1,638 research outputs found
Ni-impurity effects on the superconducting gap of LaSrCuO studied from the magnetic field and temperature dependence of the electronic specific heat
The magnetic field and temperature dependence of the electronic specific heat
have been systematically investigated in (LSCNO) in order to study Ni-impurity
effects on the superconducting (SC) gap. In LSCNO with =0.15 and =0.015,
the value of () at =0 K, , is
enhanced under the magnetic field applied along the -axis. The
increment of , , follows the Volovik relation
=, characteristic of the SC gap with line nodes,
with prefactor similar to that of a pure sample. The vs.
curve under =0 shows a d-wave-like SC anomaly with an abrupt increase at
and -linear dependence at , although the
-value in the vs. curve increases with increasing
Ni concentrations. Interestingly, as the SC part of , , decreases in LSCNO, is
reduced in proportion to the decrease of . These findings can
be explained phenomenologically by a simple model in which Ni impurities bring
about strong pair breaking at the edges of the coherent nodal part of the Fermi
surface but in the vicinity of the nodes of the SC gap. The reduction of the SC
condensation energy in LSCNO, evaluated from at
{0.3em}\raisebox{0.4ex}{} {-0.75em}\raisebox{-.7ex}{} {0.3em}, is also understood by the same model.Comment: 7 pages, 6 figures, accepted in Phys. Rev.
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
Chiral charge-density-waves
We discovered the chirality of charge density waves (CDW) in 1T-TiSe by
using scanning tunnelling microscopy (STM) and optical ellipsometry. We found
that the CDW intensity becomes , where (i =1, 2, 3) is the amplitude of the tunnelling current
contributed by the CDWs. There were two states, in which the three intensity
peaks of the CDW decrease \textit{clockwise} and \textit{anticlockwise} when we
index each nesting vector in order of intensity in the Fourier transformation
of the STM images. The chirality in CDW results in the three-fold symmetry
breaking. Macroscopically, two-fold symmetry was indeed observed in optical
measurement. We propose the new generalized CDW chirality H_{CDW} \equiv
{\boldmath q_1} \cdot ({\boldmath q_2}\times {\boldmath q_3}), where
{\boldmath q_i} are the nesting vectors, which is independent of the
symmetry of components. The nonzero - the triple-{\boldmath q}
vectors do not exist in an identical plane in the reciprocal space - should
induce a real-space chirality in CDW system.Comment: 12 pages, 4 figure
MHD Simulations of Magnetospheric Accretion, Ejection and Plasma-field Interaction
We review recent axisymmetric and three-dimensional (3D) magnetohydrodynamic
(MHD) numerical simulations of magnetospheric accretion, plasma-field
interaction and outflows from the disk-magnetosphere boundary.Comment: 11 pages, 8 figures, conference proceedings: "Physics at the
Magnetospheric Boundary", Geneva, Switzerland, 25-28 June, 201
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
Coronal structure of the cTTS V2129 Oph
The nature of the magnetic coupling between T Tauri stars and their disks
determines not only the mass accretion process but possibly the spin evolution
of the central star. We have taken a recently-published surface magnetogram of
one moderately-accreting T Tauri star (V2129 Oph) and used it to extrapolate
the geometry of its large-scale field. We determine the structure of the open
(wind-bearing) field lines, the closed (X-ray bright) field lines and those
potentially accreting field lines that pass through the equatorial plane inside
the Keplerian co-rotation radius. We consider a series of models in which the
stellar magnetic field is opened up by the outward pressure of the hot coronal
gas at a range of radii. As this radius is increased, accretion takes place
along simpler field structures and impacts on fewer sites at the stellar
surface. This is consistent with the observed variation in the Ca II IRT and
HeI lines which suggests that accretion in the visible hemisphere is confined
to a single high-latitude spot. By determining the density and velocity of the
accretion flows, we find that in order to have most of the total mass accretion
rate impacting on a single high-latitude region we need disk material to
accrete from approximately 7R*, close to the Keplerian co-rotation radius at
6.8R*. We also calculate the coronal density and X-ray emission measure. We
find that both the magnitude and rotational modulation of the emission measure
increase as the source surface is increased. For the field structure of V2129
Oph which is dominantly octupolar, the emission forms a bright, high-latitude
ring that is always in view as the star rotates. Since the accretion funnels
are not dense enough to cause significant scattering of coronal X-ray photons,
they provide only a low rotational modulation of around 10% at most.Comment: 10 pages, 9 figure
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
Spectral line profiles changed by dust scattering in heavily obscured young stellar objects
It is known that scattering of radiation by circumstellar dust can strongly
change the line profiles in stellar spectra. This hampers the analysis of
spectral lines originating in the emitting regions of heavily obscured young
stars. To calculate the line profile of the scattered radiation, we suggest to
use the approximation of remote scattering particles. This approximation
assumes that the scattering dust grains are at a distance from the star that is
much larger than the characteristic size of the emitting region. Using this
method, we calculated the line profiles of several simple models. They show the
H alpha line profiles of Herbig AeBe stars in the presence and absence of
motionless or moving dust
Combinatorial Bounds and Characterizations of Splitting Authentication Codes
We present several generalizations of results for splitting authentication
codes by studying the aspect of multi-fold security. As the two primary
results, we prove a combinatorial lower bound on the number of encoding rules
and a combinatorial characterization of optimal splitting authentication codes
that are multi-fold secure against spoofing attacks. The characterization is
based on a new type of combinatorial designs, which we introduce and for which
basic necessary conditions are given regarding their existence.Comment: 13 pages; to appear in "Cryptography and Communications
Pre-main sequence stars with disks in the Eagle Nebula observed in scattered light
NGC6611 and its parental cloud, the Eagle Nebula (M16), are well-studied
star-forming regions, thanks to their large content of both OB stars and stars
with disks and the observed ongoing star formation. We identified 834
disk-bearing stars associated with the cloud, after detecting their excesses in
NIR bands from J band to 8.0 micron. In this paper, we study in detail the
nature of a subsample of disk-bearing stars that show peculiar characteristics.
They appear older than the other members in the V vs. V-I diagram, and/or they
have one or more IRAC colors at pure photospheric values, despite showing NIR
excesses, when optical and infrared colors are compared. We confirm the
membership of these stars to M16 by a spectroscopic analysis. The physical
properties of these stars with disks are studied by comparing their spectral
energy distributions (SEDs) with the SEDs predicted by models of T-Tauri stars
with disks and envelopes. We show that the age of these stars estimated from
the V vs. V-I diagram is unreliable since their V-I colors are altered by the
light scattered by the disk into the line of sight. Only in a few cases their
SEDs are compatible with models with excesses in V band caused by optical
veiling. Candidate members with disks and photospheric IRAC colors are selected
by the used NIR disk diagnostic, which is sensitive to moderate excesses, such
as those produced by disks with low masses. In 1/3 of these cases, scattering
of stellar flux by the disks can also be invoked. The photospheric light
scattered by the disk grains into the line of sight can affect the derivation
of physical parameters of ClassII stars from photometric optical and NIR data.
Besides, the disks diagnostic we defined are useful for selecting stars with
disks, even those with moderate excesses or whose optical colors are altered by
veiling or photospheric scattered light.Comment: Accepted for publication in A&
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