6,457 research outputs found
A comprehensive population synthesis study of post-common envelope binaries
We apply population synthesis techniques to calculate the present day
population of post-common envelope binaries (PCEBs) for a range of theoretical
models describing the common envelope (CE) phase. Adopting the canonical energy
budget approach we consider models where the ejection efficiency,
\alpha_{\rmn{CE}} is either a constant, or a function of the secondary mass.
We obtain the envelope binding energy from detailed stellar models of the
progenitor primary, with and without the thermal and ionization energy, but we
also test a commonly used analytical scaling. We also employ the alternative
angular momentum budget approach, known as the -algorithm. We find that
a constant, global value of \alpha_{\rmn{CE}} \ga 0.1 can adequately account
for the observed population of PCEBs with late spectral-type secondaries.
However, this prescription fails to reproduce IK Pegasi, which has a secondary
with spectral type A8. We can account for IK Pegasi if we include thermal and
ionization energy of the giant's envelope, or if we use the -algorithm.
However, the -algorithm predicts local space densities that are 1 to 2
orders of magnitude greater than estimates from observations. In contrast, the
canonical energy budget prescription with an initial mass ratio distribution
that favours unequal initial mass ratios gives a local space density which is
in good agreement with observations, and best reproduces the observed
distribution of PCEBs. Finally, all models fail to reproduce the sharp decline
for orbital periods, P_{\rmn{orb}} \ga 1 d in the orbital period distribution
of observed PCEBs, even if we take into account selection effects against
systems with long orbital periods and early spectral-type secondaries.Comment: Accepted for publication in the Monthly Notices of the Royal
Astronomical Society. 18 pages, 10 figures. Work concerning the
reconstruction of the common envelope phase presented in the previous version
will now be submitted in a separate paper in the near futur
Superconducting phase formation in random neck syntheses: a study of the Y-Ba-Cu-O system by magneto-optics and magnetometry
Magneto-optical imaging and magnetization measurements were applied to
investigate local formation of superconducting phase effected by a random neck
synthesis in Y-Ba-Cu-O system. Polished pellets of strongly inhomogeneous
ceramic samples show clearly the appearance of superconducting material in the
intergrain zones of binary primary particles reacted under different
conditions. Susceptibility measurements allows evaluation of superconducting
critical temperature, which turned out to be close to that of optimally doped
YBCO.Comment: 6 pages, 11 figure
Angular momentum losses and the orbital period distribution of cataclysmic variables below the period gap: effects of circumbinary disks
The population synthesis of cataclysmic variables below the period is
investigated. A grid of detailed binary evolutionary sequences has been
calculated and included in the simulations to take account of additional
angular momentum losses beyond that associated with gravitational radiation and
mass loss, due to nova outbursts, from the system. As a specific example, we
consider the effect of a circumbinary disk to gain insight into the ingredients
necessary to reproduce the observed orbital period distribution. The resulting
distributions show that the period minimum lies at about 80 minutes with the
number of systems monotonically increasing with increasing orbital period to a
maximum near 90 minutes. There is no evidence for an accumulation of systems at
the period minimum which is a common feature of simulations in which only
gravitational radiation losses are considered. The period distribution is found
to be fairly flat for orbital periods ranging from about 85 to 120 minutes. The
steepness of the lower edge of the period gap can be reproduced, for example,
by an input of systems at periods near 2.25 hrs due to a flow of cataclysmic
variable binary systems from orbital periods longer than 2.75 hrs. The good
agreement with the cumulated distribution function of observed systems within
the framework of our model indicates that the angular momentum loss by a
circumbinary disk or a mechanism which mimics its features coupled with a
weighting factor to account for selection effects in the discovery of such
systems and a flow of systems from above the period gap to below the period gap
are important ingredients for understanding the overall period distribution of
cataclysmic variable binary systems.Comment: Accepted for publication in Ap
Relaxing competition through speculation: Committing to a negative supply slope
We demonstrate how suppliers can take strategic speculative positions in derivatives markets to soften competition in the spot market. In our game, suppliers first choose a portfolio of call options and then compete with supply functions. In equilibrium firms sell forward contracts and buy call options to commit to downward sloping supply functions. Although this strategy is risky, it reduces the elasticity of the residual demand of competitors, who increase their mark-ups in response. We show that this type of strategic speculation increases the level and volatility of commodity prices and decreases welfare
Deep level transient spectroscopy study for the development of ion-implanted silicon field-effect transistors for spin-dependent transport
A deep level transient spectroscopy (DLTS) study of defects created by
low-fluence, low-energy ion implantation for development of ion-implanted
silicon field-effect transistors for spin-dependent transport experiments is
presented. Standard annealing strategies are considered to activate the
implanted dopants and repair the implantation damage in test
metal-oxide-semiconductor (MOS) capacitors. Fixed oxide charge, interface
trapped charge and the role of minority carriers in DLTS are investigated. A
furnace anneal at 950 C was found to activate the dopants but did not
repair the implantation damage as efficiently as a 1000 C rapid
thermal anneal. No evidence of bulk traps was observed after either of these
anneals. The ion- implanted spin-dependent transport device is shown to have
expected characteristics using the processing strategy determined in this
study.Comment: 4 pages, 6 figure
On the detection of pre-low-mass X-ray binaries
We explore the population of candidate pre-low-mass X-ray binaries in which a
neutron star accretes mass from the wind of a low-mass companion (mass < 2Msun)
in the framework of a binary population synthesis study. The simulated
accretion-luminosity distribution shows a primary peak close to 1e31 erg/s and
a secondary peak near 1e28 erg/s. The relative contribution of the two peaks
depends primarily on the magnitude of the kick velocity imparted to the neutron
star at birth. The secondary peak is negligible for average kick velocities
larger than 200 km/s, but becomes dominant for average kick velocities smaller
than 50 km/s. Regardless of the relative contributions of the two peaks, our
calculations suggest that pre-low-mass X-ray binaries may provide a
non-negligible contribution to the population of discrete low-luminosity X-ray
sources in the Galaxy.Comment: Accepted for publication in MNRA
How HTLV-1 may subvert miRNAs for persistence and transformation
Distinct mechanisms are used by viruses to interact with cellular miRNAs. The role of microRNAs in viral replication and persistence ranges from viral-encoded microRNAs to suppressors of RNA interference. Viruses can also exploit cellular miRNAs for influencing cellular metabolism to ensure efficient replication or latency. In particular, two recent studies provide examples of how HTLV-1 may co-opt or subvert cellular miRNAs for persistent replication and oncogenic purposes. The pathways modulated by these described miRNAs are critically involved in apoptosis, proliferation and innate immune response
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