1,825 research outputs found
Entropic Inequalities for a Class of Quantum Secret Sharing States
It is well-known that von Neumann entropy is nonmonotonic unlike Shannon
entropy (which is monotonically nondecreasing). Consequently, it is difficult
to relate the entropies of the subsystems of a given quantum state. In this
paper, we show that if we consider quantum secret sharing states arising from a
class of monotone span programs, then we can partially recover the monotonicity
of entropy for the so-called unauthorized sets. Furthermore, we can show for
these quantum states the entropy of the authorized sets is monotonically
nonincreasing.Comment: LaTex, 5 page
Modeling T Tauri Winds from He I 10830 Profiles
The high opacity of He I 10830 makes it an exceptionally sensitive probe of
the inner wind geometry of accreting T Tauri stars. In this line blueshifted
absorption below the continuum results from simple scattering of stellar
photons, a situation which is readily modeled without definite knowledge of the
physical conditions and recourse to multi-level radiative transfer. We present
theoretical line profiles for scattering in two possible wind geometries, a
disk wind and a wind emerging radially from the star, and compare them to
observed He I 10830 profiles from a survey of classical T Tauri stars. The
comparison indicates that subcontinuum blueshifted absorption is characteristic
of disk winds in ~30% of the stars and of stellar winds in ~40%. We further
conclude that for many stars the emission profile of helium likely arises in
stellar winds, increasing the fraction of accreting stars inferred to have
accretion-powered stellar winds to ~60%. Stars with the highest disk accretion
rates are more likely to have stellar wind than disk wind signatures and less
likely to have redshifted absorption from magnetospheric funnel flows. This
suggests the possibility that when accretion rates are high, disks can extend
closer to the star, magnetospheric accretion zones can be reduced in size and
conditions arise that favor radially outflowing stellar winds.Comment: 41 pages, 11 figures. Accepted by Astrophysical Journa
Three-dimensional simulations of rotationally-induced line variability from a Classical T Tauri star with a misaligned magnetic dipole
We present three-dimensional (3-D) simulations of rotationally induced line
variability arising from complex circumstellar environment of classical T Tauri
stars (CTTS) using the results of the 3-D magnetohydrodynamic (MHD) simulations
of Romanova et al., who considered accretion onto a CTTS with a misaligned
dipole magnetic axis with respect to the rotational axis. The density, velocity
and temperature structures of the MHD simulations are mapped on to the
radiative transfer grid, and corresponding line source function and the
observed profiles of neutral hydrogen lines (H-beta, Pa-beta and Br-gamma) are
computed using the Sobolev escape probability method. We study the dependency
of line variability on inclination angles (i) and magnetic axis misalignment
angles (Theta). By comparing our models with the Pa-beta profiles of 42 CTTS
observed by Folha & Emerson, we find that models with a smaller misaligngment
angle (Theta<~15 deg.) are more consistent with the observations which show
that majority of Pa-beta are rather symmetric around the line centre. For a
high inclination system with a small dipole misalignment angle (Theta ~ 15
deg.), only one accretion funnel (on the upper hemisphere) is visible to an
observer at any given rotational phase. This can cause an anti-correlation of
the line equivalent width in the blue wing (v0)
over a half of a rotational period, and a positive correlation over other half.
We find a good overall agreement of the line variability behaviour predicted by
our model and those from observations. (Abridged)Comment: 15 pages, 13 figures. Accepted for publication in MNRAS. A version
with full resolution figures can be downloaded from
http://www.physics.unlv.edu/~rk/preprint/inclined_dipole.pd
Micro-crystalline inclusions analysis by PIXE and RBS
A characteristic feature of the nuclear microprobe using a 3 MeV proton beam
is the long range of particles (around 70 \mu m in light matrices). The PIXE
method, with EDS analysis and using the multilayer approach for treating the
X-ray spectrum allows the chemistry of an intra-crystalline inclusion to be
measured, provided the inclusion roof and thickness at the impact point of the
beam (Z and e, respectively) are known (the depth of the inclusion floor is Z +
e). The parameter Z of an inclusion in a mineral can be measured with a
precision of around 1 \mu m using a motorized microscope. However, this value
may significantly depart from Z if the analyzed inclusion has a complex shape.
The parameter e can hardly be measured optically. By using combined RBS and
PIXE measurements, it is possible to obtain the geometrical information needed
for quantitative elemental analysis. This paper will present measurements on
synthetic samples to investigate the advantages of the technique, and also on
natural solid and fluid inclusions in quartz. The influence of the geometrical
parameters will be discussed with regard to the concentration determination by
PIXE. In particular, accuracy of monazite micro-inclusion dating by coupled
PIXE-RBS will be presented
Accretion dynamics in the classical T Tauri star V2129 Oph
We analyze the photometric and spectroscopic variability of the classical T
Tauri star V2129 Oph over several rotational cycles to test the dynamical
predictions of magnetospheric accretion models. The photometric variability and
the radial velocity variations in the photospheric lines can be explained by
rotational modulation due to cold spots, while the radial velocity variations
of the He I (5876 \AA) line and the veiling variability are due to hot spot
rotational modulation. The hot and cold spots are located at high latitudes and
about the same phase, but the hot spot is expected to sit at the chromospheric
level, while the cold spot is at the photospheric level. Using the
dipole+octupole magnetic-field configuration previously proposed in the
literature for the system, we compute 3D MHD magnetospheric simulations of the
star-disk system. We use the simulation's density, velocity and scaled
temperature structures as input to a radiative transfer code, from which we
calculate theoretical line profiles at all rotational phases. The theoretical
profiles tend to be narrower than the observed ones, but the qualitative
behavior and the observed rotational modulation of the H\alpha and H\beta
emission lines are well reproduced by the theoretical profiles. The
spectroscopic and photometric variability observed in V2129 Oph support the
general predictions of complex magnetospheric accretion models with
non-axisymmetric, multipolar fields.Comment: Accepted by Astronomy and Astrophysic
Dynamics of Rotating Accretion Flows Irradiated by a Quasar
We study the axisymmetric, time-dependent hydrodynamics of rotating flows
that are under the influence of supermassive black hole gravity and radiation
from an accretion disk surrounding the black hole. This work is an extension of
the earlier work presented by Proga, where nonrotating flows were studied.
Here, we consider effects of rotation, a position-dependent radiation
temperature, density at large radii, and uniform X-ray background radiation. As
in the non-rotating case, the rotating flow settles into a configuration with
two components (1) an equatorial inflow and (2) a bipolar inflow/outflow with
the outflow leaving the system along the pole. However, with rotation the flow
does not always reach a steady state. In addition, rotation reduces the outflow
collimation and the outward flux of mass and kinetic energy. Moreover rotation
increases the outward flux of the thermal energy and can lead to fragmentation
and time-variability of the outflow. We also show that a position-dependent
radiation temperature can significantly change the flow solution. In
particular, the inflow in the equatorial region can be replaced by a thermally
driven outflow. Generally, as it have been discussed and shown in the past, we
find that self-consistently determined preheating/cooling from the quasar
radiation can significantly reduce the rate at which the central BH is fed with
matter. However, our results emphasize also a little appreciated feature.
Namely, quasar radiation drives a non-spherical, multi-temperature and very
dynamic flow. These effects become dominant for luminosities in excess of 0.01
of the Eddington luminosity.Comment: accepted for publication in Ap
UV excess measures of accretion onto young very low-mass stars and brown dwarfs
Low-resolution spectra from 3000-9000 AA of young low-mass stars and brown
dwarfs were obtained with LRIS on Keck I. The excess UV and optical emission
arising in the Balmer and Paschen continua yields mass accretion rates ranging
from 2e-12 to 1e-8 Mo/yr. These results are compared with {\it HST}/STIS
spectra of roughly solar-mass accretors with accretion rates that range from
2e-10 to 5e-8 Mo/yr. The weak photospheric emission from M-dwarfs at <4000 A
leads to a higher contrast between the accretion and photospheric emission
relative to higher-mass counterparts. The mass accretion rates measured here
are systematically 4-7 times larger than those from H-alpha emission line
profiles, with a difference that is consistent with but unlikely to be
explained by the uncertainty in both methods. The accretion luminosity
correlates well with many line luminosities, including high Balmer and many He
I lines. Correlations of the accretion rate with H-alpha 10% width and line
fluxes show a large amount of scatter. Our results and previous accretion rate
measurements suggest that accretion rate is proportional to M^(1.87+/-0.26) for
accretors in the Taurus Molecular Cloud.Comment: 13 pages text, 15 tables, 14 figures. Accepted by Ap
Remarks on the k-error linear complexity of p(n)-periodic sequences
Recently the first author presented exact formulas for the number of 2ⁿn-periodic binary sequences with given 1-error linear complexity, and an exact formula for the expected 1-error linear complexity and upper and lower bounds for the expected k-error linear complexity, k >2, of a random 2ⁿn-periodic binary sequence. A crucial role for the analysis played the Chan-Games algorithm. We use a more sophisticated generalization of the Chan-Games algorithm by Ding et al. to obtain exact formulas for the counting function and the expected value for the 1-error linear complexity for pⁿn-periodic sequences over Fp, p prime. Additionally we discuss the calculation of lower and upper bounds on the k-error linear complexity of pⁿn-periodic sequences over Fp
Accretion-Powered Stellar Winds II: Numerical Solutions for Stellar Wind Torques
[Abridged] In order to explain the slow rotation observed in a large fraction
of accreting pre-main-sequence stars (CTTSs), we explore the role of stellar
winds in torquing down the stars. For this mechanism to be effective, the
stellar winds need to have relatively high outflow rates, and thus would likely
be powered by the accretion process itself. Here, we use numerical
magnetohydrodynamical simulations to compute detailed 2-dimensional
(axisymmetric) stellar wind solutions, in order to determine the spin down
torque on the star. We explore a range of parameters relevant for CTTSs,
including variations in the stellar mass, radius, spin rate, surface magnetic
field strength, the mass loss rate, and wind acceleration rate. We also
consider both dipole and quadrupole magnetic field geometries.
Our simulations indicate that the stellar wind torque is of sufficient
magnitude to be important for spinning down a ``typical'' CTTS, for a mass loss
rate of yr. The winds are wide-angle,
self-collimated flows, as expected of magnetic rotator winds with moderately
fast rotation. The cases with quadrupolar field produce a much weaker torque
than for a dipole with the same surface field strength, demonstrating that
magnetic geometry plays a fundamental role in determining the torque. Cases
with varying wind acceleration rate show much smaller variations in the torque
suggesting that the details of the wind driving are less important. We use our
computed results to fit a semi-analytic formula for the effective Alfv\'en
radius in the wind, as well as the torque. This allows for considerable
predictive power, and is an improvement over existing approximations.Comment: Accepted for publication in Ap
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