268 research outputs found
Stellar interactions in dense and sparse star clusters
Stellar encounters potentially affect the evolution of the protoplanetary
discs in the Orion Nebula Cluster (ONC). However, the role of encounters in
other cluster environments is less known. We investigate the effect of the
encounter-induced disc-mass loss in different cluster environments. Starting
from an ONC-like cluster we vary the cluster size and density to determine the
correlation of collision time scale and disc-mass loss. We use the NBODY6++
code to model the dynamics of these clusters and analyze the effect of
star-disc encounters. We find that the disc-mass loss depends strongly on the
cluster density but remains rather unaffected by the size of the stellar
population. The essential outcome of the simulations are: i) Even in clusters
four times sparser than the ONC the effect of encounters is still apparent. ii)
The density of the ONC itself marks a threshold: in less dense and less massive
clusters it is the massive stars that dominate the encounter-induced disc-mass
loss whereas in denser and more massive clusters the low-mass stars play the
major role for the disc mass removal. It seems that in the central regions of
young dense star clusters -- the common sites of star formation -- stellar
encounters do affect the evolution of the protoplanetary discs. With higher
cluster density low-mass stars become more heavily involved in this process.
This finding allows for the extrapolation towards extreme stellar systems: in
case of the Arches cluster one would expect stellar encounters to destroy the
discs of most of the low- and high-mass stars in several hundred thousand
years, whereas intermediate mass stars are able to retain to some extant their
discs even under these harsh environmental conditions.Comment: accepted by Astronomy and Astrophysic
Simulations of the Hyades
Context: Using the recent observational data of R\"oser et al. we present
-body simulations of the Hyades open cluster. Aims: We make an attempt to
determine initial conditions of the Hyades cluster at the time of its formation
in order to reproduce the present-day cumulative mass profile, stellar mass and
luminosity function (LF). Methods: We performed direct -body simulations of
the Hyades in an analytic Milky Way potential that account for stellar
evolution and include primordial binaries in a few models. Furthermore, we
applied a Kroupa (2001) IMF and used extensive ensemble-averaging. Results: We
find that evolved single-star King initial models with King parameters and initial particle numbers provide good fits to the
observational present-day cumulative mass profile within the Jacobi radius. The
best-fit King model has an initial mass of and an average mass
loss rate of . The K-band LFs of models and
observations show a reasonable agreement. Mass segregation is detected in both
observations and models. If 33% primordial binaries are included the initial
particle number is reduced by 5% as compared to the model without primordial
binaries. Conclusions: The present-day properties of the Hyades can be well
reproduced by a standard King or Plummer initial model when choosing
appropriate initial conditions. The degeneracy of good-fitting models can be
quite high due to the large dimension of the parameter space. More simulations
with different Roche-lobe filling factors and primordial binary fractions are
required to explore this degeneracy in more detail.Comment: 14 pages, 16+1 figures, hopefully final version, contains a note
added in proo
Methane regulation in the EU: Stakeholder perspectives on MRV and emissions reductions
Published online 19 September 2022Methane is potent greenhouse gas (GHG) accounting for 11% of all EU emissions, but in contrast to CO2 it has received relatively little attention. Although methane is regulated under the EU Effort Sharing framework, this policy lacks methane-specific regulations or targets, leaving the Member States considerable discretion over whether to prioritize methane reduction or not. The European Commission presented a proposal for EU methane regulation on 15 December 2021. However, our understanding of how to design measurement, reporting and verification (MRV) regulation for methane is limited. MRV involves many stakeholders at different steps in the process (policymakers, industry, civil society, MRV service providers, etc.), whose perspectives may differ, and our study aims to gain an insight into what constitutes an effective MRV by garnering the different stakeholders’ perspectives. The study reveals that: (1) the limits of voluntary MRV initiatives justify regulatory intervention, (2) the major barrier to the implementation of methane-specific MRV is not economic, but relates to an incomplete understanding of methane sources and available measurement technologies, (3) verification is likely to be the most challenging MRV element to implement, partly due to the limited number of accredited verifiers and overlapping tasks (4) MRV needs to be accompanied by methane mitigation policies incentivising continuous improvement of companies’ performance. The study recommends enhancing the proposed regulation by: introducing equal requirements for operated and non-operated assets; an obligation to report measurement uncertainties; a closer integration of MRV and LDAR; clear verification rules; and an introduction of minimum and optimum methane control standards
A natural formation scenario for misaligned and short-period eccentric extrasolar planets
Recent discoveries of strongly misaligned transiting exoplanets pose a
challenge to the established planet formation theory which assumes planetary
systems to form and evolve in isolation. However, the fact that the majority of
stars actually do form in star clusters raises the question how isolated
forming planetary systems really are. Besides radiative and tidal forces the
presence of dense gas aggregates in star-forming regions are potential sources
for perturbations to protoplanetary discs or systems. Here we show that
subsequent capture of gas from large extended accretion envelopes onto a
passing star with a typical circumstellar disc can tilt the disc plane to
retrograde orientation, naturally explaining the formation of strongly inclined
planetary systems. Furthermore, the inner disc regions may become denser, and
thus more prone to speedy coagulation and planet formation. Pre-existing
planetary systems are compressed by gas inflows leading to a natural occurrence
of close-in misaligned hot Jupiters and short-period eccentric planets. The
likelihood of such events mainly depends on the gas content of the cluster and
is thus expected to be highest in the youngest star clusters.Comment: 7 pages, 4 figures. Accepted for publication in MNRAS. Updated to
match published versio
An increasing number of convolutional neural networks for fracture recognition and classification in orthopaedics:are these externally validated and ready for clinical application?
Aims: The number of convolutional neural networks (CNN) available for fracture detection and classification is rapidly increasing. External validation of a CNN on a temporally separate (separated by time) or geographically separate (separated by location) dataset is crucial to assess generalizability of the CNN before application to clinical practice in other institutions. We aimed to answer the following questions: are current CNNs for fracture recognition externally valid?; which methods are applied for external validation (EV)?; and, what are reported performances of the EV sets compared to the internal validation (IV) sets of these CNNs? Methods: The PubMed and Embase databases were systematically searched from January 2010 to October 2020 according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement. The type of EV, characteristics of the external dataset, and diagnostic performance characteristics on the IV and EV datasets were collected and compared. Quality assessment was conducted using a seven-item checklist based on a modified Methodologic Index for NOn-Randomized Studies instrument (MINORS). Results: Out of 1,349 studies, 36 reported development of a CNN for fracture detection and/or classification. Of these, only four (11%) reported a form of EV. One study used temporal EV, one conducted both temporal and geographical EV, and two used geographical EV. When comparing the CNN’s performance on the IV set versus the EV set, the following were found: AUCs of 0.967 (IV) versus 0.975 (EV), 0.976 (IV) versus 0.985 to 0.992 (EV), 0.93 to 0.96 (IV) versus 0.80 to 0.89 (EV), and F1-scores of 0.856 to 0.863 (IV) versus 0.757 to 0.840 (EV). Conclusion: The number of externally validated CNNs in orthopaedic trauma for fracture recognition is still scarce. This greatly limits the potential for transfer of these CNNs from the developing institute to another hospital to achieve similar diagnostic performance. We recommend the use of geographical EV and statements such as the Consolidated Standards of Reporting Trials–Artificial Intelligence (CONSORT-AI), the Standard Protocol Items: Recommendations for Interventional Trials–Artificial Intelligence (SPIRIT-AI) and the Transparent Reporting of a multivariable prediction model for Individual Prognosis or Diagnosis–Machine Learning (TRIPOD-ML) to critically appraise performance of CNNs and improve methodological rigor, quality of future models, and facilitate eventual implementation in clinical practice
The effects of supernovae on the dynamical evolution of binary stars and star clusters
In this chapter I review the effects of supernovae explosions on the
dynamical evolution of (1) binary stars and (2) star clusters.
(1) Supernovae in binaries can drastically alter the orbit of the system,
sometimes disrupting it entirely, and are thought to be partially responsible
for `runaway' massive stars - stars in the Galaxy with large peculiar
velocities. The ejection of the lower-mass secondary component of a binary
occurs often in the event of the more massive primary star exploding as a
supernova. The orbital properties of binaries that contain massive stars mean
that the observed velocities of runaway stars (10s - 100s km s) are
consistent with this scenario.
(2) Star formation is an inherently inefficient process, and much of the
potential in young star clusters remains in the form of gas. Supernovae can in
principle expel this gas, which would drastically alter the dynamics of the
cluster by unbinding the stars from the potential. However, recent numerical
simulations, and observational evidence that gas-free clusters are observed to
be bound, suggest that the effects of supernova explosions on the dynamics of
star clusters are likely to be minimal.Comment: 16 pages, to appear in the 'Handbook of Supernovae', eds. Paul Murdin
and Athem Alsabti. This version replaces an earlier version that contained
several typo
Comparative analysis of involvement of UGT1 and UGT2 splice variants of UDP-galactose transporter in glycosylation of macromolecules in MDCK and CHO cell lines
Nucleotide sugar transporters deliver nucleotide sugars into the Golgi apparatus and endoplasmic reticulum. This study aimed to further characterize mammalian UDP-galactose transporter (UGT) in MDCK and CHO cell lines. MDCK-RCAr and CHO-Lec8 mutant cell lines are defective in UGT transporter, although they exhibit some level of galactosylation. Previously, only single forms of UGT were identified in both cell lines, UGT1 in MDCK cells and UGT2 in CHO cells. We have identified the second UGT splice variants in CHO (UGT1) and MDCK (UGT2) cells. Compared to UGT1, UGT2 is more abundant in nearly all examined mammalian tissues and cell lines, but MDCK cells exhibit different relative distribution of both splice variants. Complementation analysis demonstrated that both UGT splice variants are necessary for N- and O-glycosylation of proteins. Both mutant cell lines produce chondroitin-4-sulfate at only a slightly lower level compared to wild-type cells. This defect is corrected by overexpression of both UGT splice variants. MDCK-RCAr mutant cells do not produce keratan sulfate and this effect is not corrected by either UGT splice variant, overexpressed either singly or in combination. Here we demonstrate that both UGT splice variants are important for glycosylation of proteins. In contrast to MDCK cells, MDCK-RCAr mutant cells may possess an additional defect within the keratan sulfate biosynthesis pathway
Unique Structure and Stability of HmuY, a Novel Heme-Binding Protein of Porphyromonas gingivalis
Infection, survival, and proliferation of pathogenic bacteria in humans depend on their capacity to impair host responses and acquire nutrients in a hostile environment. Among such nutrients is heme, a co-factor for oxygen storage, electron transport, photosynthesis, and redox biochemistry, which is indispensable for life. Porphyromonas gingivalis is the major human bacterial pathogen responsible for severe periodontitis. It recruits heme through HmuY, which sequesters heme from host carriers and delivers it to its cognate outer-membrane transporter, the TonB-dependent receptor HmuR. Here we report that heme binding does not significantly affect the secondary structure of HmuY. The crystal structure of heme-bound HmuY reveals a new all-β fold mimicking a right hand. The thumb and fingers pinch heme iron through two apical histidine residues, giving rise to highly symmetric octahedral iron co-ordination. The tetrameric quaternary arrangement of the protein found in the crystal structure is consistent with experiments in solution. It shows that thumbs and fingertips, and, by extension, the bound heme groups, are shielded from competing heme-binding proteins from the host. This may also facilitate heme transport to HmuR for internalization. HmuY, both in its apo- and in its heme-bound forms, is resistant to proteolytic digestion by trypsin and the major secreted proteases of P. gingivalis, gingipains K and R. It is also stable against thermal and chemical denaturation. In conclusion, these studies reveal novel molecular properties of HmuY that are consistent with its role as a putative virulence factor during bacterial infection
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