12,804 research outputs found
Chemical abundances for Hf 2-2, a planetary nebula with the strongest known heavy element recombination lines
We present high quality optical spectroscopic observations of the planetary
nebula (PN) Hf 2-2. The spectrum exhibits many prominent optical recombination
lines (ORLs) from heavy element ions. Analysis of the H {\sc i} and He {\sc i}
recombination spectrum yields an electron temperature of K, a factor
of ten lower than given by the collisionally excited [O {\sc iii}] forbidden
lines. The ionic abundances of heavy elements relative to hydrogen derived from
ORLs are about a factor of 70 higher than those deduced from collisionally
excited lines (CELs) from the same ions, the largest abundance discrepancy
factor (adf) ever measured for a PN. By comparing the observed O {\sc ii}
4089/4649 ORL ratio to theoretical value as a function of
electron temperature, we show that the O {\sc ii} ORLs arise from ionized
regions with an electron temperature of only K. The current
observations thus provide the strongest evidence that the nebula contains
another previously unknown component of cold, high metallicity gas, which is
too cool to excite any significant optical or UV CELs and is thus invisible via
such lines. The existence of such a plasma component in PNe provides a natural
solution to the long-standing dichotomy between nebular plasma diagnostics and
abundance determinations using CELs on the one hand and ORLs on the other.Comment: 12 pages, 5 figures, accepted for publication in the Monthly Notices
of the Royal Astronomical Societ
Unravelling the chemical inhomogeneity of PNe with VLT FLAMES integral-field unit spectroscopy
Recent weak emission-line long-slit surveys and modelling studies of PNe have
convincingly argued in favour of the existence of an unknown component in the
planetary nebula plasma consisting of cold, hydrogen-deficient gas, as an
explanation for the long-standing recombination-line versus forbidden-line
temperature and abundance discrepancy problems. Here we describe the rationale
and initial results from a detailed spectroscopic study of three Galactic PNe
undertaken with the VLT FLAMES integral-field unit spectrograph, which advances
our knowledge about the small-scale physical properties, chemical abundances
and velocity structure of these objects across a two-dimensional field of view,
and opens up for exploration an uncharted territory in the study and modelling
of PNe and photoionized nebulae in general.Comment: 4 pages; 3 figures; invited paper to appear in proceedings of IAU
Symp. No. 234, 2006, Planetary Nebulae in our Galaxy and Beyond (held in
Hawaii, April 2006
Stress/strain Distributions and Role of Sheathing in Partition Wall Panels Subjected to Compression
Built upon a companion study [1] on the load carrying capacity of cold-formed steel wall panels, the stress/strain distributions in each constituent of a panel - middle and side studs, top and bottom tracks, boards, and screws - are examined in detail with extensive use of strain gauges. These are subsequently synthesized to analyse the structural performance of the panel as a whole. Panels with 1-side sheathing and one middle stud were tested under vertical compressive loading. The main variables considered are screw spacing (300 mm, 400 rnm, or 600 mm) in the middle stud, board type (oriented strand board - OSB, cement particle board - CPB, or calcium silicate board - CSB), board number (no sheathing, one-side sheathing, or two-side sheathing), and loading type (1 or 3-point loading). The test results show that, at a given cross-section of the stud, the strains and stresses experienced by the flanges are significantly different from those in the web. Along the vertical direction, the stresses in the stud are not uniform, decreasing as the bottom track is approached. Screw connections between stud and board not only restrain the lateral displacement of the stud, but also support and re-distribute a portion of the machine load to the board and then to the bottom track. The axial forces experienced by the screws are negligibly small during the initial stage of loading, increasing slowly as the load is further increased until substantial stud buckling occurs. Buckling drastically increases the forces acting on a screw, often resulting in its pulling-out from the board and studs/tracks. The results also show that the role of board in a partition wall panel is multi-fold. It acts as a shearing member to steady the whole structure, as a supporting member to enhance the overall and local buckling performance of middle and side studs, and as a structural member to support part of the machine load. It is important account for the contributions of load-sharing boards (via screw connections) when designing cold-formed steel wall panels against minimum weight
Crossing Statistic: Bayesian interpretation, model selection and resolving dark energy parametrization problem
By introducing Crossing functions and hyper-parameters I show that the
Bayesian interpretation of the Crossing Statistics [1] can be used trivially
for the purpose of model selection among cosmological models. In this approach
to falsify a cosmological model there is no need to compare it with other
models or assume any particular form of parametrization for the cosmological
quantities like luminosity distance, Hubble parameter or equation of state of
dark energy. Instead, hyper-parameters of Crossing functions perform as
discriminators between correct and wrong models. Using this approach one can
falsify any assumed cosmological model without putting priors on the underlying
actual model of the universe and its parameters, hence the issue of dark energy
parametrization is resolved. It will be also shown that the sensitivity of the
method to the intrinsic dispersion of the data is small that is another
important characteristic of the method in testing cosmological models dealing
with data with high uncertainties.Comment: 14 pages, 4 figures, discussions extended, 1 figure and two
references added, main results unchanged, matches the final version to be
published in JCA
Biased tomography schemes: an objective approach
We report on an intrinsic relationship between the maximum-likelihood
quantum-state estimation and the representation of the signal. A quantum
analogy of the transfer function determines the space where the reconstruction
should be done without the need for any ad hoc truncations of the Hilbert
space. An illustration of this method is provided by a simple yet practically
important tomography of an optical signal registered by realistic binary
detectors.Comment: 4 pages, 3 figures, accepted in PR
The Alexander-Orbach conjecture holds in high dimensions
We examine the incipient infinite cluster (IIC) of critical percolation in
regimes where mean-field behavior has been established, namely when the
dimension d is large enough or when d>6 and the lattice is sufficiently spread
out. We find that random walk on the IIC exhibits anomalous diffusion with the
spectral dimension d_s=4/3, that is, p_t(x,x)= t^{-2/3+o(1)}. This establishes
a conjecture of Alexander and Orbach. En route we calculate the one-arm
exponent with respect to the intrinsic distance.Comment: 25 pages, 2 figures. To appear in Inventiones Mathematica
Parent-infant psychotherapy: a systematic review of the evidence for improving parental and infant mental health
BACKGROUND: Parentâinfant psychotherapy (PIP) is a psychodynamic intervention with parentâinfant dyads, designed to address regulatory disturbances in infancy and problems in the parentâinfant relationship.
AIMS: This systematic review aimed to examine whether PIP is effective in improving the parentâinfant relationship or other aspects of parent or infant functioning.
METHODS: A systematic review was undertaken. Electronic databases were searched for randomised controlled trials in which participants had been allocated to a PIP intervention or control group/other treatment.
RESULTS: Eight studies were identified that provided data comparing parentâinfant psychotherapy with a no-treatment control group (four studies) or comparing PIP with other kinds of treatment (four studies). Meta-analyses indicated that parents who received PIP were more likely to have an infant who was rated as being securely attached to the parent after the intervention; however, there were no significant differences in studies comparing outcomes of PIP with another model of treatment.
CONCLUSIONS: Although PIP appears to be a promising method of improving infant attachment security, there is inconclusive evidence of its benefits in terms of other outcomes, and no evidence to show that it is more effective than other interventions for parents and infants. Many studies had limitations in their design or implementation, and findings must be interpreted with caution
Transition density of diffusion on Sierpinski gasket and extension of Flory's formula
Some problems related to the transition density u(t,x) of the diffusion on
the Sierpinski gasket are considerd, based on recent rigorous results and
detailed numerical calculations. The main contents are an extension of Flory's
formula for the end-to-end distance exponent of self-avoiding walks on the
fractal spaces, and an evidence of the oscillatory behavior of u(t,x) on the
Sierpinski gasket.Comment: 11 pages, REVTEX, 2 postscript figure
Accelerator system for the PRISM based muon to electron conversion experiment
The next generation of lepton flavor violation experiments need high
intensity and high quality muon beams. Production of such beams requires
sending a short, high intensity proton pulse to the pion production target,
capturing pions and collecting the resulting muons in the large acceptance
transport system. The substantial increase of beam quality can be obtained by
applying the RF phase rotation on the muon beam in the dedicated FFAG ring,
which was proposed for the PRISM project.This allows to reduce the momentum
spread of the beam and to purify from the unwanted components like pions or
secondary protons. A PRISM Task Force is addressing the accelerator and
detector issues that need to be solved in order to realize the PRISM
experiment. The parameters of the required proton beam, the principles of the
PRISM experiment and the baseline FFAG design are introduced. The spectrum of
alternative designs for the PRISM FFAG ring are shown. Progress on ring main
systems like injection and RF are presented. The current status of the study
and its future directions are discussed.Comment: Studies performed within the PRISM Task Force initiativ
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