4,714 research outputs found
Rotational modulation of X-ray emission in Orion Nebula young stars
We investigate the spatial distribution of X-ray emitting plasma in a sample
of young Orion Nebula Cluster stars by modulation of their X-ray light-curves
due to stellar rotation. The study, part of the Chandra Orion Ultradeep Project
(COUP), is made possible by the exceptional length of the observation: 10 days
of ACIS integration during a time span of 13 days, yielding a total of 1616
detected sources in the 17x17 arcmin field of view. We here focus on a
subsample of 233 X-ray-bright stars with known rotational periods. We search
for X-ray modulation using the Lomb Normalized Periodogram method.
X-ray modulation related to the rotation period is detected in at least 23
stars with periods between 2 and 12 days and relative amplitudes ranging from
20% to 70%. In 16 cases, the X-ray modulation period is similar to the stellar
rotation period while in seven cases it is about half that value, possibly due
to the presence of X-ray emitting structures at opposite stellar longitudes.
These results constitute the largest sample of low mass stars in which X-ray
rotational modulation has been observed. The detection of rotational modulation
indicates that the X-ray emitting regions are distributed inhomogeneneously in
longitude and do not extend to distances significantly larger than the stellar
radius. Modulation is observed in stars with saturated activity levels
(L_X/L_bol ~ 10^(-3)) showing that saturation is not due to the filling of the
stellar surface with X-ray emitting regions.Comment: 41 pages, 15 figures, ApJS in press. Figure 15 (34 panels) is an
on-line only figure and is not included. A pdf file which includes figure 15
as well as full resolution versions of figure 10 and 11 is available at:
http://www.astropa.unipa.it/~ettoref/COUP_RotMod.pd
Extremely Metal-Poor Stars. VII. The Most Metal-Poor Dwarf, CS 22876-032
We report high-resolution, high-signal-to-noise, observations of the
extremely metal-poor double-lined spectroscopic binary CS 22876-032. The system
has a long period : P = 424.7 0.6 days. It comprises two main sequence
stars having effective temperatures 6300 K and 5600 K, with a ratio of
secondary to primary mass of 0.89 0.04. The metallicity of the system is
[Fe/H] = -3.71 0.11 0.12 (random and systematic errors) -- somewhat
higher than previous estimates. We find [Mg/Fe] = 0.50, typical of values of
less extreme halo material. [Si/Fe], [Ca/Fe], and [Ti/Fe], however, all have
significantly lower values, ~ 0.0-0.1, suggesting that the heavier elements
might have been underproduced relative to Mg in the material from which this
object formed. In the context of the hypothesis that the abundance patterns of
extremely metal-poor stars are driven by individual enrichment events and the
models of Woosley and Weaver (1995), the data for CS 22876-032 are consistent
with its having been enriched by a zero-metallicity supernova of mass 30
M. As the most metal-poor near-main-sequence-turnoff star currently
known, the primary of the system has the potential to strongly constrain the
primordial lithium abundance. We find A(Li) (= log(N(Li)/N(H)) + 12.00) = 2.03
0.07, which is consistent with the finding of Ryan et al. (1999) that for
stars of extremely low metallicity A(Li) is a function of [Fe/H].Comment: 27 pages, 9 figures, accepted for publication in The Astrophysical
Journal, Sept. 1, 2000 issu
New order parameters in the Potts model on a Cayley tree
For the state Potts model new order parameters projecting on a group of
spins instead of a single spin are introduced. On a Cayley tree this allows the
physical interpretation of the Potts model at noninteger values q of the number
of states. The model can be solved recursively. This recursion exhibits chaotic
behaviour changing qualitatively at critical values of . Using an
additional order parameter belonging to a group of zero extrapolated size the
additional ordering is related to a percolation problem. This percolation
distinguishes different phases and explains the critical indices of percolation
class occuring at the Peierls temperature.Comment: 16 pages TeX, 5 figures PostScrip
Crystal structures and proton dynamics in potassium and cesium hydrogen bistrifluoroacetate salts with strong symmetric hydrogen bonds
The crystal structures of potassium and cesium bistrifluoroacetates were
determined at room temperature and at 20 K and 14 K, respectively, with the
single crystal neutron diffraction technique. The crystals belong to the I2/a
and A2/a monoclinic space groups, respectively, and there is no visible phase
transition. For both crystals, the trifluoroacetate entities form dimers linked
by very short hydrogen bonds lying across a centre of inversion. Any proton
disorder or double minimum potential can be rejected. The inelastic neutron
scattering spectral profiles in the OH stretching region between 500 and 1000
cm^{-1} previously published [Fillaux and Tomkinson, Chem. Phys. 158 (1991)
113] are reanalyzed. The best fitting potential has the major characteristics
already reported for potassium hydrogen maleate [Fillaux et al. Chem. Phys. 244
(1999) 387]. It is composed of a narrow well containing the ground state and a
shallow upper part corresponding to dissociation of the hydrogen bond.Comment: 31 pages, 7 figure
Two-dimensional wetting layer structures of reduced ternary oxides on Ru(0001) and Pt(111)
Long-range ordered structures of reduced oxide films with monolayer thickness derived from BaTiO3 and SrTiO3 on Ru(0001) and Pt(111) are investigated by scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED). Upon ultrahigh vacuum annealing at 1100 K, a hexagonal phase is observed for BaTiO3 on Ru(0001), which forms similarly from SrTiO3 on Pt(111). At higher temperatures, a triangle–square tiling called σ-phase develops in the BaTiO3/Ru(0001) system, with a unit cell rotation of 15° against the Ru(0001) substrate. Furthermore, it is shown that this 15° rotated σ-phase also forms in the BaTiO3/Pt(111) system in addition to the already known 8° rotated σ-phase. The results emphasize a strong flexibility in the structural parameters of the reduced oxide wetting layers in response to the substrate interaction strength
Do columnar defects produce bulk pinning?
From magneto-optical imaging performed on heavy-ion irradiated YBaCuO single
crystals, it is found that at fields and temperatures where strong single
vortex pinning by individual irradiation-induced amorphous columnar defects is
to be expected, vortex motion is limited by the nucleation of vortex kinks at
the specimen surface rather than by half-loop nucleation in the bulk. In the
material bulk, vortex motion occurs through (easy) kink sliding. Depinning in
the bulk determines the screening current only at fields comparable to or
larger than the matching field, at which the majority of moving vortices is not
trapped by an ion track.Comment: 5 pages, 5 figures, submitted to Physical Review Letter
A Practical Guide for X-Ray Diffraction Characterization of Ga(Al, In)N Alloys
Ga(In, Al)N alloys are used as an active layer or cladding layer in light
emitting diodes and laser diodes. x-ray diffraction is extensively used to
evaluate the crystalline quality, the chemical composition and the residual
strain in Ga(Al,In)N thin films, which directly determine the emission
wavelength and the device performance. Due to the minor mismatch in lattice
parameters between Ga(Al, In)N alloy and a GaN virtual substrate, x-ray
diffraction comes to a problem to separate the signal from Ga(Al,In)N alloy and
GaN. We give a detailed comparison on different diffraction planes. In order to
balance the intensity and peak separation between Ga(Al,In)N alloy and GaN,
(0004) and (1015) planes make the best choice for symmetric scan and asymmetric
scan, respectively.Comment: 9 pages, 5 figure
Transformation of dynamical fluctuation into coherent energy
Studies of noise-induced motions are showing that coherent energy can be
extracted from some kinds of noise in a periodic ratchet.
Recently, energetics of Langevin dynamics is formulated by Sekimoto
[J.Phys.Soc.Jpn, 66 1234 (1997)], which can be applied to ratchet systems
described by Fokker-Planck equation. In this paper, we derive an energetics of
ratchet systems that can be applied to dynamical-noise-induced motion in a
static potential. Analytical efficiency of the energy transformation is derived
for the dynamical noise in an overdumping limit of the system.
Comparison between analytical and numerical studies is performed for chaotic
noise.Comment: 3 pages, 2 figures; submitted to Phys. Rev. Let
Likelihood scan of the Super-Kamiokande I time series data
In this work a detailed spectral analysis of the time series of the 8B solar
neutrino flux published by the Super-Kamiokande Collaboration is presented,
performed through a likelihood scan approach. Preliminarily a careful review of
the analysis methodology is given, showing that the traditional periodicity
search via the Lomb-Scargle periodogram is a special case of a more general
likelihood based method. Since the data are published together with the
relevant asymmetric errors, it is then shown how the likelihood analysis can be
performed either with or without a prior error averaging. A key point of this
work is the detailed illustration of the mathematical model describing the
statistical properties of the estimated spectra obtained in the various cases,
which is also validated through extensive Monte Carlo computations; the model
includes a calculation for the prediction of the possible alias effects. In the
successive investigation of the data, such a model is used to derive objective,
mathematical predictions which are quantitatively compared with the features
observed in the experimental spectra. This article clearly demonstrates that
the handling of the errors is the origin of the discrepancy between published
null observations and claimed significant periodicity in the same SK-I data
sample. Moreover, the comprehensive likelihood analysis with asymmetric errors
developed in this work provides results which cannot exclude the null
hypothesis of constant rate, even though some indications stemming from the
model at odd with such conclusion point towards the desirability of additional
investigations with alternative methods to shed further light on the
characteristics of the data.Comment: 49 pages, 38 figures. Calculation of the asymmetric likelihood
revised. Accepted fo publication on Physical Review
Image of the Energy Gap Anisotropy in the Vibrational Spectum of a High Temperature Superconductor
We present a new method of determining the anisotropy of the gap function in
layered high-Tc superconductors. Careful inelastic neutron scattering
measurements at low temperature of the phonon dispersion curves in the (100)
direction in La_(1.85)Sr_(.15)CuO_4 would determine whether the gap is
predominately s-wave or d-wave. We also propose an experiment to determine the
gap at each point on a quasi-two-dimensional Fermi surface.Comment: 12 pages + 2 figures (included
- …