1,642 research outputs found
Substellar Objects in Nearby Young Clusters VII: The substellar mass function revisited
The abundance of brown dwarfs (BDs) in young clusters is a diagnostic of star
formation theory. Here we revisit the issue of determining the substellar
initial mass function (IMF), based on a comparison between NGC1333 and IC348,
two clusters in the Perseus star-forming region. We derive their mass
distributions for a range of model isochrones, varying distances, extinction
laws and ages, with comprehensive assessments of the uncertainties. We find
that the choice of isochrone and other parameters have significant effects on
the results, thus we caution against comparing IMFs obtained using different
approaches. For NGC1333, we find that the star/BD ratio R is between 1.9 and
2.4, for all plausible scenarios, consistent with our previous work. For IC348,
R is between 2.9 and 4.0, suggesting that previous studies have overestimated
this value. Thus, the star forming process generates about 2.5-5 substellar
objects per 10 stars. The derived star/BD ratios correspond to a slope of the
power-law mass function of alpha=0.7-1.0 for the 0.03-1.0Msol mass range. The
median mass in these clusters - the typical stellar mass - is between
0.13-0.30Msol. Assuming that NGC1333 is at a shorter distance than IC348, we
find a significant difference in the cumulative distribution of masses between
the two clusters, resulting from an overabundance of very low mass objects in
NGC1333. Gaia astrometry will constrain the cluster distances better and will
lead to a more definitive conclusion. Furthermore, ratio R is somewhat larger
in IC348 compared with NGC1333, although this difference is still within the
margins of error. Our results indicate that environments with higher object
density may produce a larger fraction of very low mass objects, in line with
predictions for brown dwarf formation through gravitational fragmentation of
filaments falling into a cluster potential.Comment: 16 pages, 4 figures, accepted for publication in Ap
Sustainable approaches for stormwater quality improvements with experimental geothermal paving systems
This article has been made available through the Brunel Open Access Publishing Fund.This research assesses the next generation of permeable pavement systems (PPS) incorporating ground source heat pumps (geothermal paving systems). Twelve experimental pilot-scaled pavement systems were assessed for its stormwater treatability in Edinburgh, UK. The relatively high variability of temperatures during the heating and cooling cycle of a ground source heat pump system embedded into the pavement structure did not allow the ecological risk of pathogenic microbial expansion and survival. Carbon dioxide monitoring indicated relatively high microbial activity on a geotextile layer and within the pavement structure. Anaerobic degradation processes were concentrated around the geotextile zone, where carbon dioxide concentrations reached up to 2000 ppm. The overall water treatment potential was high with up to 99% biochemical oxygen demand removal. The pervious pavement systems reduced the ecological risk of stormwater discharges and provided a low risk of pathogen growth
Posterior accuracy and calibration under misspecification in Bayesian generalized linear models
Generalized linear models (GLMs) are popular for data-analysis in almost all
quantitative sciences, but the choice of likelihood family and link function is
often difficult. This motivates the search for likelihoods and links that
minimize the impact of potential misspecification. We perform a large-scale
simulation study on double-bounded and lower-bounded response data where we
systematically vary both true and assumed likelihoods and links. In contrast to
previous studies, we also study posterior calibration and uncertainty metrics
in addition to point-estimate accuracy. Our results indicate that certain
likelihoods and links can be remarkably robust to misspecification, performing
almost on par with their respective true counterparts. Additionally, normal
likelihood models with identity link (i.e., linear regression) often achieve
calibration comparable to the more structurally faithful alternatives, at least
in the studied scenarios. On the basis of our findings, we provide practical
suggestions for robust likelihood and link choices in GLMs
Prediction can be safely used as a proxy for explanation in causally consistent Bayesian generalized linear models
Bayesian modeling provides a principled approach to quantifying uncertainty
in model parameters and model structure and has seen a surge of applications in
recent years. Within the context of a Bayesian workflow, we are concerned with
model selection for the purpose of finding models that best explain the data,
that is, help us understand the underlying data generating process. Since we
rarely have access to the true process, all we are left with during real-world
analyses is incomplete causal knowledge from sources outside of the current
data and model predictions of said data. This leads to the important question
of when the use of prediction as a proxy for explanation for the purpose of
model selection is valid. We approach this question by means of large-scale
simulations of Bayesian generalized linear models where we investigate various
causal and statistical misspecifications. Our results indicate that the use of
prediction as proxy for explanation is valid and safe only when the models
under consideration are sufficiently consistent with the underlying causal
structure of the true data generating process
Identification and Analysis of Patterns of Machine Learning Systems in the Connected, Adaptive Production
Over the past six decades, many companies have discovered the potential of computer-controlled systems in the manufacturing industry. Overall, digitization can be identified as one of the main drivers of cost reduction in the manufacturing industry. However, recent advances in Artificial Intelligence indicate that there is still untapped potential in the use and analysis of data in industry. Many reports and surveys indicate that machine learning solutions are slowly adapted and that the process of implementation is decelerated by inefficiencies. The goal of this paper is the systematic analysis of successfully implemented machine learning solutions in manufacturing as well as the derivation of a more efficient implementation approach. For this, three use cases have been identified for in-depth analysis and a framework for systematic comparisons between differently implemented solutions is developed. In all three use cases it is possible to derive implementation patterns as well as to identify key variables which determine the success of implementation. The identified patterns show that similar machine learning problems within the same use case can be solved with similar solutions. The results provide a heuristic for future implementation attempts tackling problems of similar nature
Enhanced light emission from top-emitting organic light-emitting diodes by optimizing surface plasmon polariton losses
We demonstrate enhanced light extraction for monochrome top-emitting organic
light-emitting diodes (OLEDs). The enhancement by a factor of 1.2 compared to a
reference sample is caused by the use of a hole transport layer (HTL) material
possessing a low refractive index (1.52). The low refractive index reduces the
in-plane wave vector of the surface plasmon polariton (SPP) excited at the
interface between the bottom opaque metallic electrode (anode) and the HTL. The
shift of the SPP dispersion relation decreases the power dissipated into lost
evanescent excitations and thus increases the outcoupling efficiency, although
the SPP remains constant in intensity. The proposed method is suitable for
emitter materials owning isotropic orientation of the transition dipole moments
as well as anisotropic, preferentially horizontal orientation, resulting in
comparable enhancement factors. Furthermore, for sufficiently low refractive
indices of the HTL material, the SPP can be modeled as a propagating plane wave
within other organic materials in the optical microcavity. Thus, by applying
further extraction methods, such as micro lenses or Bragg gratings, it would
become feasible to obtain even higher enhancements of the light extraction.Comment: 11 pages, 6 figures, will be submitted to PR
Some models are useful, but how do we know which ones? Towards a unified Bayesian model taxonomy
Probabilistic (Bayesian) modeling has experienced a surge of applications in
almost all quantitative sciences and industrial areas. This development is
driven by a combination of several factors, including better probabilistic
estimation algorithms, flexible software, increased computing power, and a
growing awareness of the benefits of probabilistic learning. However, a
principled Bayesian model building workflow is far from complete and many
challenges remain. To aid future research and applications of a principled
Bayesian workflow, we ask and provide answers for what we perceive as two
fundamental questions of Bayesian modeling, namely (a) "What actually is a
Bayesian model?" and (b) "What makes a good Bayesian model?". As an answer to
the first question, we propose the PAD model taxonomy that defines four basic
kinds of Bayesian models, each representing some combination of the assumed
joint distribution of all (known or unknown) variables (P), a posterior
approximator (A), and training data (D). As an answer to the second question,
we propose ten utility dimensions according to which we can evaluate Bayesian
models holistically, namely, (1) causal consistency, (2) parameter
recoverability, (3) predictive performance, (4) fairness, (5) structural
faithfulness, (6) parsimony, (7) interpretability, (8) convergence, (9)
estimation speed, and (10) robustness. Further, we propose two example utility
decision trees that describe hierarchies and trade-offs between utilities
depending on the inferential goals that drive model building and testing
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