6,226 research outputs found
Leptonic and Semileptonic Decays of Pseudoscalar Mesons
We employ the relativistic constituent quark model to give a unified
description of the leptonic and semileptonic decays of pseudoscalar mesons
(\pi, K, D, D_s, B, B_s). The calculated leptonic decay constants and form
factors are found to be in good agreement with available experimental data and
other approaches. We reproduce the results of spin-flavor symmetry in the heavy
quark limit.Comment: 12 pages LaTeX (elsart.sty) + 4 figures; added references, to appear
in Phys. Lett.
Brood cells like conifer cones: the peculiar nesting biology of the osmiine bee Hoplitis (Alcidamea) curvipes (Morawitz, 1871) (Hymenoptera, Megachilidae)
Two nests of Hoplitis curvipes are described from Apulia (Italy) and Dagestan (Russia). Both nests consisted of two brood cells placed side by side under a stone. The cells were neither attached to each other nor to the substrate. They were constructed from leaf fragments, which were imbricately arranged, forming a cone-like structure; each leaf fragment consisted of a basal part that was masticated to leaf pulp and an apical part that protruded freely from the cell wall. The cell wall was formed by the fusion of the masticated basal parts of the leaf fragments and thus entirely consisted of leaf pulp. The cell was sealed with a closing plug made of pure leaf pulp; a few leaf fragments were glued to its outer surface. The cocoon consisted of two layers: the outer layer was restricted to the anterior portion of the cell and had several longitudinal air-exchange slits on its lateral surface, while the inner layer had an air-exchange orifice in its most anterior dome-shaped top. Results of measurements of brood cell dimensions and contents are provided. The nesting biology of species of the H. curvipes group is discussed
On equilibrium tides in fully convective planets and stars
We consider the tidal interaction of a fully convective primary star and a
point mass. Using a normal mode decomposition we calculate the evolution of the
primary angular velocity and orbit for arbitrary eccentricity e. The
dissipation acting on the tidal perturbation is associated with convective
turbulence. A novel feature of the Paper is that, to take into account of the
fact that there is a relaxation time t_{c}, being the turn-over time of
convective eddies, associated with the process, this is allowed to act non
locally in time, producing a dependence of the dissipation on tidal forcing
frequency. Results are expressed in terms of the Fourier coefficients of the
tidal potential. We find analytical approximations for these valid for .
When the tidal response is frequency independent, our results are equivalent to
those obtained in the standard constant time lag approximation. When there is
the frequency dependence of the dissipative response, the evolution can differ
drastically. In that case the system can evolve through a sequence of
spin-orbit corotation resonances with Omega_{r}/Omega_{orb}=n/2, where
Omega_{r} and Omega_{orb} are the rotation and orbital frequencies and n is an
integer. We study this case analytically and numerically.Comment: The size of the shown abstract is reduced. Submitted to MNRA
Mathematical retroreflectors
Retroreflectors are optical devices that reverse the direction of incident
beams of light. Here we present a collection of billiard type retroreflectors
consisting of four objects; three of them are asymptotically perfect
retroreflectors, and the fourth one is a retroreflector which is very close to
perfect. Three objects of the collection have recently been discovered and
published or submitted for publication. The fourth object - notched angle - is
a new one; a proof of its retroreflectivity is given.Comment: 32 pages, 19 figure
Influence of a stellar cusp on the dynamics of young stellar discs and the origin of the S-stars in the Galactic Centre
Observations of the Galactic Centre show evidence of one or two disc-like
structures of very young stars orbiting the central super-massive black hole
within a distance of a few 0.1 pc. A number of analyses have been carried out
to investigate the dynamical behaviour and consequences of these discs,
including disc thickness and eccentricity growth as well as mutual interaction
and warping. However, most of these studies have neglected the influence of the
stellar cusp surrounding the black hole, which is believed to be 1-2 orders of
magnitude more massive than the disc(s).
By means of N-body integrations using our bhint code, we study the impact of
stellar cusps of different compositions. We find that although the presence of
a cusp does have an important effect on the evolution of an otherwise isolated
flat disc, its influence on the evolution of disc thickness and warping is
rather mild in a two-disc configuration. However, we show that the creation of
highly eccentric orbits strongly depends on the graininess of the cusp (i.e.
the mean and maximum stellar masses): While Chang (2009) recently found that
full cycles of Kozai resonance are prevented by the presence of an analytic
cusp, we show that relaxation processes play an important role in such highly
dense regions and support short-term resonances. We thus find that young disc
stars on initially circular orbits can achieve high eccentricities by resonant
effects also in the presence of a cusp of stellar remnants, yielding a
mechanism to create S-stars and hyper-velocity stars.
Furthermore, we discuss the underlying initial mass function (IMF) of the
young stellar discs and find no definite evidence for a non-canonical IMF.Comment: 10 pages, 7 figures, 1 table, accepted for publication in MNRA
Heavy- to light-meson transition form factors
Semileptonic heavy -> heavy and heavy -> light meson transitions are studied
as a phenomenological application of a heavy-quark limit of Dyson-Schwinger
equations. Employing two parameters: E, the difference between the mass of the
heavy meson and the effective-mass of the heavy quark; and Lambda, the width of
the heavy-meson Bethe-Salpeter amplitude, we calculate f_+(t) for all decays on
their entire kinematically accessible t-domain. Our study favours f_B in the
range 0.135-0.17 GeV and with E=0.44 GeV and 1/Lambda = 0.14 fm we obtain
f_+^{B pi}(0) = 0.46. As a result of neglecting 1/m_c-corrections, we estimate
that our calculated values of \rho^2 = 0.87 and f_+^{DK}(0)=0.62 are too low by
approximately 15%. However, the bulk of these corrections should cancel in our
calculated values of Br(D -> \pi l nu)/Br(D -> K l nu)=0.13 and f_+^{D
pi}(0)/f_+^{DK}(0) = 1.16.Comment: 26 pages, 3 figures, REVTE
Aerothermodynamic Analysis of a Reentry Brazilian Satellite
This work deals with a computational investigation on the small ballistic
reentry Brazilian vehicle SARA (acronyms for SAt\'elite de Reentrada
Atmosf\'erica). Hypersonic flows over the vehicle SARA at zero-degree angle of
attack in a chemical equilibrium and thermal non-equilibrium are modeled by the
Direct Simulation Monte Carlo (DSMC) method, which has become the main
technique for studying complex multidimensional rarefied flows, and that
properly accounts for the non-equilibrium aspects of the flows. The emphasis of
this paper is to examine the behavior of the primary properties during the high
altitude portion of SARA reentry. In this way, velocity, density, pressure and
temperature field are investigated for altitudes of 100, 95, 90, 85 and 80 km.
In addition, comparisons based on geometry are made between axisymmetric and
planar two-dimensional configurations. Some significant differences between
these configurations were noted on the flowfield structure in the reentry
trajectory. The analysis showed that the flow disturbances have different
influence on velocity, density, pressure and temperature along the stagnation
streamline ahead of the capsule nose. It was found that the stagnation region
is a thermally stressed zone. It was also found that the stagnation region is a
zone of strong compression, high wall pressure. Wall pressure distributions are
compared with those of available experimental data and good agreement is found
along the spherical nose for the altitude range investigated.Comment: The paper will be published in Vol. 42 of the Brazilian Journal of
Physic
Simulations of Direct Collisions of Gas Clouds with the Central Black Hole
We perform numerical simulations of clouds in the Galactic Centre (GC)
engulfing the nuclear super-massive black hole and show that this mechanism
leads to the formation of gaseous accretion discs with properties that are
similar to the expected gaseous progenitor discs that fragmented into the
observed stellar disc in the GC. As soon as the cloud hits the black hole, gas
with opposite angular momentum relative to the black hole collides downstream.
This process leads to redistribution of angular momentum and dissipation of
kinetic energy, resulting in a compact gaseous accretion disc. A parameter
study using thirteen high resolution simulations of homogeneous clouds falling
onto the black hole and engulfing it in parts demonstrates that this mechanism
is able to produce gaseous accretion discs that could potentially be the
progenitor of the observed stellar disc in the GC. A comparison of simulations
with different equations of state (adiabatic, isothermal and full cooling)
demonstrates the importance of including a detailed thermodynamical
description. However the simple isothermal approach already yields good results
on the radial mass transfer and accretion rates, as well as disc eccentricities
and sizes. We find that the cloud impact parameter strongly influences the
accretion rate whereas the impact velocity has a small affect on the accretion
rate.Comment: 21 pages, 18 figures, Accepted for publication in MNRA
The tidal disruption rate in dense galactic cusps containing a supermassive binary black hole
We consider the problem of tidal disruption of stars in the centre of a
galaxy containing a supermassive binary black hole with unequal masses. We
assume that over the separation distance between the black holes the
gravitational potential is dominated by the more massive black hole. Also, we
assume that the number density of stars is concentric with the primary black
hole and has a power law cusp.We show that the bulk of stars with a small
angular-momentum component normal to the black hole binary orbit can reach a
small value of total angular momentum through secular evolution in the
gravitational field of the binary, and hence they can be tidally disrupted.
This effect is analogous to the so-called Kozai effect (Kozai, 1962, Lidov,
1961,1962) well known in celestial mechanics. We develop an analytical theory
of secular evolution of the stellar orbits and calculate the rate of tidal
disruption. We confront our analytical theory with a simple numerical model and
find very good agreement.
Our results show that for primary black-hole mass \sim
10^{6}-10^{7}M_{\odot}, the black- hole mass ratio q > 10^{-2}, cusp size \sim
1pc, the tidal disruption rate can be as large as \sim 10^{-2}-1M_{\odot}/yr.
This is at least 10^{2}-10^{4} times larger than estimated for the case of a
single supermassive black hole. The duration of the phase of enhanced tidal
disruption is determined by the dynamical friction time scale, and it is rather
short: \sim 10^{5}yr. The dependence of the tidal disruption rate on the mass
ratio, as well as on the size of the cusp, is also discussed.Comment: This version has been published in MNRA
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