14,109 research outputs found
The Effects of radial inflow of gas and galactic fountains on the chemical evolution of M31
Galactic fountains and radial gas flows are very important ingredients in
modeling the chemical evolution of galactic disks. Our aim here is to study the
effects of galactic fountains and radial gas flows in the chemical evolution of
the disk of M31. We adopt a ballistic method to study the effects of galactic
fountains on the chemical enrichment of the M31 disk. We find that the landing
coordinate for the fountains in M31 is no more than 1 kpc from the starting
point, thus producing negligible effect on the chemical evolution of the disk.
We find that the delay time in the enrichment process due to fountains is no
longer than 100 Myr and this timescale also produces negligible effects on the
results. Then, we compute the chemical evolution of the M31 disk with radial
gas flows produced by the infall of extragalactic material and fountains. We
find that a moderate inside-out formation of the disk coupled with radial flows
of variable speed can very well reproduce the observed gradient. We discuss
also the effects of other parameters such a threshold in the gas density for
star formation and an efficiency of star formation varying with the galactic
radius. We conclude that the most important physical processes in creating disk
gradients are the inside-out formation and the radial gas flows. More data on
abundance gradients both locally and at high redshift are necessary to confirm
this conclusion.Comment: Accepted by A&
Chemical evolution of the bulge of M31: predictions about abundance ratios
We aim at reproducing the chemical evolution of the bulge of M31 by means of
a detailed chemical evolution model, including radial gas flows coming from the
disk. We study the impact of the initial mass function, the star formation rate
and the time scale for bulge formation on the metallicity distribution function
of stars. We compute several models of chemical evolution using the metallicity
distribution of dwarf stars as an observational constraint for the bulge of
M31. Then, by means of the model which best reproduces the metallicity
distribution function, we predict the [X/Fe] vs. [Fe/H] relations for several
chemical elements (O, Mg, Si, Ca, C, N). Our best model for the bulge of M31 is
obtained by means of a robust statistical method and assumes a Salpeter initial
mass function, a Schmidt-Kennicutt law for star formation with an exponent
k=1.5, an efficiency of star formation of , and an
infall timescale of Gyr. Our results suggest that the bulge
of M31 formed very quickly by means of an intense star formation rate and an
initial mass function flatter than in the solar vicinity but similar to that
inferred for the Milky Way bulge. The [/Fe] ratios in the stars of the
bulge of M31 should be high for most of the [Fe/H] range, as is observed in the
Milky Way bulge. These predictions await future data to be proven.Comment: Accepted for publication by MNRA
Formation and observation of a quasi-two-dimensional electron liquid in epitaxially stabilized SrLaTiO thin films
We report the formation and observation of an electron liquid in
SrLaTiO, the quasi-two-dimensional counterpart of SrTiO,
through reactive molecular-beam epitaxy and {\it in situ} angle-resolved
photoemission spectroscopy. The lowest lying states are found to be comprised
of Ti 3 orbitals, analogous to the LaAlO/SrTiO interface and
exhibit unusually broad features characterized by quantized energy levels and a
reduced Luttinger volume. Using model calculations, we explain these
characteristics through an interplay of disorder and electron-phonon coupling
acting co-operatively at similar energy scales, which provides a possible
mechanism for explaining the low free carrier concentrations observed at
various oxide heterostructures such as the LaAlO/SrTiO interface
Recent Advances in Percutaneous Cardioscopy
Percutaneous cardioscopy, using high-resolution fiberoptic imaging, enables direct visualization of the cardiac interior, thereby enabling macroscopic pathological diagnosis. Percutaneous cardioscopy has demonstrated that the endocardial surface exhibits various colors characteristic of different heart diseases. This imaging modality can now be used for evaluation of the severity of myocardial ischemia, and staging of myocarditis. Myocardial blood flow recovery induced by vasodilating agents or percutaneous coronary interventions can be clearly visualized. Morphological and functional changes in the cardiac valves can also be evaluated. Cardioscope-guided endomyocardial biopsy enables pin-point biopsy of the diseased myocardium. Recently, dye-image cardioscopy and fluorescence cardioscopy were developed for evaluation of the subendocardial microcirculation. Cardioscope-guided intracardiac therapies such as myotomy, myectomy, valvulotomy, and transendocardial angiogenic and myogenic therapy have been trialed using animal models in anticipation of future clinical applications. Percutaneous cardioscopy has the potential to contribute to our understanding of heart disease, and to assist in guidance for intracardiac therapies
Magnetohydrodynamic jets from different magnetic field configurations
Using axisymmetric MHD simulations we investigate how the overall jet
formation is affected by a variation in the disk magnetic flux profile and/or
the existence of a central stellar magnetosphere. Our simulations evolve from
an initial, hydrostatic equilibrium state in a force-free magnetic field
configuration. We find a unique relation between the collimation degree and the
disk wind magnetization power law exponent. The collimation degree decreases
for steeper disk magnetic field profiles. Highly collimated outflows resulting
from a flat profile tend to be unsteady. We further consider a magnetic field
superposed of a stellar dipole and a disk field in parallel or anti-parallel
alignment. Both stellar and disk wind may evolve in a pair of outflows,
however, a reasonably strong disk wind component is essential for jet
collimation. Strong flares may lead to a sudden change in mass flux by a factor
two. We hypothesize that such flares may eventually trigger jet knots.Comment: 5 pages, 4 figures; proceedings from conference: Protostellar Jets in
Context, held in Rhodes, July 7-12, 200
Twisted Superspace for N=D=2 Super BF and Yang-Mills with Dirac-K\"ahler Fermion Mechanism
We propose a twisted D=N=2 superspace formalism. The relation between the
twisted super charges including the BRST charge, vector and pseudo scalar super
charges and the N=2 spinor super charges is established. We claim that this
relation is essentially related with the Dirac-K\"ahler fermion mechanism. We
show that a fermionic bilinear form of twisted N=2 chiral and anti-chiral
superfields is equivalent to the quantized version of BF theory with the Landau
type gauge fixing while a bosonic bilinear form leads to the N=2 Wess-Zumino
action. We then construct a Yang-Mills action described by the twisted N=2
chiral and vector superfields, and show that the action is equivalent to the
twisted version of the D=N=2 super Yang-Mills action, previously obtained from
the quantized generalized topological Yang-Mills action with instanton gauge
fixing.Comment: 36 page
Highly anisotropic interlayer magnetoresistance in ZrSiS nodal-line Dirac semimetal
We instigate the angle-dependent magnetoresistance (AMR) of the layered
nodal-line Dirac semimetal ZrSiS for the in-plane and out-of-plane current
directions. This material has recently revealed an intriguing butterfly-shaped
in-plane AMR that is not well understood. Our measurements of the polar
out-of-plane AMR show a surprisingly different response with a pronounced
cusp-like feature. The maximum of the cusp-like anisotropy is reached when the
magnetic field is oriented in the - plane. Moreover, the AMR for the
azimuthal out-of-plane current direction exhibits a very strong four-fold
- plane anisotropy. Combining the Fermi surfaces calculated from first
principles with the Boltzmann's semiclassical transport theory we reproduce and
explain all the prominent features of the unusual behavior of the in-plane and
out-of-plane AMR. We are also able to clarify the origin of the strong
non-saturating transverse magnetoresistance as an effect of imperfect
charge-carrier compensation and open orbits. Finally, by combining our
theoretical model and experimental data we estimate the average relaxation time
of ~s and the mean free path of ~nm at 1.8~K in our
samples of ZrSiS.Comment: 8 pages, 4 figure
A cascade of magnetic field induced spin transitions in LaCoO3
We present magnetization and magnetostriction studies of the insulating
perovskite LaCoO3 in magnetic fields approaching 100 T. In marked contrast with
expectations from single-ion models, the data reveal two distinct first-order
spin transitions and well-defined magnetization plateaux. The magnetization at
the higher plateau is only about half the saturation value expected for spin-1
Co3+ ions. These findings strongly suggest collective behavior induced by
strong interactions between different electronic -- and therefore spin --
configurations of Co3+ ions. We propose a model of these interactions that
predicts crystalline spin textures and a cascade of four magnetic phase
transitions at high fields, of which the first two account for the experimental
data.Comment: 5 pages + supplementary materials, 5 figure
Evidence for forward scattering and coupling to acoustic phonon modes in high-T cuprate superconductors
Recent laser angle-resolved photoemission spectroscopy studies have
established the presence of a new kink in the low-energy nodal dispersion of
BiSrCaCuO (Bi-2212). The energy scale (~8-15 meV) of
this kink appears below the maximum of the superconducting gap .
Therefore it is difficult to interpret this feature in terms of the usual
coupling to a sharp dispersionless mode. In this paper we examine
electron-phonon coupling to the in-plane acoustic phonon branch arising from
the modulation of the screened Coulomb potential. We demonstrate that such a
coupling has a strong forward scattering peak, and as a consequence, a kink
occurs in the dispersion at an energy scale shifted by the local gap
. In addition, considerations for the reduction of screening with
underdoping naturally explains the observed doping dependence of the low-energy
kink. These results point to a strong coupling to the acoustic branch which is
peaked in the forward scattering direction and has important implications for
transport and pairing in the high-T cuprates.Comment: 4.5 pages, 4 figures, Submitted to PR
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