clocks. The use
of natural clocks as tests of physics under the most extreme conditions is
entirely appropriate; however, the contention that these natural clocks,
particularly white dwarfs, can compete as timekeepers against devices
constructed by mankind is shown to be doubtful.Comment: 9 pages, 2 figures; presented at the International Frequency Control
Symposium, Newport Beach, Calif., June, 2010; presented at Pulsar Conference
2010, October 12th, Sardinia; accepted 13th September 2010 for publication in
Reviews of Modern Physic
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