3,868 research outputs found
Time-resolved lidar fluorosensor for sea pollution detection
A contemporary time and spectral analysis of oil fluorescence is useful for the detection and the characterization of oil spills on the sea surface. Nevertheless the fluorosensor lidars, which were realized up to now, have only partial capability to perform this double analysis. The main difficulties are the high resolution required (of the order of 1 nanosecond) and the complexity of the detection system for the recording of a two-dimensional matrix of data for each laser pulse. An airborne system whose major specifications were: time range, 30 to 75 ns; time resolution, 1 ns; spectral range, 350 to 700 nm; and spectral resolution, 10 nm was designed and constructed. The designed system of a short pulse ultraviolet laser source and a streak camera based detector are described
The Most Magnetic Stars
Observations of magnetic A, B and O stars show that the poloidal magnetic
flux per unit mass has an upper bound of 10^-6.5 G cm^2/g. A similar upper
bound is found for magnetic white dwarfs even though the highest magnetic field
strengths at their surfaces are much larger. For magnetic A and B stars there
also appears to be a well defined lower bound below which the incidence of
magnetism declines rapidly. According to recent hypotheses, both groups of
stars may result from merging stars and owe their strong magnetism to fields
generated by a dynamo mechanism as they merge. We postulate a simple dynamo
that generates magnetic field from differential rotation. The growth of
magnetic fields is limited by the requirement that the poloidal field
stabilizes the toroidal and vice versa. While magnetic torques dissipate the
differential rotation, toroidal field is generated from poloidal by an Omega
dynamo. We further suppose that mechanisms that lead to the decay of toroidal
field lead to the generation of poloidal. Both poloidal and toroidal fields
reach a stable configuration which is independent of the size of small initial
seed fields but proportional to the initial differential rotation. We pose the
hypothesis that strongly magnetic stars form from the merging of two stellar
objects. The highest fields are generated when the merge introduces
differential rotation that amounts to critical break up velocity within the
condensed object. Calibration of a simplistic dynamo model with the observed
maximum flux per unit mass for main-sequence stars and white dwarfs indicates
that about 1.5x10^-4 of the decaying toroidal flux must appear as poloidal. The
highest fields in single white dwarfs are generated when two degenerate cores
merge inside a common envelope or when two white dwarfs merge by
gravitational-radiation angular momentum loss.Comment: accepted by MNRAS 8 pages, 3 figure
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Mechanisms producing different precipitation patterns over northâeastern Italy: insights from HyMeXâSOP1 and previous events
During the first HyMeX Special Observation Period (SOP1) field campaign, the target site of northâeastern Italy (NEI) experienced a large amount of precipitation, locally exceeding the climatological values and distributed among several heavyârainfall episodes. In particular, two events that occurred during the last period of the campaign drew our attention. These events had common largeâscale patterns and a similar mesoscale setting, characterised by southerly lowâlevel flow interacting with the Alpine orography, but the precipitation distribution was very different. During Intensive Observing Period IOP18 (31 Octoberâ1 November 2012), convective systems were responsible for intense rainfall mainly located over a flat area of the eastern Po Valley, well upstream of the orography. Conversely, during IOP19 (4/5 November 2012), heavy precipitation affected only the Alpine area. In addition to IOP18 and IOP19, the present study analyses other heavyâprecipitation episodes that display similar characteristics and which occurred over NEI during the autumn of recent years. A highâresolution (2 km grid spacing) nonâhydrostatic NWP model and available observations are used for this purpose.
The two different observed precipitation patterns are explained in terms of interaction between the impinging flow and the Alps. Depending on the thermodynamic profile, convection can be triggered when the impinging flow is forced to rise over a preâexisting coldâair layer at the base of the orography. In this situation a persistent blockedâflow condition and upstream convergence are responsible for heavy rain localized over the plain. Conversely, if convection does not develop, flowâover conditions are established and heavy rain affects the Alps. Numerical parameters proposed in the literature are used to support the analysis.
Finally, the role of evaporative cooling beneath the convective systems is evaluated. It turns out that the stationarity of the systems upstream of the Alps is mainly attributable to persistent blockedâflow conditions, while convective outflow slightly modifies the location of precipitation
The open cluster initial-final mass relationship and the high-mass tail of the white dwarf distribution
Recent studies of white dwarfs in open clusters have provided new constraints
on the initial - final mass relationship (IFMR) for main sequence stars with
masses in the range 2.5 - 6.5 Mo. We re-evaluate the ensemble of data that
determines the IFMR and argue that the IFMR can be characterised by a mean
initial-final mass relationship about which there is an intrinsic scatter. We
investigate the consequences of the IFMR for the observed mass distribution of
field white dwarfs using population synthesis calculations. We show that while
a linear IFMR predicts a mass distribution that is in reasonable agreement with
the recent results from the PG survey, the data are better fitted by an IFMR
with some curvature. Our calculations indicate that a significant (~28%)
percentage of white dwarfs originating from single star evolution have masses
in excess of ~0.8 Mo, obviating the necessity for postulating the existence of
a dominant population of high-mass white dwarfs that arise from binary star
mergers.Comment: 5 pages, 2 color Postscript figures. Accepted for publication in
MNRA
High-mass star formation in southern disk galaxies
As part of a major study of the physical processes of star formation and the evolution of galactic discs, the detailed distribution of high-mass star formation within southern late-type spirals and Magellanic-type galaxies is being measured by means of narrow-band imaging in Ha and the continuum, spectroscopic studies of prominent HII regions identified in the Ha images, and by radio mapping in neutral hydrogen and the continuum. The radio mapping will be undertaken with the Southern Hemisphere's first large, multi-frequency synthesis array, the Australia Telescope. Some optical imaging and spectroscopic data has already been acquired; the optical data and some preliminary results are described
Bimodules and branes in deformation quantization
We prove a version of Kontsevich's formality theorem for two subspaces
(branes) of a vector space . The result implies in particular that the
Kontsevich deformation quantizations of and
associated with a quadratic Poisson structure are Koszul dual. This answers an
open question in Shoikhet's recent paper on Koszul duality in deformation
quantization.Comment: 40 pages, 15 figures; a small change of notations in the definition
of the 4-colored propagators; an Addendum about the appearance of loops in
the -quasi-isomorphism and a corresponding change in the proof of
Theorem 7.2; several changes regarding completions, when dealing with general
-structure
Massive color-octet bosons and the charge asymmetries of top quarks at hadron colliders
Several models predict the existence of heavy colored resonances decaying to
top quarks in the TeV energy range that might be discovered at the LHC. In some
of those models, moreover, a sizable charge asymmetry of top versus antitop
quarks might be generated. The detection of these exotic resonances, however,
requires selecting data samples where the top and the antitop quarks are highly
boosted, which is experimentally very challenging. We asses that the
measurement of the top quark charge asymmetry at the LHC is very sensitive to
the existence of excited states of the gluon with axial-vector couplings to
quarks. We use a toy model with general flavour independent couplings, and show
that a signal can be detected with relatively not too energetic top and antitop
quarks. We also compare the results with the asymmetry predicted by QCD, and
show that its highest statistical significance is achieved with data samples of
top-antitop quark pairs of low invariant masses.Comment: 20 page
Superconducting Superstructure for the TESLA Collider
We discuss the new layout of a cavity chain (superstructure) allowing, we
hope, significant cost reduction of the RF system of both linacs of the TESLA
linear collider. The proposed scheme increases the fill factor and thus makes
an effective gradient of an accelerator higher. We present mainly computations
we have performed up to now and which encouraged us to order the copper model
of the scheme, still keeping in mind that experiments with a beam will be
necessary to prove if the proposed solution can be used for the acceleration.Comment: 11 page
V405 Aurigae: A High Magnetic Field Intermediate Polar
Our simultaneous multicolor (UBVRI) circular polarimetry has revealed nearly
sinusoidal variation over the WD spin cycle, and almost symmetric positive and
negative polarization excursions. Maximum amplitudes are observed in the B and
V bands (+-3 %). This is the first time that polarization peaking in the blue
has been discovered in an IP, and suggests that V405 Aur is the highest
magnetic field IP found so far. The polarized flux spectrum is similar to those
found in polars with magnetic fields in the range B ~ 25-50 MG. Our low
resolution circular spectropolarimetry has given evidence of transient features
which can be fitted by cyclotron harmonics n = 6, 7, and 8, at a field of B =
31.5 +- 0.8 MG, consistent with the broad-band polarized flux spectrum. Timings
of the circular polarization zero crossovers put strict upper limits on WD spin
period changes and indicate that the WD in V405 Aur is currently accreting
closely at the spin equilibrium rate, with very long synchronization
timescales, T_s > 10^9 yr. For the observed spin to orbital period ratio,
P_{spin}/P_{orb} = 0.0365, and P_{orb} ~ 4.15 hr, existing numerical accretion
models predict spin equilibrium condition with B ~ 30 MG if the mass ratio of
the binary components is q_1 ~ 0.4. The high magnetic field makes V405 Aur a
likely candidate as a progenitor of a polar.Comment: To appear in The Astrophysical Journal, September 1 Issue (2008), 9
pages, 10 figure
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