3,068 research outputs found
Decoding sequential vs non-sequential two-photon double ionization of helium using nuclear recoil
Above 54.4 eV, two-photon double ionization of helium is dominated by a
sequential absorption process, producing characteristic behavior in the single
and triple differential cross sections. We show that the signature of this
process is visible in the nuclear recoil cross section, integrated over all
energy sharings of the ejected electrons, even below the threshold for the
sequential process. Since nuclear recoil momentum imaging does not require
coincident photoelectron measurement, the predicted images present a viable
target for future experiments with new short-pulse VUV and soft X-ray sources.Comment: 4 pages, 3 figure
The Populations of Comet-Like Bodies in the Solar system
A new classification scheme is introduced for comet-like bodies in the Solar
system. It covers the traditional comets as well as the Centaurs and
Edgeworth-Kuiper belt objects. At low inclinations, close encounters with
planets often result in near-constant perihelion or aphelion distances, or in
perihelion-aphelion interchanges, so the minor bodies can be labelled according
to the planets predominantly controlling them at perihelion and aphelion. For
example, a JN object has a perihelion under the control of Jupiter and aphelion
under the control of Neptune, and so on. This provides 20 dynamically distinct
categories of outer Solar system objects in the Jovian and trans-Jovian
regions. The Tisserand parameter with respect to the planet controlling
perihelion is also often roughly constant under orbital evolution. So, each
category can be further sub-divided according to the Tisserand parameter. The
dynamical evolution of comets, however, is dominated not by the planets nearest
at perihelion or aphelion, but by the more massive Jupiter. The comets are
separated into four categories -- Encke-type, short-period, intermediate and
long-period -- according to aphelion distance. The Tisserand parameter
categories now roughly correspond to the well-known Jupiter-family comets,
transition-types and Halley-types. In this way, the nomenclature for the
Centaurs and Edgeworth-Kuiper belt objects is based on, and consistent with,
that for comets.Comment: MNRAS, in press, 11 pages, 6 figures (1 available as postscript, 5 as
gif). Higher resolution figures available at
http://www-thphys.physics.ox.ac.uk/users/WynEvans/preprints.pd
Discovery of a very X-ray luminous galaxy cluster at z=0.89 in the WARPS survey
We report the discovery of the galaxy cluster ClJ1226.9+3332 in the Wide
Angle ROSAT Pointed Survey (WARPS). At z=0.888 and L_X=1.1e45 erg/s (0.5-2.0
keV, h_0=0.5) ClJ1226.9+3332 is the most distant X-ray luminous cluster
currently known. The mere existence of this system represents a huge problem
for Omega_0=1 world models.
At the modest (off-axis) resolution of the ROSAT PSPC observation in which
the system was detected, ClJ1226.9+3332 appears relaxed; an off-axis HRI
observation confirms this impression and rules out significant contamination
from point sources. However, in moderately deep optical images (R and I band)
the cluster exhibits signs of substructure in its apparent galaxy distribution.
A first crude estimate of the velocity dispersion of the cluster galaxies based
on six redshifts yields a high value of 1650 km/s, indicative of a very massive
cluster and/or the presence of substructure along the line of sight. While a
more accurate assessment of the dynamical state of this system requires much
better data at both optical and X-ray wavelengths, the high mass of the cluster
has already been unambiguously confirmed by a very strong detection of the
Sunyaev-Zel'dovich effect in its direction (Joy et al. 2001).
Using ClJ1226.9+3332 and ClJ0152.7-1357 (z=0.835), the second-most distant
X-ray luminous cluster currently known and also a WARPS discovery, we obtain a
first estimate of the cluster X-ray luminosity function at 0.8<z<1.4 and
L_X>5e44 erg/s. Using the best currently available data, we find the comoving
space density of very distant, massive clusters to be in excellent agreement
with the value measured locally (z<0.3), and conclude that negative evolution
is not required at these luminosities out to z~1. (truncated)Comment: accepted for publication in ApJ Letters, 6 pages, 2 figures, uses
emulateapj.st
A New Kinematic Distance Estimator to the LMC
The distance to the Large Magellanic Cloud (LMC) can be directly determined
by measuring three of its properties, its radial-velocity field, its mean
proper motion, and the position angle \phi_ph of its photometric line of nodes.
Statistical errors of 2% are feasible based on proper motions obtained with any
of several proposed astrometry satellites, the first possibility being the
Full-Sky Astrometric Mapping Explorer (FAME). The largest source of systematic
error is likely to be in the determination of \phi_ph. I suggest two
independent methods to measure \phi_ph, one based on counts of clump giants and
the other on photometry of clump giants. I briefly discuss a variety of methods
to test for other sources of systematic errors.Comment: submitted to ApJ, 13 page
Dynamics of Learning with Restricted Training Sets I: General Theory
We study the dynamics of supervised learning in layered neural networks, in
the regime where the size of the training set is proportional to the number
of inputs. Here the local fields are no longer described by Gaussian
probability distributions and the learning dynamics is of a spin-glass nature,
with the composition of the training set playing the role of quenched disorder.
We show how dynamical replica theory can be used to predict the evolution of
macroscopic observables, including the two relevant performance measures
(training error and generalization error), incorporating the old formalism
developed for complete training sets in the limit as a
special case. For simplicity we restrict ourselves in this paper to
single-layer networks and realizable tasks.Comment: 39 pages, LaTe
The WARPS survey - IV: The X-ray luminosity-temperature relation of high redshift galaxy clusters
We present a measurement of the cluster X-ray luminosity-temperature relation
out to high redshift (z~0.8). Combined ROSAT PSPC spectra of 91 galaxy clusters
detected in the Wide Angle ROSAT Pointed Survey (WARPS) are simultaneously fit
in redshift and luminosity bins. The resulting temperature and luminosity
measurements of these bins, which occupy a region of the high redshift L-T
relation not previously sampled, are compared to existing measurements at low
redshift in order to constrain the evolution of the L-T relation. We find a
best fit to low redshift (z1 keV, to be L proportional
to T^(3.15\pm0.06). Our data are consistent with no evolution in the
normalisation of the L-T relation up to z~0.8. Combining our results with ASCA
measurements taken from the literature, we find eta=0.19\pm0.38 (for Omega_0=1,
with 1 sigma errors) where L_Bol is proportional to (1 + z)^eta T^3.15, or
eta=0.60\pm0.38 for Omega_0=0.3. This lack of evolution is considered in terms
of the entropy-driven evolution of clusters. Further implications for
cosmological constraints are also discussed.Comment: 11 pages, 7 figures, accepted for publication in MNRA
Exoplanets or Dynamic Atmospheres? The Radial Velocity and Line Shape Variations of 51 Pegasi and Tau Bootis
Because of our relatively low spectral resolution, we compare our
observations with Gray's line bisector data by fitting observed line profiles
to an expansion in terms of orthogonal (Hermite) functions. To obtain an
accurate comparison, we model the emergent line profiles from rotating and
pulsating stars, taking the instrumental point spread function into account. We
describe this modeling process in detail.
We find no evidence for line profile or strength variations at the radial
velocity period in either 51 Peg or in Tau Boo. For 51 Peg, our upper limit for
line shape variations with 4.23-day periodicity is small enough to exclude with
10 sigma confidence the bisector curvature signal reported by Gray & Hatzes;
the bisector span and relative line depth signals reported by Gray (1997) are
also not seen, but in this case with marginal (2 sigma) confidence. We cannot,
however, exclude pulsations as the source of 51 Peg's radial velocity
variation, because our models imply that line shape variations associated with
pulsations should be much smaller than those computed by Gray & Hatzes; these
smaller signals are below the detection limits both for Gray & Hatzes' data and
for our own.
Tau Boo's large radial velocity amplitude and v*sin(i) make it easier to test
for pulsations in this star. Again we find no evidence for periodic line-shape
changes, at a level that rules out pulsations as the source of the radial
velocity variability. We conclude that the planet hypothesis remains the most
likely explanation for the existing data.Comment: 44 pages, 19 figures, plain TeX, accepted to ApJS (companion to
letter astro-ph/9712279
Winds of Planet Hosting Stars
The field of exoplanetary science is one of the most rapidly growing areas of
astrophysical research. As more planets are discovered around other stars, new
techniques have been developed that have allowed astronomers to begin to
characterise them. Two of the most important factors in understanding the
evolution of these planets, and potentially determining whether they are
habitable, are the behaviour of the winds of the host star and the way in which
they interact with the planet. The purpose of this project is to reconstruct
the magnetic fields of planet hosting stars from spectropolarimetric
observations, and to use these magnetic field maps to inform simulations of the
stellar winds in those systems using the Block Adaptive Tree Solar-wind Roe
Upwind Scheme (BATS-R-US) code. The BATS-R-US code was originally written to
investigate the behaviour of the Solar wind, and so has been altered to be used
in the context of other stellar systems. These simulations will give
information about the velocity, pressure and density of the wind outward from
the host star. They will also allow us to determine what influence the winds
will have on the space weather environment of the planet. This paper presents
the preliminary results of these simulations for the star Bo\"otis,
using a newly reconstructed magnetic field map based on previously published
observations. These simulations show interesting structures in the wind
velocity around the star, consistent with the complex topology of its magnetic
field.Comment: 8 pages, 2 figures, accepted for publication in the peer-reviewed
proceedings of the 14th Australian Space Research Conference, held at the
University of South Australia, 29th September - 1st October 201
Observing Strategies for the Detection of Jupiter Analogs
To understand the frequency, and thus the formation and evolution, of planetary systems like our own solar system, it is critical to detect Jupiter-like planets in Jupiter-like orbits. For long-term radial-velocity monitoring, it is useful to estimate the observational effort required to reliably detect such objects, particularly in light of severe competition for limited telescope time. We perform detailed simulations of observational campaigns, maximizing the realism of the sampling of a set of simulated observations. We then compute the detection limits for each campaign to quantify the effect of increasing the number of observational epochs and varying their time coverage. We show that once there is sufficient time baseline to detect a given orbital period, it becomes less effective to add further time coverage-rather, the detectability of a planet scales roughly as the square root of the number of observations, independently of the number of orbital cycles included in the data string. We also show that no noise floor is reached, with a continuing improvement in detectability at the maximum number of observations N = 500 tested here.Peer reviewe
- …