314 research outputs found
Second harmonic beam analysis, a sensitive technique to determine the duration of single ultrashort laser pulses
The second harmonic beam generated in a noncollinear arrangement allows the observation of the autocorrelation functio: A compact optical beam splitter and imaging system eliminates alignment problems. Single pulses of 1 ps duration and approximately 10−8 joule energy are readily observed using an optical multichannel analyser
A fluid analysis framework for a Markovian process algebra
Markovian process algebras, such as PEPA and stochastic π-calculus, bring a powerful compositional approach to the performance modelling of complex systems. However, the models generated by process algebras, as with other interleaving formalisms, are susceptible to the state space explosion problem. Models with only a modest number of process algebra terms can easily generate so many states that they are all but intractable to traditional solution techniques. Previous work aimed at addressing this problem has presented a fluid-flow approximation allowing the analysis of systems which would otherwise be inaccessible. To achieve this, systems of ordinary differential equations describing the fluid flow of the stochastic process algebra model are generated informally. In this paper, we show formally that for a large class of models, this fluid-flow analysis can be directly derived from the stochastic process algebra model as an approximation to the mean number of component types within the model. The nature of the fluid approximation is derived and characterised by direct comparison with the Chapman–Kolmogorov equations underlying the Markov model. Furthermore, we compare the fluid approximation with the exact solution using stochastic simulation and we are able to demonstrate that it is a very accurate approximation in many cases. For the first time, we also show how to extend these techniques naturally to generate systems of differential equations approximating higher order moments of model component counts. These are important performance characteristics for estimating, for instance, the variance of the component counts. This is very necessary if we are to understand how precise the fluid-flow calculation is, in a given modelling situation
Surface science and stability of networks prepared from hydroxy-terminated polydimethylsiloxane and methyltriethoxysilane
Polydimethylsiloxane (PDMS) hybrid networks have been prepared by the reaction of PDMS(OH) 2 , average molecular weight 26 14× 1410 3 , 43.6 14× 1410 3 and 58 14× 1410 3 , and methyltriethoxysilane (MeTEOS, 10–60 wt%) using a dibutyltin dilaurate or dibutyltin diacetate catalyst. By hydrolysis and homo- and co-condensation, MeTEOS forms a siliceous domain (MeSD) and acts as a crosslinker for the PDMS domain. Kinetic studies showed that high MeTEOS and catalyst concentrations and reduction of free surface area favor fast gelation and efficiency in converting MeTEOS to the MeSD. Under the water-sparse conditions utilized, cure was slow and substantial evaporative loss of MeTEOS occurred. © 1998 John Wiley & Sons, Ltd.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/38313/1/784_ftp.pd
CLASH: Precise New Constraints on the Mass Profile of Abell 2261
We precisely constrain the inner mass profile of Abell 2261 (z=0.225) for the
first time and determine this cluster is not "over-concentrated" as found
previously, implying a formation time in agreement with {\Lambda}CDM
expectations. These results are based on strong lensing analyses of new 16-band
HST imaging obtained as part of the Cluster Lensing and Supernova survey with
Hubble (CLASH). Combining this with revised weak lensing analyses of Subaru
wide field imaging with 5-band Subaru + KPNO photometry, we place tight new
constraints on the halo virial mass M_vir = 2.2\pm0.2\times10^15 M\odot/h70
(within r \approx 3 Mpc/h70) and concentration c = 6.2 \pm 0.3 when assuming a
spherical halo. This agrees broadly with average c(M,z) predictions from recent
{\Lambda}CDM simulations which span 5 <~ 8. Our most significant
systematic uncertainty is halo elongation along the line of sight. To estimate
this, we also derive a mass profile based on archival Chandra X-ray
observations and find it to be ~35% lower than our lensing-derived profile at
r2500 ~ 600 kpc. Agreement can be achieved by a halo elongated with a ~2:1 axis
ratio along our line of sight. For this elongated halo model, we find M_vir =
1.7\pm0.2\times10^15 M\odot/h70 and c_vir = 4.6\pm0.2, placing rough lower
limits on these values. The need for halo elongation can be partially obviated
by non-thermal pressure support and, perhaps entirely, by systematic errors in
the X-ray mass measurements. We estimate the effect of background structures
based on MMT/Hectospec spectroscopic redshifts and find these tend to lower
Mvir further by ~7% and increase cvir by ~5%.Comment: Submitted to the Astrophysical Journal. 19 pages, 14 figure
Observations of the pulsating subdwarf B star Feige 48: Constraints on evolution and companions
Since pulsating subdwarf B (sdBV or EC14026) stars were first discovered
(Kilkenny et al, 1997), observational efforts have tried to realize their
potential for constraining the interior physics of extreme horizontal branch
(EHB) stars. Difficulties encountered along the way include uncertain mode
identifications and a lack of stable pulsation mode properties. Here we report
on Feige 48, an sdBV star for which follow-up observations have been obtained
spanning more than four years, which shows some stable pulsation modes.
We resolve the temporal spectrum into five stable pulsation periods in the
range 340 to 380 seconds with amplitudes less than 1%, and two additional
periods that appear in one dataset each. The three largest amplitude
periodicities are nearly equally spaced, and we explore the consequences of
identifying them as a rotationally split l=1 triplet by consulting with a
representative stellar model.
The general stability of the pulsation amplitudes and phases allows us to use
the pulsation phases to constrain the timescale of evolution for this sdBV
star. Additionally, we are able to place interesting limits on any stellar or
planetary companion to Feige 48.Comment: accepted for publication in MNRA
Mugsy: fast multiple alignment of closely related whole genomes
Motivation: The relative ease and low cost of current generation sequencing technologies has led to a dramatic increase in the number of sequenced genomes for species across the tree of life. This increasing volume of data requires tools that can quickly compare multiple whole-genome sequences, millions of base pairs in length, to aid in the study of populations, pan-genomes, and genome evolution
Asteroseismology and Interferometry
Asteroseismology provides us with a unique opportunity to improve our
understanding of stellar structure and evolution. Recent developments,
including the first systematic studies of solar-like pulsators, have boosted
the impact of this field of research within Astrophysics and have led to a
significant increase in the size of the research community. In the present
paper we start by reviewing the basic observational and theoretical properties
of classical and solar-like pulsators and present results from some of the most
recent and outstanding studies of these stars. We centre our review on those
classes of pulsators for which interferometric studies are expected to provide
a significant input. We discuss current limitations to asteroseismic studies,
including difficulties in mode identification and in the accurate determination
of global parameters of pulsating stars, and, after a brief review of those
aspects of interferometry that are most relevant in this context, anticipate
how interferometric observations may contribute to overcome these limitations.
Moreover, we present results of recent pilot studies of pulsating stars
involving both asteroseismic and interferometric constraints and look into the
future, summarizing ongoing efforts concerning the development of future
instruments and satellite missions which are expected to have an impact in this
field of research.Comment: Version as published in The Astronomy and Astrophysics Review, Volume
14, Issue 3-4, pp. 217-36
Fast Statistical Alignment
We describe a new program for the alignment of multiple biological sequences that is both statistically motivated and fast enough for problem sizes that arise in practice. Our Fast Statistical Alignment program is based on pair hidden Markov models which approximate an insertion/deletion process on a tree and uses a sequence annealing algorithm to combine the posterior probabilities estimated from these models into a multiple alignment. FSA uses its explicit statistical model to produce multiple alignments which are accompanied by estimates of the alignment accuracy and uncertainty for every column and character of the alignment—previously available only with alignment programs which use computationally-expensive Markov Chain Monte Carlo approaches—yet can align thousands of long sequences. Moreover, FSA utilizes an unsupervised query-specific learning procedure for parameter estimation which leads to improved accuracy on benchmark reference alignments in comparison to existing programs. The centroid alignment approach taken by FSA, in combination with its learning procedure, drastically reduces the amount of false-positive alignment on biological data in comparison to that given by other methods. The FSA program and a companion visualization tool for exploring uncertainty in alignments can be used via a web interface at http://orangutan.math.berkeley.edu/fsa/, and the source code is available at http://fsa.sourceforge.net/
Adaptation of mammalian host-pathogen interactions in a changing arctic environment
Many arctic mammals are adapted to live year-round in extreme environments with low winter temperatures and great seasonal variations in key variables (e.g. sunlight, food, temperature, moisture). The interaction between hosts and pathogens in high northern latitudes is not very well understood with respect to intra-annual cycles (seasons). The annual cycles of interacting pathogen and host biology is regulated in part by highly synchronized temperature and photoperiod changes during seasonal transitions (e.g., freezeup and breakup). With a warming climate, only one of these key biological cues will undergo drastic changes, while the other will remain fixed. This uncoupling can theoretically have drastic consequences on host-pathogen interactions. These poorly understood cues together with a changing climate by itself will challenge host populations that are adapted to pathogens under the historic and current climate regime. We will review adaptations of both host and pathogens to the extreme conditions at high latitudes and explore some potential consequences of rapid changes in the Arctic
Understanding the Cool DA White Dwarf, G29-38
The white dwarfs are promising laboratories for the study of cosmochronology
and stellar evolution. Through observations of the pulsating white dwarfs, we
can measure their internal structures and compositions, critical to
understanding post main sequence evolution, along with their cooling rates,
allowing us to calibrate their ages directly. The most important set of white
dwarf variables to measure are the oldest of the pulsators, the cool DAVs,
which have not previously been explored through asteroseismology due to their
complexity and instability. Through a time-series photometry data set spanning
ten years, we explore the pulsation spectrum of the cool DAV, G29-38 and find
an underlying structure of 19 (not including multiplet components) normal-mode,
probably l=1 pulsations amidst an abundance of time variability and linear
combination modes. Modelling results are incomplete, but we suggest possible
starting directions and discuss probable values for the stellar mass and
hydrogen layer size. For the first time, we have made sense out of the
complicated power spectra of a large-amplitude DA pulsator. We have shown its
seemingly erratic set of observed frequencies can be understood in terms of a
recurring set of normal-mode pulsations and their linear combinations. With
this result, we have opened the interior secrets of the DAVs to future
asteroseismological modelling, thereby joining the rest of the known white
dwarf pulsators.Comment: 29 pages including 5 figures To appear in ApJ 1 Mar 9
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