21,662 research outputs found
Feedback local optimality principle applied to rocket vertical landing VTVL
Vertical landing is becoming popular in the last fifteen years, a technology known under the acronym VTVL, Vertical Takeoff and Vertical Landing [1,2]. The interest in such landing technology is dictated by possible cost reductions [3,4], that impose spaceship’s recycling. The rockets are not generally de- signed to perform landing operations, rather their design is aimed at takeoff operations, guaranteeing a very high forward acceleration to gain the velocity needed to escape the gravitational force. In this paper a new control method based on Feedback Local Optimality Principle, named FLOP is applied to the rocket landing problem. The FLOP belongs to a special class of optimal controllers, developed by the mechatronic and vehicle dynamics lab of Sapienza, named Variational Feedback Controllers - VFC, that are part of an ongoing research and are recently applied in different field: nonlinear system [5], marine and terrestrial autonomous vehicles [6,7,8], multi agents interactions and vibration control [9, 10]. The paper is devoted to show the robustness of the nonlinear controlled system, comparing the performances with the LQR, one of the most acknowledged methods in optimal control
Scattered light mapping of protoplanetary disks
High-contrast scattered light observations have revealed the surface
morphology of several dozens of protoplanetary disks at optical and
near-infrared wavelengths. Inclined disks offer the opportunity to measure part
of the phase function of the dust grains that reside in the disk surface which
is essential for our understanding of protoplanetary dust properties and the
early stages of planet formation. We aim to construct a method which takes into
account how the flaring shape of the scattering surface of an (optically thick)
protoplanetary disk projects onto the image plane of the observer. This allows
us to map physical quantities (scattering radius and scattering angle) onto
scattered light images and retrieve stellar irradiation corrected (r^2-scaled)
images and dust phase functions. We apply the method on archival polarized
intensity images of the protoplanetary disk around HD 100546 that were obtained
with VLT/SPHERE in R'-band and VLT/NACO in H- and Ks-band. The brightest side
of the r^2-scaled R'-band polarized intensity image of HD 100546 changes from
the far to the near side of the disk when a flaring instead of a geometrically
flat disk surface is used for the r^2-scaling. The decrease in polarized
surface brightness in the scattering angle range of ~40-70 deg is likely a
result of the dust phase function and degree of polarization which peak in
different scattering angle regimes. The derived phase functions show part of a
forward scattering peak which indicates that large, aggregate dust grains
dominate the scattering opacity in the disk surface. Projection effects of a
protoplanetary disk surface need to be taken into account to correctly
interpret scattered light images. Applying the correct scaling for the
correction of stellar irradiation is crucial for the interpretation of the
images and the derivation of the dust properties in the disk surface layer.Comment: Accepted for publication in A&A, 6 pages, 3 figure
The D0 same-charge dimuon asymmetry and possibile new CP violation sources in the system
Recently, the D0 collaboration reported a large CP violation in the same-sign
dimuon charge asymmetry which has the deviation from the value
estimated in the Standard Model. In this paper, several new physics models are
considered: the MSSM, two Higgs doublet model, the recent dodeca model, and a
new model. Generally, it is hard to achieve such a large CP violation
consistently with other experimental constraints. We find that a scheme with
extra non-anomalous U(1) gauge symmetry is barely consistent. In general,
the extra gauge boson induces the flavor changing neutral current
interactions at tree level, which is the basic reason allowing a large new
physics CP violation. To preserve the U(1) symmetry at high energy,
SU(2) singlet exotic heavy quarks of mass above 1 TeV and the Standard
Model gauge singlet scalars are introduced.Comment: 12 pages, 13 figure
The First Calculation for the Mass of the Ground Glueball State on Lattice
Under the quenched approximation, we perform a lattice calculation for the
mass of the ground glueball state in channel on a
lattice. Our calculation shows that the mass of this state is
, which rules out the or mainly
glueball interpretation for .Comment: 10 pages and 1 figur
Chromatic transit light curves of disintegrating rocky planets
Context. Kepler observations have revealed a class of short period
exoplanets, of which Kepler-1520 b is the prototype, which have comet-like dust
tails thought to be the result of small, rocky planets losing mass. The shape
and chromaticity of the transits constrain the properties of the dust particles
originating from the planet's surface, offering a unique opportunity to probe
the composition and geophysics of rocky exoplanets.
Aims. We aim to approximate the average Kepler long-cadence light curve of
Kepler-1520 b and investigate how the optical thickness and transit
cross-section of a general dust tail can affect the observed wavelength
dependence and depth of transit light curves.
Methods. We developed a new 3D model that ejects sublimating particles from
the planet surface to build up a dust tail, assuming it to be optically thin,
and used 3D radiative transfer computations that fully treat scattering using
the distribution of hollow spheres (DHS) method, to generate transit light
curves between 0.45 and 2.5 m.
Results. We show that the transit depth is wavelength independent for
optically thick tails, potentially explaining why only some observations
indicate a wavelength dependence. From the 3D nature of our simulated tails, we
show that their transit cross-sections are related to the component of particle
ejection velocity perpendicular to the planet's orbital plane and use this to
derive a minimum ejection velocity of 1.2 kms. To fit the average
transit depth of Kepler-1520 b of 0.87%, we require a high dust mas-loss rate
of 7 80 M Gyr which implies planet lifetimes that may be
inconsistent with the observed sample. Therefore, these mass-loss rates should
be considered to be upper limits.Comment: 22 pages, 22 figures, accepted for publication in A&
Study of Radiative Leptonic D Meson Decays
We study the radiative leptonic meson decays of D^+_{(s)}\to
\l^+\nu_{\l}\gamma (\l=e,\mu,\tau), and D^0\to
\l^+\l^-\gamma () within the light front quark model. In the standard
model, we find that the decay branching ratios of , and
are (), (), and
(), and that of D^0\to\l^+\l^-\gamma (\l=e,\mu) and
are and ,
respectively.Comment: 23 pages, 6 Figures, LaTex file, a reference added, to be published
in Mod. Phys. Lett.
Resolving HD 100546 disc in the mid-infrared: Small inner disc and asymmetry near the gap
A region of roughly half of the solar system scale around the star HD 100546
is largely cleared of gas and dust, in contrast to the bright outer disc.
However, some material is observed in the immediate vicinity of the star. We
investigate how the dust is distributed within and outside the gap, and
constrain the disc geometry with mid-infrared interferometric observations
using VLTI/MIDI. With baseline lengths of 40m, our long baseline observations
are sensitive to the inner few AU from the star, and we combined them with
observations at shorter, 15m baselines, to probe emission beyond the gap at up
to 20AU from the star. We modelled the mid-infrared emission using radial
temperature profiles. Our model is composed of infinitesimal concentric annuli
emitting as black bodies, and it has distinct inner and outer disc components.
We derived an upper limit of 0.7AU for the radial size of the inner disc, from
our longest baseline data. This small dusty disc is separated from the edge of
the outer disc by a large, roughly 10AU wide gap. Our short baseline data place
a bright ring of emission at 11+-1AU, consistent with prior observations of the
transition region between the gap and the outer disc, known as the disc wall.
The inclination and position angle are constrained by our data to i=53+-8deg
and PA=145+-5deg. Compared to the rim and outer disc geometry this suggests
co-planarity. Brightness asymmetry is evident in both short and long baseline
data, and it is unequivocally discernible from any atmospheric or instrumental
effects. The origin of the asymmetry is consistent with the bright disc wall,
which we find to be 1-2AU wide. The gap is cleared of micron-sized dust, but we
cannot rule out the presence of larger particles and/or perturbing bodies.Comment: 12 pages, 9 figures, accepted for publication in A&
Monte Carlo modeling of spin injection through a Schottky barrier and spin transport in a semiconductor quantum well
We develop a Monte Carlo model to study injection of spin-polarized electrons
through a Schottky barrier from a ferromagnetic metal contact into a
non-magnetic low-dimensional semiconductor structure. Both mechanisms of
thermionic emission and tunneling injection are included in the model. Due to
the barrier shape, the injected electrons are non-thermalized. Spin dynamics in
the semiconductor heterostructure is controlled by the Rashba and Dresselhaus
spin-orbit interactions and described by a single electron spin density matrix
formalism. In addition to the linear term, the third order term in momentum for
the Dresselhaus interaction is included. Effect of the Schottky potential on
the spin dynamics in a 2 dimensional semiconductor device channel is studied.
It is found that the injected current can maintain substantial spin
polarization to a length scale in the order of 1 micrometer at room temperature
without external magnetic fields.Comment: 18 pages, 4 figures, J. Appl. Phys., accepted for publicatio
Location and sizes of forsterite grains in protoplanetary disks: interpretation from the Herschel DIGIT programme
The spectra of protoplanetary disks contain mid- and far- infrared emission
features produced by forsterite dust grains. The spectral features contain
information about the forsterite temperature, chemical composition and grain
size. We aim to characterize how the 23 and 69 micron features can be used to
constrain the physical locations of forsterite in disks. We check for
consistency between two independent forsterite temperature measurements: the
23/69 feature strength ratio and the shape of the 69 micron band. We performed
radiative transfer modeling to study the effect of disk properties to the
forsterite spectral features. Temperature-dependent forsterite opacities were
considered in self-consistent models to compute forsterite emission from
protoplanetary disks. Modelling grids are presented to study the effects of
grain size, disk gaps, radial mixing and optical depth to the forsterite
features. Independent temperature estimates derived from the 23/69 feature
strength ratio and the 69 micron band shape are most inconsistent for HD141569
and Oph IRS 48. A case study of the disk of HD141569 shows two solutions to fit
the forsterite spectrum. A model with T ~ 40 K, iron-rich (~0-1 % Fe) and 1
micron forsterite grains, and a model with warmer (T ~ 100 K), iron-free, and
larger (10 micron) grains. We find that for disks with low upper limits of the
69 micron feature (most notably in flat, self-shadowed disks), the forsterite
must be hot, and thus close to the star. We find no correlation between disk
gaps and the presence or absence of forsterite features. We argue that the 69
micron feature of the evolved transitional disks HD141569 and Oph IRS 48 is
most likely a tracer of larger (i.e. ~10 micron) forsterite grains.Comment: Accepted for publication in A&A. 14 pages, 9 figure
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