550,937 research outputs found
X-ray Observations of Parsec-Scale Tails behind Two Middle-Aged Pulsars
Chandra and XMM-Newton resolved extremely long tails behind two middle-aged
pulsars, J1509-5850 and J1740+1000. The tail of PSR J1509-5850 is discernible
up to 5.6' from the pulsar (6.5 pc at a distance of 4 kpc), with a flux of
2*10^{-13} erg s^{-1} cm^{-2} in 0.5-8 keV. The tail spectrum fits an absorbed
power-law (PL) model with the photon index of 2.3\pm0.2, corresponding to the
0.5-8 keV luminosity of 1*10^{33} ergs s^{-1}, for n_H= 2.1*10^{22} cm^{-2}.
The tail of PSR J1740+1000 is firmly detected up to 5' (2 pc at a 1.4 kpc
distance), with a flux of 6*10^{-14} ergs cm^{-2} s^{-1} in 0.4-10 keV. The PL
fit yields photon index of 1.4-1.5 and n_H=1*10^{21} cm^{-2}. The large extent
of the tails suggests that the bulk flow in the tails starts as mildly
relativistic downstream of the termination shock, and then gradually
decelerates. Within the observed extent of the J1509-5850 tail, the average
flow speed exceeds 5,000 km s^{-1}, and the equipartition magnetic field is a
few times 10^{-5} G. For the J1740+1000 tail, the equipartition field is a
factor of a few lower. The harder spectrum of the J1740+1000 tail implies
either less efficient cooling or a harder spectrum of injected electrons. For
the high-latitude PSR J1740+1000, the orientation of the tail on the sky shows
that the pulsar is moving toward the Galactic plane, which means that it was
born from a halo-star progenitor. The comparison between the J1509 and J1740
tails and the X-ray tails of other pulsars shows that the X-ray radiation
efficiency correlates poorly with the pulsar spin-down luminosity or age. The
X-ray efficiencies of the ram-pressure confined pulsar wind nebulae (PWNe) are
systematically higher than those of PWNe around slowly moving pulsars with
similar spin-down parameters.Comment: 14 pages, 16 figures and 5 table
Abundance anomalies in pre-main-sequence stars: Stellar evolution models with mass loss
The effects of atomic diffusion on internal and surface abundances of A and F
pre-main-sequence stars with mass loss are studied in order to determine at
what age the effects materialize, as well as to further understand the
processes at play in HAeBe and young ApBp stars. Self-consistent stellar
evolution models of 1.5 to 2.8Msun with atomic diffusion (including radiative
accelerations) for all species within the OPAL opacity database were computed
and compared to observations of HAeBe stars. Atomic diffusion in the presence
of weak mass loss can explain the observed abundance anomalies of
pre-main-sequence stars, as well as the presence of binary systems with metal
rich primaries and chemically normal secondaries such as V380 Ori and HD72106.
This is in contrast to turbulence models which do not allow for abundance
anomalies to develop on the pre-main-sequence. The age at which anomalies can
appear depends on stellar mass. For A and F stars, the effects of atomic
diffusion can modify both the internal and surface abundances before the onset
of the MS. The appearance of important surface abundance anomalies on the
pre-main-sequence does not require mass loss, though the mass loss rate affects
their amplitude. Observational tests are suggested to decipher the effects of
mass loss from those of turbulent mixing. If abundance anomalies are confirmed
in pre-main-sequence stars they would severely limit the role of turbulence in
these stars.Comment: 9 pages, 6 figures, accepeted for publicatio
Phonon spectral function for an interacting electron-phonon system
Using exact diagonalzation techniques, we study a model of interacting
electrons and phonons. The spectral width of the phonons is found to be reduced
as the Coulomb interaction U is increased. For a system with two modes per
site, we find a transfer of coupling strength from the upper to the lower mode.
This transfer is reduced as U is increased. These results give a qualitative
explanation of differences between Raman and photoemission estimates of the
electron-phonon coupling constants for A3C60 (A= K, Rb).Comment: 4 pages, RevTeX, 2 eps figur
Mg I emission lines at 12 and 18 micrometer in K giants
The solar Mg I emission lines at 12 micrometer have already been observed and
analyzed well. Previous modeling attempts for other stars have, however, been
made only for Procyon and two cool evolved stars, with unsatisfactory results
for the latter. We present high-resolution observational spectra for the K
giants Pollux, Arcturus, and Aldebaran, which show strong Mg I emission lines
at 12 micrometer as compared to the Sun. We also present the first observed
stellar emission lines from Mg I at 18 micrometer and from Al I, Si I, and
presumably Ca I at 12 micrometer. To produce synthetic line spectra, we employ
standard non-LTE modeling for trace elements in cool stellar photospheres. We
compute model atmospheres with the MARCS code, apply a comprehensive magnesium
model atom, and use the radiative transfer code MULTI to solve for the
magnesium occupation numbers in statistical equilibrium. We successfully
reproduce the observed Mg I emission lines simultaneously in the giants and in
the Sun, but show how the computed line profiles depend critically on atomic
input data and how the inclusion of energy levels with n > 9 and collisions
with neutral hydrogen are necessary to obtain reasonable fits.Comment: 9 pages, 6 figures, accepted for publication in Astronomy &
Astrophysic
Global constraints on muon-neutrino non-standard interactions
The search for new interactions of neutrinos beyond those of the Standard
Model may help to elucidate the mechanism responsible for neutrino masses. Here
we combine existing accelerator neutrino data with restrictions coming from a
recent atmospheric neutrino data analysis in order to lift parameter
degeneracies and improve limits on new interactions of muon neutrinos with
quarks. In particular we re-consider the results of the NuTeV experiment in
view of a new evaluation of its systematic uncertainties. We find that,
although constraints for muon neutrinos are better than those applicable to tau
or electron neutrinos, they lie at the few level, not as
strong as previously believed. We briefly discuss prospects for further
improvement.Comment: 10 pages, 5 figures, 2 table
The main transition in the Pink membrane model: finite-size scaling and the influence of surface roughness
We consider the main transition in single-component membranes using computer
simulations of the Pink model [D. Pink {\it et al.}, Biochemistry {\bf 19}, 349
(1980)]. We first show that the accepted parameters of the Pink model yield a
main transition temperature that is systematically below experimental values.
This resolves an issue that was first pointed out by Corvera and co-workers
[Phys. Rev. E {\bf 47}, 696 (1993)]. In order to yield the correct transition
temperature, the strength of the van der Waals coupling in the Pink model must
be increased; by using finite-size scaling, a set of optimal values is
proposed. We also provide finite-size scaling evidence that the Pink model
belongs to the universality class of the two-dimensional Ising model. This
finding holds irrespective of the number of conformational states. Finally, we
address the main transition in the presence of quenched disorder, which may
arise in situations where the membrane is deposited on a rough support. In this
case, we observe a stable multi-domain structure of gel and fluid domains, and
the absence of a sharp transition in the thermodynamic limit.Comment: submitted to PR
Scaling behavior of interactions in a modular quantum system and the existence of local temperature
We consider a quantum system of fixed size consisting of a regular chain of
-level subsystems, where is finite. Forming groups of subsystems
each, we show that the strength of interaction between the groups scales with
. As a consequence, if the total system is in a thermal state with
inverse temperature , a sufficient condition for subgroups of size
to be approximately in a thermal state with the same temperature is , where is the width of the occupied
level spectrum of the total system. These scaling properties indicate on what
scale local temperatures may be meaningfully defined as intensive variables.
This question is particularly relevant for non-equilibrium scenarios such as
heat conduction etc.Comment: 7 pages, accepted for publication in Europhysics Letter
- …