Discerning the Form of the Dense Core Mass Function

We investigate the ability to discern between lognormal and powerlaw forms for the observed mass function of dense cores in star forming regions. After testing our fitting, goodness-of-fit, and model selection procedures on simulated data, we apply our analysis to 14 datasets from the literature. Whether the core mass function has a powerlaw tail or whether it follows a pure lognormal form cannot be distinguished from current data. From our simulations it is estimated that datasets from uniform surveys containing more than approximately 500 cores with a completeness limit below the peak of the mass distribution are needed to definitively discern between these two functional forms. We also conclude that the width of the core mass function may be more reliably estimated than the powerlaw index of the high mass tail and that the width may also be a more useful parameter in comparing with the stellar initial mass function to deduce the statistical evolution of dense cores into stars.Comment: 6 pages, 2 figures, accepted for publication in PAS

Complete Monte Carlo RF analysis of 'real' short-channel compound FET's

A comprehensive RF analysis technique based on ensemble Monte Carlo (EMC) simulation of compound FET's with realistic device geometry is presented. Y-parameters are obtained through Fourier transformation of the EMC transients in response to small changes in the terminal voltages. The terminal currents are statistically enhanced and filtered to allow for reliable y-parameters extraction. Improved analytic procedure for extracting the intrinsic device small-signal circuit components is described. As a result, stable y-parameters and reliable circuit components can he extracted for the whole range of device operation voltages. Parasitic components like contact and gate resistances are included in the y-parameters at a post processing stage to facilitate the forecast of the performance figures of merit of real devices. The developed RF technique has been applied in the EMC simulation of pseudomorphic HEMT's (pHEMT's) fabricated at the Glasgow Nanoelectronics Research Center. Good agreement has been achieved between the simulated and measured small-signal circuit components and performance figures of meri

Noisy Monte Carlo: Convergence of Markov chains with approximate transition kernels

Monte Carlo algorithms often aim to draw from a distribution $\pi$ by simulating a Markov chain with transition kernel $P$ such that $\pi$ is invariant under $P$. However, there are many situations for which it is impractical or impossible to draw from the transition kernel $P$. 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 $P$ by an approximation $\hat{P}$. 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 $\hat{P}$ is to the chain given by $P$. We apply these results to several examples from spatial statistics and network analysis.Comment: This version: results extended to non-uniformly ergodic Markov chain

Transient thermal effects in solid noble gases as materials for the detection of Dark Matter

The transient phenomena produced in solid noble gases by the stopping of the recoils resulting from the elastic scattering processes of WIMPs from the galactic halo were modelled, as dependencies of the temperatures of lattice and electronic subsystems on the distance to the recoil's trajectory, and time from its passage. The peculiarities of these thermal transients produced in Ar, Kr and Xe were analysed for different initial temperatures and WIMP energies, and were correlated with the characteristics of the targets and with the energy loss of the recoils. The results were compared with the thermal spikes produced by the same WIMPs in Si and Ge. In the range of the energy of interest, up to tens of keV for the self-recoil, local phase transitions solid - liquid and even liquid - gas were found possible, and the threshold parameters were established.Comment: Minor corrections and updated references; accepted to JCA

Nonlinear electron transport in normally pinched-off quantum wire

Nonlinear electron transport in normally pinched-off quantum wires was studied. The wires were fabricated from AlGaAs/GaAs heterostructures with high-mobility two-dimensional electron gas by electron beam lithography and following wet etching. At certain critical source-drain voltage the samples exhibited a step rise of the conductance. The differential conductance of the open wires was noticeably lower than e^2/h as far as only part of the source-drain voltage dropped between source contact and saddle-point of the potential relief along the wire. The latter limited the electron flow injected to the wire. At high enough source-drain voltages the decrease of the differential conductance due to the real space transfer of electrons from the wire in GaAs to the doped AlGaAs layer was found. In this regime the sign of differential magnetoconductance was changed with reversing the direction of the current in the wire or the magnetic field, whet the magnetic field lies in the heterostructure plane and is directed perpendicular to the current. The dependence of the differential conductance on the magnetic field and its direction indicated that the real space transfer events were mainly mediated by the interface scattering.Comment: LaTeX 2e (epl.cls) 6 pages, 3 figure

A Simple Perspective on the Mass-Area Relationship in Molecular Clouds

Despite over 30 yr of study, the mass–area relationship within and among clouds is still poorly understood both observationally and theoretically. Modern extinction data sets should have sufficient resolution and dynamic range to characterize this relationship for nearby molecular clouds, although recent papers using extinction data seem to yield different interpretations regarding the nature and universality of this aspect of cloud structure. In this paper we try to unify these various results and interpretations by accounting for the different ways cloud properties are measured and analysed. We interpret the mass–area relationship in terms of the column density distribution function and its possible variation within and among clouds. We quantitatively characterize regional variations in the column density probability distribution function (PDF). We show that structures both within and among clouds possess the same degree of ‘universality’, in that their PDF means do not systematically scale with structure size. Because of this, mass scales linearly with area.Astronom

Efficacy and toxicity of intravenous iron in a mouse model of critical care anemia

OBJECTIVE: Anemia is common in critically ill patients, due to inflammation and blood loss. Anemia can be associated with iron deficiency and low serum hepcidin levels. However, iron administration in this setting remains controversial because of its potential toxicity, including oxidative stress induction and sepsis facilitation. The objective of this work was to determine the efficacy and toxicity of iron administration using a mouse model mimicking critical care anemia as well as a model of acute septicemia. DESIGN: Prospective, randomized, open label controlled animal study. SETTING: University-based research laboratory. SUBJECTS: C57BL/6 and OF1 mice. INTERVENTIONS: Intraperitoneal injection of zymosan inducing generalized inflammation in C57BL/6 mice, followed in our full model by repeated phlebotomies. A dose equivalent to 15 mg/kg of ferric carboxymaltose was injected intravenously on day 5. To assess the toxicity of iron in a septicemia model, OF1 mice were simultaneously injected with iron and different Escherichia coli strains. MEASUREMENTS AND MAIN RESULTS: To investigate the effect of iron on oxidative stress, we measured reactive oxygen species production in the blood using luminol-amplified chemiluminescence and superoxide dismutase 2 messenger RNA levels in the liver. These markers of oxidative stress were increased after iron administration in control mice but not in zymosan-treated mice. Liver catalase messenger RNA levels decreased in iron-treated control mice. Iron administration was not associated with increased mortality in the septicemia model or in the generalized inflammation model. Iron increased hemoglobin levels in mice fed with a low iron diet and subjected to phlebotomies and zymosan 2 wks after treatment administration. CONCLUSIONS: Adverse effects of intravenous iron supplementation by ferric carboxymaltose seem to be minimal in our animal models. Furthermore, iron appears to be effective in correcting anemia, despite inflammation. Studies of efficacy and safety of iron in critically ill patients are warranted

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