848 research outputs found
Noisy Monte Carlo: Convergence of Markov chains with approximate transition kernels
Monte Carlo algorithms often aim to draw from a distribution by
simulating a Markov chain with transition kernel such that is
invariant under . However, there are many situations for which it is
impractical or impossible to draw from the transition kernel . For instance,
this is the case with massive datasets, where is it prohibitively expensive to
calculate the likelihood and is also the case for intractable likelihood models
arising from, for example, Gibbs random fields, such as those found in spatial
statistics and network analysis. A natural approach in these cases is to
replace by an approximation . Using theory from the stability of
Markov chains we explore a variety of situations where it is possible to
quantify how 'close' the chain given by the transition kernel is to
the chain given by . We apply these results to several examples from spatial
statistics and network analysis.Comment: This version: results extended to non-uniformly ergodic Markov chain
Extended Stromgren Photoelectric Photometry in NGC 752
Photoelectric photometry on the extended Stromgren system (uvbyCa) is
presented for 7 giants and 21 main sequence stars in the old open cluster, NGC
752. Analysis of the hk data for the turnoff stars yields a new determination
of the cluster mean metallicity. From 10 single-star members, [Fe/H] = -0.06
+/- 0.03, where the error quoted is the standard error of the mean and the
Hyades abundance is set at [Fe/H] = +0.12. This result is unchanged if all 20
stars within the limits of the hk metallicity calibration are included. The
derived [Fe/H] is in excellent agreement with past estimates using
properly-zeroed m1 data, transformed moderate-dispersion spectroscopy, and
recent high dispersion spectroscopy.Comment: 14 tex'd pages including 2 tables; 2 separate files with eps figures
Accepted for PASP March 200
Mine Fire Detection in the Presence Of Diesel Emissions
A series of four coal combustion experiments was conducted at the National Institute for Occupational Safety and Health\u27s (NIOSH) Pittsburgh Research Laboratory (PRL) in the Safety Research Coal Mine (SRCM) to evaluate the response of fire sensors to a small 0.61 m square smoldering coal fire which transitions to flaming combustion in the presence of diesel emissions. An optical path smoke sensor alarmed earlier than a point type diffusion mode ionization smoke sensor, which alarmed prior to a co alert value of 5 PPM above ambient. The presence of steady state diesel emissions resulted in a decrease in the optical smoke sensor analog output voltage signal by less than 1.4 pet for the three coal fire experiments in which a diesel engine was operating, whereas the ionization smoke sensor output decreased between 10.8 and 26.7 pet after the initial surge of the diesel engine. A commercial diesel discriminating fire sensor did not alarm for a fire in the one experiment for which it was used. The results of the experiments demonstrated that an optical path smoke sensor might be used to detect a coal fire under the experimental conditions considered of starting a diesel engine followed by a slowly developing coal fire
Stellar Evolution in NGC 6791: Mass Loss on the Red Giant Branch and the Formation of Low Mass White Dwarfs
We present the first detailed study of the properties (temperatures,
gravities, and masses) of the NGC 6791 white dwarf population. This unique
stellar system is both one of the oldest (8 Gyr) and most metal-rich ([Fe/H] ~
0.4) open clusters in our Galaxy, and has a color-magnitude diagram (CMD) that
exhibits both a red giant clump and a much hotter extreme horizontal branch.
Fitting the Balmer lines of the white dwarfs in the cluster, using Keck/LRIS
spectra, suggests that most of these stars are undermassive, = 0.43 +/-
0.06 Msun, and therefore could not have formed from canonical stellar evolution
involving the helium flash at the tip of the red giant branch. We show that at
least 40% of NGC 6791's evolved stars must have lost enough mass on the red
giant branch to avoid the flash, and therefore did not convert helium into
carbon-oxygen in their core. Such increased mass loss in the evolution of the
progenitors of these stars is consistent with the presence of the extreme
horizontal branch in the CMD. This unique stellar evolutionary channel also
naturally explains the recent finding of a very young age (2.4 Gyr) for NGC
6791 from white dwarf cooling theory; helium core white dwarfs in this cluster
will cool ~3 times slower than carbon-oxygen core stars and therefore the
corrected white dwarf cooling age is in fact ~7 Gyr, consistent with the well
measured main-sequence turnoff age. These results provide direct empirical
evidence that mass loss is much more efficient in high metallicity environments
and therefore may be critical in interpreting the ultraviolet upturn in
elliptical galaxies.Comment: 15 pages, 9 figures, 2 tables. Accepted for publication in Astrophys.
J. Very minor changes from first versio
Abundance Gradients and the Formation of the Milky Way
In this paper we adopt a chemical evolution model, which is an improved
version of the Chiappini, Matteucci and Gratton (1997) model, assuming two main
accretion episodes for the formation of the Galaxy. The present model takes
into account in more detail than previously the halo density distribution and
explores the effects of a threshold density in the star formation process,
during both the halo and disk phases. In the comparison between model
predictions and available data, we have focused our attention on abundance
gradients as well as gas, stellar and star formation rate distributions along
the disk. We suggest that the mechanism for the formation of the halo leaves
detectable imprints on the chemical properties of the outer regions of the
disk, whereas the evolution of the halo and the inner disk are almost
completely disentangled. This is due to the fact that the halo and disk
densities are comparable at large Galactocentric distances and therefore the
gas lost from the halo can substantially contribute to building up the outer
disk. We also show that the existence of a threshold density for the star
formation rate, both in the halo and disk phase, is necessary to reproduce the
majority of observational data in the solar vicinity and in the whole disk.
Moreover, we predict that the abundance gradients along the Galactic disk must
have increased with time and that the average [alpha/Fe] ratio in stars (halo
plus disk) slightly decrease going from 4 to 10 Kpcs from the Galactic center.
We also show that the same ratios increase substantially towards the outermost
disk regions and the expected scatter in the stellar ages decreases, because
the outermost regions are dominated by halo stars.Comment: 41 pages (including the figures), To be published in Ap
Bayesian Parameter Estimation for Latent Markov Random Fields and Social Networks
Undirected graphical models are widely used in statistics, physics and
machine vision. However Bayesian parameter estimation for undirected models is
extremely challenging, since evaluation of the posterior typically involves the
calculation of an intractable normalising constant. This problem has received
much attention, but very little of this has focussed on the important practical
case where the data consists of noisy or incomplete observations of the
underlying hidden structure. This paper specifically addresses this problem,
comparing two alternative methodologies. In the first of these approaches
particle Markov chain Monte Carlo (Andrieu et al., 2010) is used to efficiently
explore the parameter space, combined with the exchange algorithm (Murray et
al., 2006) for avoiding the calculation of the intractable normalising constant
(a proof showing that this combination targets the correct distribution in
found in a supplementary appendix online). This approach is compared with
approximate Bayesian computation (Pritchard et al., 1999). Applications to
estimating the parameters of Ising models and exponential random graphs from
noisy data are presented. Each algorithm used in the paper targets an
approximation to the true posterior due to the use of MCMC to simulate from the
latent graphical model, in lieu of being able to do this exactly in general.
The supplementary appendix also describes the nature of the resulting
approximation.Comment: 26 pages, 2 figures, accepted in Journal of Computational and
Graphical Statistics (http://www.amstat.org/publications/jcgs.cfm
Hubble Space Telescope Observations of Element Abundances in Low-redshift Damped Lyman-alpha Galaxies and Implications for the Global Metallicity-Redshift Relation
Most models of cosmic chemical evolution predict that the mass-weighted mean
interstellar metallicity of galaxies should rise with time from a low value
solar at to a nearly solar value at . In the
absence of any selection effects, the damped Lyman-alpha absorbers (DLAs) in
quasar spectra are expected to show such a rise in global metallicity. However,
it has been difficult to determine whether or not DLAs show this effect,
primarily because of the very small number of DLA metallicity measurements at
low redshifts. In an attempt to put tighter constraints on the low-redshift end
of the DLA metallicity-redshift relation, we have observed Zn II and Cr II
lines in four DLAs at , using the Space Telescope Imaging
Spectrograph (STIS) onboard the Hubble Space Telescope (HST). These
observations have provided the first constraints on Zn abundances in DLAs with
. In all the three DLAs for which our observations offer meaningful
constraints on the metallicity, the data suggest that the metallicities are
much lower than the solar value. These results are consistent with recent
imaging studies indicating that these DLAs may be associated with dwarf or low
surface brightness galaxies. We combine our results with higher redshift data
from the literature to estimate the global mean metallicity-redshift relation
for DLAs. We find that the global mean metallicity shows at most a slow
increase with decreasing redshift. ...(Please see the paper for the complete
abstract).Comment: 56 pages, including 13 figures. Accepted for publication in the
Astrophysical Journa
Transport behavior of holes in boron delta-doped diamond structures
Boron delta-doped diamond structures have been synthesized using microwave plasma chemical vapor deposition and fabricated into FET and gated Hall bar devices for assessment of the electrical characteristics. A detailed study of variable temperature Hall, conductivity, and field-effect mobility measurements was completed. This was supported by SchrâŹdinger-Poisson and relaxation time o calculations based upon application of Fermiâs golden rule. A two carrier-type model was developed with an activation energy of 1 cm2/Vs and the bulk valence band with high mobility. This new understanding of the transport of holes in such boron delta-doped structures has shown that although Hall mobility as high as 900 cm2/Vs was measured at room temperature, this dramatically overstates the actual useful performance of the device
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