3,274 research outputs found
Photodissociation and the Morphology of HI in Galaxies
Young massive stars produce Far-UV photons which dissociate the molecular gas
on the surfaces of their parent molecular clouds. Of the many dissociation
products which result from this ``back-reaction'', atomic hydrogen \HI is one
of the easiest to observe through its radio 21-cm hyperfine line emission. In
this paper I first review the physics of this process and describe a simplified
model which has been developed to permit an approximate computation of the
column density of photodissociated \HI which appears on the surfaces of
molecular clouds. I then review several features of the \HI morphology of
galaxies on a variety of length scales and describe how photodissociation might
account for some of these observations. Finally, I discuss several consequences
which follow if this view of the origin of HI in galaxies continues to be
successful.Comment: 18 pages, 7 figures in 8 files, invited review paper for the
conference "Penetrating Bars Through Masks of Cosmic Dust: The Hubble Tuning
Fork Strikes a New Note", South Africa, June 2004. Proceedings to be
published by Kluwer, eds. D.L. Block, K.C. Freeman, I. Puerari, R. Groess, &
E.K. Bloc
Total-evidence framework reveals complex morphological evolution in nightbirds (Strisores)
Strisores is a clade of neoavian birds that include diurnal aerial specialists such as swifts and hummingbirds, as well as several predominantly nocturnal lineages such as nightjars and potoos. Despite the use of genome-scale molecular datasets, the phylogenetic interrelationships among major strisorean groups remain controversial. Given the availability of next-generation sequence data for Strisores and the clade’s rich fossil record, we reassessed the phylogeny of Strisores by incorporating a large-scale sequence dataset with anatomical data from living and fossil strisoreans within a Bayesian total-evidence framework. Combined analyses of molecular and morphological data resulted in a phylogenetic topology for Strisores that is congruent with the findings of two recent molecular phylogenomic studies, supporting nightjars (Caprimulgidae) as the extant sister group of the remainder of Strisores. This total-evidence framework allowed us to identify morphological synapomorphies for strisorean clades previously recovered using molecular-only datasets. However, a combined analysis of molecular and morphological data highlighted strong signal conflict between sequence and anatomical data in Strisores. Furthermore, simultaneous analysis of molecular and morphological data recovered differing placements for some fossil taxa compared with analyses of morphological data under a molecular scaffold, highlighting the importance of analytical decisions when conducting morphological phylogenetic analyses of taxa with molecular phylogenetic data. We suggest that multiple strisorean lineages have experienced convergent evolution across the skeleton, obfuscating the phylogenetic position of certain fossils, and that many distinctive specializations of strisorean subclades were acquired early in their evolutionary history. Despite this apparent complexity in the evolutionary history of Strisores, our results provide fossil support for aerial foraging as the ancestral ecological strategy of Strisores, as implied by recent phylogenetic topologies derived from molecular data.Parts of this project were supported by Systematics Research Fund awards to A.C. and D.J.F. by the Linnean Society of London and the Systematics Association. D.J.F. also acknowledges support from the Isaac Newton Trust early career support scheme. Parts of this project were also funded by the European Union’s Horizon 2020 research and innovation program 2014–2018 under grant agreement 677774 (European Research Council [ERC] Starting Grant: TEMPO) to R.B.J.B. N.D.W. and M.J.B. received Smithsonian Institution support through the Predoctoral Fellowship Program (N.D.W.), the Frontiers in Phylogenetics Program, the Scholarly Studies Program, and the Consortium for Understanding and Sustaining a Biodiverse Planet
Numerical Hermitian Yang-Mills Connections and Vector Bundle Stability in Heterotic Theories
A numerical algorithm is presented for explicitly computing the gauge
connection on slope-stable holomorphic vector bundles on Calabi-Yau manifolds.
To illustrate this algorithm, we calculate the connections on stable monad
bundles defined on the K3 twofold and Quintic threefold. An error measure is
introduced to determine how closely our algorithmic connection approximates a
solution to the Hermitian Yang-Mills equations. We then extend our results by
investigating the behavior of non slope-stable bundles. In a variety of
examples, it is shown that the failure of these bundles to satisfy the
Hermitian Yang-Mills equations, including field-strength singularities, can be
accurately reproduced numerically. These results make it possible to
numerically determine whether or not a vector bundle is slope-stable, thus
providing an important new tool in the exploration of heterotic vacua.Comment: 52 pages, 15 figures. LaTex formatting of figures corrected in
version 2
Numerical Hermitian Yang-Mills Connections and Kahler Cone Substructure
We further develop the numerical algorithm for computing the gauge connection
of slope-stable holomorphic vector bundles on Calabi-Yau manifolds. In
particular, recent work on the generalized Donaldson algorithm is extended to
bundles with Kahler cone substructure on manifolds with h^{1,1}>1. Since the
computation depends only on a one-dimensional ray in the Kahler moduli space,
it can probe slope-stability regardless of the size of h^{1,1}. Suitably
normalized error measures are introduced to quantitatively compare results for
different directions in Kahler moduli space. A significantly improved numerical
integration procedure based on adaptive refinements is described and
implemented. Finally, an efficient numerical check is proposed for determining
whether or not a vector bundle is slope-stable without computing its full
connection.Comment: 38 pages, 10 figure
Needles in the Haystack: Identifying Individuals Present in Pooled Genomic Data
Recent publications have described and applied a novel metric that quantifies the
genetic distance of an individual with respect to two population samples, and
have suggested that the metric makes it possible to infer the presence of an
individual of known genotype in a sample for which only the marginal allele
frequencies are known. However, the assumptions, limitations, and utility of
this metric remained incompletely characterized. Here we present empirical tests
of the method using publicly accessible genotypes, as well as analytical
investigations of the method's strengths and limitations. The results
reveal that the null distribution is sensitive to the underlying assumptions,
making it difficult to accurately calibrate thresholds for classifying an
individual as a member of the population samples. As a result, the
false-positive rates obtained in practice are considerably higher than
previously believed. However, despite the metric's inadequacies for
identifying the presence of an individual in a sample, our results suggest
potential avenues for future research on tuning this method to problems of
ancestry inference or disease prediction. By revealing both the strengths and
limitations of the proposed method, we hope to elucidate situations in which
this distance metric may be used in an appropriate manner. We also discuss the
implications of our findings in forensics applications and in the protection of
GWAS participant privacy
Optimal Torque and Stiffness Control in Compliantly Actuated Robots
Abstract — Anthropomorphic robots that aim to approach human performance agility and efficiency are typically highly redundant not only in their kinematics but also in actuation. Variable-impedance actuators, used to drive many of these devices, are capable of modulating torque and passive impedance (stiffness and/or damping) simultaneously and independently. Here, we propose a framework for simultaneous optimisation of torque and impedance (stiffness) profiles in order to optimise task performance, tuned to the complex hardware and incorporating real-world constraints. Simulation and hardware experiments validate the viability of this approach to complex, state dependent constraints and demonstrate task performance benefits of optimal temporal impedance modulation. Index Terms — Variable-stiffness actuation, physical constraints, optimal control
Supersymmetric Hidden Sectors for Heterotic Standard Models
Within the context of the weakly coupled E 8 × E 8 heterotic string, we study the hidden sector of heterotic standard model compactifications to four-dimensions. Specifically, we present a class of hidden sector vector bundles — composed of the direct sum of line bundles only — that, together with an effective bulk five-brane, renders the heterotic standard model entirely N = 1 supersymmetric. Two explicit hidden sectors are constructed and analyzed in this context; one with the gauge group E 7 × U(1) arising from a single line bundle and a second with an SO(12) × U(1) × U(1) gauge group constructed from the direct sum of two line bundles. Each hidden sector bundle is shown to satisfy all requisite physical constraints within a finite region of the Kähler cone. We also clarify that the first Chern class of the line bundles need not be even in our context, as has often been imposed in the model building literature
Internet-based psychoeducation for bipolar disorder: a qualitative analysis of feasibility, acceptability and impact
<p>Background: In a recent exploratory randomised trial we found that a novel, internet-based psychoeducation programme for bipolar disorder (Beating Bipolar) was relatively easy to deliver and had a modest effect on psychological quality of life. We sought to explore the experiences of participants with respect to feasibility, acceptability and impact of Beating Bipolar.</p>
<p>Methods: Participants were invited to take part in a semi-structured interview. Thematic analysis techniques were employed; to explore and describe participants’ experiences, the data were analysed for emerging themes which were identified and coded.</p>
<p>Results: The programme was feasible to deliver and acceptable to participants where they felt comfortable using a computer. It was found to impact upon insight into illness, health behaviour, personal routines and positive attitudes towards medication. Many participants regarded the programme as likely to be most beneficial for those recently diagnosed.</p>
<p>Conclusions: An online psychoeducation package for bipolar disorder, such as Beating Bipolar, is feasible and acceptable to patients, has a positive impact on self-management behaviours and may be particularly suited to early intervention. Alternative (non-internet) formats should also be made available to patients.</p>
Understanding innovators' experiences of barriers and facilitators in implementation and diffusion of healthcare service innovations: A qualitative study
This article is made available through the Brunel Open Access Publishing Fund - Copyright @ 2011 Barnett et al.Background: Healthcare service innovations are considered to play a pivotal role in improving organisational efficiency and responding effectively to healthcare needs. Nevertheless, healthcare organisations encounter major difficulties in sustaining and diffusing innovations, especially those which concern the organisation and delivery of healthcare services. The purpose of the present study was to explore how healthcare innovators of process-based initiatives perceived and made sense of factors that either facilitated or obstructed the innovation implementation and diffusion. Methods: A qualitative study was designed. Fifteen primary and secondary healthcare organisations in the UK, which had received health service awards for successfully generating and implementing service innovations, were studied. In-depth, semi structured interviews were conducted with the organisational representatives who conceived and led the development process. The data were recorded, transcribed and thematically analysed. Results: Four main themes were identified in the analysis of the data: the role of evidence, the function of inter-organisational partnerships, the influence of human-based resources, and the impact of contextual factors. "Hard" evidence operated as a proof of effectiveness, a means of dissemination and a pre-requisite for the initiation of innovation. Inter-organisational partnerships and people-based resources, such as champions, were considered an integral part of the process of developing, establishing and diffusing the innovations. Finally, contextual influences, both intra-organisational and extra-organisational were seen as critical in either impeding or facilitating innovators' efforts. Conclusions: A range of factors of different combinations and co-occurrence were pointed out by the innovators as they were reflecting on their experiences of implementing, stabilising and diffusing novel service initiatives. Even though the innovations studied were of various contents and originated from diverse organisational contexts, innovators' accounts converged to the significant role of the evidential base of success, the inter-personal and inter-organisational networks, and the inner and outer context. The innovators, operating themselves as important champions and being often willing to lead constructive efforts of implementation to different contexts, can contribute to the promulgation and spread of the novelties significantly.This research was supported financially by the Multidisciplinary Assessment of
Technology Centre for Healthcare (MATCH)
Non-supersymmetric heterotic model building
We investigate orbifold and smooth Calabi-Yau compactifications of the
non-supersymmetric heterotic SO(16)xSO(16) string. We focus on such Calabi-Yau
backgrounds in order to recycle commonly employed techniques, like index
theorems and cohomology theory, to determine both the fermionic and bosonic 4D
spectra. We argue that the N=0 theory never leads to tachyons on smooth
Calabi-Yaus in the large volume approximation. As twisted tachyons may arise on
certain singular orbifolds, we conjecture that such tachyonic states are lifted
in the full blow-up. We perform model searches on selected orbifold geometries.
In particular, we construct an explicit example of a Standard Model-like theory
with three generations and a single Higgs field.Comment: 1+30 pages latex, 11 tables; v2: references and minor revisions
added, matches version published in JHE
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