33,337 research outputs found
The Struve-Sahade effect in the optical spectra of O-type binaries I. Main-sequence systems
We present a spectroscopic analysis of four massive binary systems that are
known or are good candidates to display the Struve-Sahade effect (defined as
the apparent strengthening of the secondary spectrum of the binary when the
star is approaching, and the corresponding weakening of the lines when it is
receding).
We use high resolution optical spectra to determine new orbital solutions and
spectral types of HD 165052, HD 100213, HD 159176 and DH Cep. As good knowledge
of the fundamental parameters of the considered systems is necessary to examine
the Struve-Sahade effect. We then study equivalent width variations in the
lines of both components of these binaries during their orbital cycle.
In the case of these four systems, variations appear in the equivalent widths
of some lines during the orbital cycle, but the definition given above can any
longer be valid, since it is now clear that the effect modifies the primary
spectrum as much as the secondary spectrum. Furthermore, the lines affected,
and the way in which they are affected, depend on the considered system. For at
least two of them (HD 100213 and HD 159176) these variations probably reflect
the ellipsoidal variable nature of the system.Comment: 12 pages, 20 figures, in press A&
The cosmic dust analyzer: Experimental evaluation of an impact ionization model
A thermal equilibrium plasma model is used to process data from an impact ionization time-of-flight mass spectrometer in order to convert the raw ion data to relative abundances of the elemental constituents of cosmic dust particles
Monte Carlo simulation of the transmission of measles: Beyond the mass action principle
We present a Monte Carlo simulation of the transmission of measles within a
population sample during its growing and equilibrium states by introducing two
different vaccination schedules of one and two doses. We study the effects of
the contact rate per unit time as well as the initial conditions on the
persistence of the disease. We found a weak effect of the initial conditions
while the disease persists when lies in the range 1/L-10/L ( being
the latent period). Further comparison with existing data, prediction of future
epidemics and other estimations of the vaccination efficiency are provided.
Finally, we compare our approach to the models using the mass action
principle in the first and another epidemic region and found the incidence
independent of the number of susceptibles after the epidemic peak while it
strongly fluctuates in its growing region. This method can be easily applied to
other human, animals and vegetable diseases and includes more complicated
parameters.Comment: 15 pages, 4 figures, 1 table, Submitted to Phys.Rev.
State education as high-yield investment: human capital theory in European policy discourse
Human Capital Theory has been an increasingly important phenomenon in economic thought over the last 50 years. The central role it affords to education has become even more marked in recent years as the concept of the ‘knowledge economy’ has become a global concern. In this paper, the prevalence of Human Capital Theory within European educational policy discourse is explored. The paper examines a selection of policy documents from a number of disparate European national contexts and considers the extent to which the ideas of Human Capital Theory can be seen to be influential. In the second part of the paper, the implications of Human Capital Theory for education are considered, with a particular focus on the possible ramifications at a time of economic austerity. The paper argues that Human Capital Theory risks offering a diminished view of the person, a diminished view of education, but that with its sole focus on economic goals leaves room for educationists and others to argue for the educational, social, and moral values it ignores, and for the conception of the good life and good society it fails to mention
Spectroscopic study of the O-type runaway supergiant HD 195592
The scope of this paper is to perform a detailed spectroscopic study of the
northern O-type supergiant HD 195592. We use a large sample of high quality
spectra in order to investigate its multiplicity, and to probe the line profile
variability. Our analysis reveals a clear spectroscopic binary signature in the
profile of the He {\sc i} 6678 line, pointing to a probable O + B
system. We report on low amplitude radial velocity variations in every strong
absorption line in the blue spectrum of HD 195592. These variations are ruled
by two time-scales respectively of 5.063 and about 20 days. The former is
firmly established, whilst the latter is poorly constrained. We report also on
a very significant line profile variability of the H line, with time
scales strongly related to those of the radial velocities. Our results provide
significant evidence that HD 195592 is a binary system, with a period that
might be the variability time-scale of about 5 days. The second time scale may
be the signature of an additional star moving along a wider orbit provided its
mass is low enough, even though direct evidence for the presence of a third
star is still lacking. Alternatively, the second time-scale may be the
signature of a variability intrinsic to the stellar wind of the primary,
potentially related to the stellar rotation.Comment: 9 pages, 5 postscript figures, accepted for publication in New
Astronom
High-energy Emission from Pulsar Outer Magnetospheres
We investigate a stationary pair production cascade in the outer
magnetosphere of an isolated, spinning neutron star. The charge depletion due
to global flows of charged particles, causes a large electric field along the
magnetic field lines. Migratory electrons and/or positrons are accelerated by
this field to radiate gamma-rays via curvature and inverse-Compton processes.
Some of such gamma-rays collide with the X-rays to materialize as pairs in the
gap. The replenished charges partially screen the electric field, which is
self-consistently solved together with the energy distribution of particles and
gamma-rays at each point along the field lines. By solving the set of Maxwell
and Boltzmann equations, we demonstrate that an external injection of charged
particles at nearly Goldreich-Julian rate does not quench the gap but shifts
its position and that the particle energy distribution cannot be described by a
power-law. The injected particles are accelerated in the gap and escape from it
with large Lorentz factors. We show that such escaping particles migrating
outside of the gap contribute significantly to the gamma-ray luminosity for
young pulsars and that the soft gamma-ray spectrum between 100 MeV and 3 GeV
observed for the Vela pulsar can be explained by this component. We also
discuss that the luminosity of the gamma-rays emitted by the escaping particles
is naturally proportional to the square root of the spin-down luminosity.Comment: 24 pages, 11 figures; to appear in the inaugural (Sep) issue of
Progress in Astrophysics Researches (a new book series
Testing common classical LTE and NLTE model atmosphere and line-formation codes for quantitative spectroscopy of early-type stars
It is generally accepted that the atmospheres of cool/lukewarm stars of
spectral types A and later are described well by LTE model atmospheres, while
the O-type stars require a detailed treatment of NLTE effects. Here model
atmosphere structures, spectral energy distributions and synthetic spectra
computed with ATLAS9/SYNTHE and TLUSTY/SYNSPEC, and results from a hybrid
method combining LTE atmospheres and NLTE line-formation with DETAIL/SURFACE
are compared. Their ability to reproduce observations for effective
temperatures between 15000 and 35000 K are verified. Strengths and weaknesses
of the different approaches are identified. Recommendations are made as to how
to improve the models in order to derive unbiased stellar parameters and
chemical abundances in future applications, with special emphasis on Gaia
science.Comment: 12 pages, 8 figures; accepted for publication in Journal of Physics:
Conference Series, GREAT-ESF Workshop: Stellar Atmospheres in the Gaia Er
Young Crab-like pulsars and luminous X-ray sources in starbursts and optically dull galaxies
Recent Chandra observations of nearby galaxies have revealed a number of
ultraluminous X-ray sources (ULXs) with super-Eddington luminosities, away from
the central regions of non-active galaxies. The nature of these sources is
still debated. We argue that a fraction of them could be young, Crab-like
pulsars, the X-ray luminosity of which is powered by rotation. We use the
pulsar birth parameters estimated from radio pulsar data to compute the
steady-state pulsar X-ray luminosity distribution as a function of the star
formation rate (SFR) in the galaxy. We find that ~10% of optically dull
galaxies are expected to have a source with L_x >~ 10^{39} erg/s, while
starbursts galaxies should each have several of these sources. We estimate that
the X-ray luminosity of a few percents of galaxies is dominated by a single
bright pulsar with L_x >~10^{39} erg/s, roughly independently of its SFR. We
discuss observational diagnostics that can help distinguish the young pulsar
population in ULXs.Comment: 17 pages, 4 figures, accepted to Ap
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