11,343 research outputs found
Planetesimal disk evolution driven by embryo-planetesimal gravitational scattering
The process of gravitational scattering of planetesimals by a massive
protoplanetary embryo is explored theoretically. We propose a method to
describe the evolution of the disk surface density, eccentricity, and
inclination caused by the embryo-planetesimal interaction. It relies on the
analytical treatment of the scattering in two extreme regimes of the
planetesimal epicyclic velocities: shear-dominated (dynamically ``cold'') and
dispersion-dominated (dynamically ``hot''). In the former, planetesimal
scattering can be treated as a deterministic process. In the latter, scattering
is mostly weak because of the large relative velocities of interacting bodies.
This allows one to use the Fokker-Planck approximation and the two-body
approximation to explore the disk evolution. We compare the results obtained by
this method with the outcomes of the direct numerical integrations of
planetesimal orbits and they agree quite well. In the intermediate velocity
regime an approximate treatment of the disk evolution is proposed based on
interpolation between the two extreme regimes. We also calculate the rate of
embryo's mass growth in an inhomogeneous planetesimal disk and demonstrate that
it is in agreement with both the simulations and earlier calculations. Finally
we discuss the question of the direction of the embryo-planetesimal interaction
in the dispersion-dominated regime and demonstrate that it is repulsive. This
means that the embryo always forms a gap in the disk around it, which is in
contrast with the results of other authors. The machinery developed here will
be applied to realistic protoplanetary systems in future papers.Comment: 40 pages, 9 figures, submitted to A
Frequency stability of maser oscillators operated with cavity Q
The short term frequency stability of masers equipped with an external feedback loop to increase the cavity quality factor was studied. The frequency stability of a hydrogen and a rubidium maser were measured and compared with theoretical evaluation. It is shown that the frequency stability passes through an optimum when the cavity Q is varied. Long term fluctuations are discussed and the optimum mid term frequency stability achievably by small size active and passive H-masers is considered
Conductance fluctuations in metallic nanogaps made by electromigration
We report on low temperature conductance measurements of gold nanogaps
fabricated by controlled electromigration. Fluctuations of the conductance due
to quantum interferences and depending both on bias voltage and magnetic field
are observed. By analyzing the voltage and magnetoconductance correlation
functions we determine the type of electron trajectories generating the
observed quantum interferences and the effective characteristic time of phase
coherence in our device.Comment: 5 pages, 4 figures, to appear in J. Appl. Phy
The Effects of Workplace Union on the Gender Wage Gap
The Roudy Law in 1983 tried to promote wage equality between men and women by reinforcing union power in this field. A reexamination of CMOSS data from 1992 allows measuring what was its impact after nearly ten years of implementation. The methodology consists in computing how the presence of union representatives at the workplace affects wage gaps between men and women of similar characteristics. Taking into account selection bias and unobserved individual and establishment heterogeneity, our results show a positive impact of unions on wages for both men and women working full time in the industrial sector. This union premium is all the more important as the worker earns a low wage. Nevertheless, we find no significant impact of unions on the unexplained gender wage gap: unions tend to increase both mens and womens wages in similar proportions.trade union, gender wage gap, gender discrimination, linked employer-employee French data
Surface Josephson plasma waves in layered superconductors
We predict the existence of surface waves in layered superconductors in the
THz frequency range, below the Josephson plasma frequency . This wave
propagates along the vacuum-superconductor interface and dampens in both
transverse directions out of the surface (i.e., towards the superconductor and
towards the vacuum). This is the first prediction of propagating surface waves
in any superconductor. These predicted surface Josephson plasma waves are
important for different phenomena, including the complete suppression of the
specular reflection from a sample (Wood's anomalies) and a huge enhancement of
the wave absorption (which can be used as a THz detector).Comment: 4 pages, 2 figure
Simulated CII observations for SPICA/SAFARI
We investigate the case of CII 158 micron observations for SPICA/SAFARI using
a three-dimensional magnetohydrodynamical (MHD) simulation of the diffuse
interstellar medium (ISM) and the Meudon PDR code. The MHD simulation consists
of two converging flows of warm gas (10,000 K) within a cubic box 50 pc in
length. The interplay of thermal instability, magnetic field and self-gravity
leads to the formation of cold, dense clumps within a warm, turbulent
interclump medium. We sample several clumps along a line of sight through the
simulated cube and use them as input density profiles in the Meudon PDR code.
This allows us to derive intensity predictions for the CII 158 micron line and
provide time estimates for the mapping of a given sky area.Comment: 4 pages, 5 figures, to appear in the proceedings of the workshop "The
Space Infrared Telescope for Cosmology & Astrophysics: Revealing the Origins
of Planets and Galaxies" (July 2009, Oxford, United Kingdom
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
A polarity reversal in the large-scale magnetic field of the rapidly rotating Sun HD 190771
Aims. We investigate the long-term evolution of the large-scale photospheric
magnetic field geometry of the solar-type star HD 190771. With fundamental
parameters very close to those of the Sun except for a shorter rotation period
of 8.8 d, HD 190771 provides us with a first insight into the specific impact
of the rotation rate in the dynamo generation of magnetic fields in 1
stars.
Methods. We use circularly polarized, high-resolution spectra obtained with
the NARVAL spectropolarimeter (Observatoire du Pic du Midi, France) and compute
cross-correlation line profiles with high signal-to-noise ratio to detect
polarized Zeeman signatures. From three phase-resolved data sets collected
during the summers of 2007, 2008, and 2009, we model the large-scale
photospheric magnetic field of the star by means of Zeeman-Doppler imaging and
follow its temporal evolution.
Results. The comparison of the magnetic maps shows that a polarity reversal
of the axisymmetric component of the large-scale magnetic field occurred
between 2007 and 2008, this evolution being observed in both the poloidal and
toroidal magnetic components. Between 2008 and 2009, another type of global
evolution occured, characterized by a sharp decrease of the fraction of
magnetic energy stored in the toroidal component. These changes were not
accompanied by significant evolution in the total photospheric magnetic energy.
Using our spectra to perform radial velocity measurements, we also detect a
very low-mass stellar companion to HD 190771.Comment: Accepted by Astronomy and Astrophysics (Letter to the Editor
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