430 research outputs found
Reaction of Massive Clusters to Gas Expulsion - The cluster density dependence
The expulsion of the unconverted gas at the end of the star formation process
potentially leads to the expansion of the just formed stellar cluster and
membership loss. The degree of expansion and mass loss depends largely on the
star formation efficiency and scales with the mass and size of the stellar
group as long as stellar interactions can be neglected. We investigate under
which circumstances stellar interactions between cluster members become so
important that the fraction of bound stars after gas expulsion is significantly
altered. The Nbody6 code is used to simulate the cluster dynamics after gas
expulsion for different SFEs. Concentrating on the most massive clusters
observed in the Milky Way, we test to what extend the results depend on the
model, i.e. stellar mass distribution, stellar density profile etc., and the
cluster parameters, such as cluster density and size.We find that stellar
interactions are responsible for up to 20% mass loss in the most compact
massive clusters in the Milky Way, making ejections the prime mass loss process
in such systems. Even in the loosely bound OB associations stellar interactions
are responsible for at least ~5% mass loss. The main reason why the importance
of encounters for massive clusters has been largely overlooked is the often
used approach of a single-mass representation instead of a realistic
distribution for the stellar masses. The density-dependence of the
encounter-induced mass loss is shallower than expected because of the
increasing importance of few-body interactions in dense clusters compared to
sparse clusters where 2-body encounters dominate.Comment: 9 pages, 6 figures, A&A accepte
The expansion of massive young star clusters - observation meets theory
Most stars form as part of a star cluster. The most massive clusters in the
Milky Way exist in two groups - loose and compact clusters - with significantly
different sizes at the end of the star formation process. After their formation
both types of clusters expand up to a factor 10-20 within the first 20 Myr. Gas
expulsion at the end of the star formation process is usually regarded as only
possible process that can lead to such an expansion.We investigate the effect
of gas expulsion by a direct comparison between numerical models and observed
clusters concentrating on clusters with masses >10^3 M(sun). For these clusters
the initial conditions before gas expulsion, the characteristic cluster
development, its dependence on cluster mass, and the star formation efficiency
(SFE) are investigated. We perform N-body simulations of the cluster expansion
process after gas expulsion and compare the results with observations. We find
that the expansion processes of the observed loose and compact massive clusters
are driven by completely different physical processes. As expected the
expansion of loose massive clusters is largely driven by the gas loss due to
the low SFE of ~30%. One new revelation is that all the observed massive
clusters of this group seem to have a very similar size of 1-3 pc at the onset
of expansion. It is demonstrated that compact clusters have a much higher
effective SFE of 60-70% and are as a result much less affected by gas
expulsion. Their expansion is mainly driven by stellar ejections caused by
interactions between the cluster members. The reason why ejections are so
efficient in driving cluster expansion is that they occur dominantly from the
cluster centre and over an extended period of time. Thus during the first 10
Myr the internal dynamics of loose and compact clusters differ fundamentally.Comment: 10 pages, 9 figures, accepted by A&
The Evolution of Binary Populations in Young Star Clusters: From the ONC to OB associations
Observations of the binary populations in young, sparse clusters have shown, that almost all stars are part of a binary system at the end of the star formation process. By contrast, the binary frequency of field stars only ~50%. Most stars, and therefore most binaries, are formed in dense star clusters. This rises the question if the natal environments lead to the observed reduction of the binary frequency. In this thesis this question is addressed using numerical Nbody simulations of binary populations in different star cluster environments.
First the evolution of binaries in ONC-like star clusters has been investigated. It was found that there the evolution of the normalised number of binaries is independent from the initial binary frequency. This allows to predict the evolution of binary populations in ONC-like clusters without the need of further numerical simulations. In addition it was found, that dynamical interactions preferentially destroy low-mass binaries resulting in a higher binary frequency for high-mass stars in the simulated clusters, in accordance with observation in the ONC. The combination of dynamical evolution with gas-induced orbital decay of embedded binaries is capable to reshape a log-uniform period distribution, as observed in young star clusters, to a log-normal period distribution as observed in the field today.
The modelling has been generalised to clusters with arbitrary densities. Performing simulations for clusters with up to eight times higher densities than before with two different initial binary frequencies, it was shown that the evolution of the normalised number of binaries remains independent of the initial binary frequency. The higher the density in a cluster the more binaries are destroyed as to be expected. However, this effect levels out for clusters with central densities exceeding ~3 * 10^4 pc^{-3}. This means that there is a limit beyond which increasing the binary frequency in the clusters does not lead to significantly more binaries being destroyed.
Finally it was investigated how the binary population evolves in star clusters that have undergone instantaneous gas expulsion with a resultant fast decrease in stellar density. It was found that the lower the star formation efficiency of a cluster and therefore the faster the decrease in stellar density, the less binaries are destroyed during the evolution while at the same time the more very wide binaries are formed. Comparison of the evolution of the simulated clusters and the observed leaky cluster sequence shows that clusters including a binary population and a SFE = 0.3 match the observations of leaky-star clusters
remarkably well
COSMIC: US-based Conversion Master\u27s Degree in Computing
COSMIC is an NSF S-STEM graduate curriculum initiative/conversion program that strives to provide an accelerated pathway to a Master of Science (MS) degree for individuals who do not have an undergraduate degree in computing, but who wish to cross over into the computing field. The structure of our conversion program, the context that motivated it, and insights from conversion students\u27 instructors are presented. Program successes with students from under-represented populations and the limitations that are also experienced are discussed. Our conversion program is based on a highly focused summer bridge course, combined with a customized curriculum pathway that enables people without undergraduate computing degrees to merge quickly and efficiently into a professional MS in computing degree program. The program is similar in concept to post-baccalaureate conversion programs in New Zealand (e.g., the Master of Software Development at the Victoria University of Wellington) and the extensive conversion choices in the UK. Undergraduate and graduate student enrollment statistics from past and current (2018) CRA Taulbee Surveys strongly suggest the computing profession has a moral obligation to seek out and encourage individuals from under-represented populations to become a significant part of the computing professional community. We encourage other institutions to join in the effort to recruit and provide pathways for post-baccalaureate individuals from under-represented populations to become a significant part of the computing community
Multiple blood-brain barrier transport mechanisms limit bumetanide accumulation, and therapeutic potential, in the mammalian brain
There is accumulating evidence that bumetanide, which has been used over decades as a potent loop diuretic, also exerts effects on brain disorders, including autism, neonatal seizures, and epilepsy, which are not related to its effects on the kidney but rather mediated by inhibition of the neuronal Na-K-C1 cotransporter isoform NKCC1. However, following systemic administration, brain levels of bumetanide are typically below those needed to inhibit NKCC1, which critically limits its clinical use for treating brain disorders. Recently, active efflux transport at the blood-brain barrier (BBB) has been suggested as a process involved in the low brain:plasma ratio of bumetanide, but it is presently not clear which transporters are involved. Understanding the processes explaining the poor brain penetration of bumetanide is needed for developing strategies to improve the brain delivery of this drug. In the present study, we administered probenecid and more selective inhibitors of active transport carriers at the BBB directly into the brain of mice to minimize the contribution of peripheral effects on the brain penetration of bumetanide. Furthermore, in vitro experiments with mouse organic anion transporter 3 (Oat3)-overexpressing Chinese hamster ovary cells were performed to study the interaction of bumetanide, bumetanide derivatives, and several known inhibitors of Oats on Oat3-mediated transport. The in vivo experiments demonstrated that the uptake and efflux of bumetanide at the BBB is much more complex than previously thought. It seems that both restricted passive diffusion and active efflux transport, mediated by Oat3 but also organic anion-transporting polypeptide (Oatp) Oatpla4 and multidrug resistance protein 4 explain the extremely low brain concentrations that are achieved after systemic administration of bumetanide, limiting the use of this drug for targeting abnormal expression of neuronal NKCC1 in brain diseases
Regulation of Nucleotide Excision Repair by UV-DDB: Prioritization of Damage Recognition to Internucleosomal DNA
This study reveals the molecular mechanism by which the nucleotide excision repair protein DDB2 prioritises excision of UV-induced DNA lesions in the nucleosome landscape
Theory of noise suppression in {\Lambda}-type quantum memories by means of a cavity
Quantum memories, capable of storing single photons or other quantum states
of light, to be retrieved on-demand, offer a route to large-scale quantum
information processing with light. A promising class of memories is based on
far-off-resonant Raman absorption in ensembles of -type atoms. However
at room temperature these systems exhibit unwanted four-wave mixing, which is
prohibitive for applications at the single-photon level. Here we show how this
noise can be suppressed by placing the storage medium inside a moderate-finesse
optical cavity, thereby removing the main roadblock hindering this approach to
quantum memory.Comment: 10 pages, 3 figures. This paper provides the theoretical background
to our recent experimental demonstration of noise suppression in a
cavity-enhanced Raman-type memory ( arXiv:1510.04625 ). See also the related
paper arXiv:1511.05448, which describes numerical modelling of an atom-filled
cavity. Comments welcom
High-speed noise-free optical quantum memory
Quantum networks promise to revolutionise computing, simulation, and
communication. Light is the ideal information carrier for quantum networks, as
its properties are not degraded by noise in ambient conditions, and it can
support large bandwidths enabling fast operations and a large information
capacity. Quantum memories, devices that store, manipulate, and release on
demand quantum light, have been identified as critical components of photonic
quantum networks, because they facilitate scalability. However, any noise
introduced by the memory can render the device classical by destroying the
quantum character of the light. Here we introduce an intrinsically noise-free
memory protocol based on two-photon off-resonant cascaded absorption (ORCA). We
consequently demonstrate for the first time successful storage of GHz-bandwidth
heralded single photons in a warm atomic vapour with no added noise; confirmed
by the unaltered photon statistics upon recall. Our ORCA memory platform meets
the stringent noise-requirements for quantum memories whilst offering technical
simplicity and high-speed operation, and therefore is immediately applicable to
low-latency quantum networks
Frequency tunability of solid-core photonic crystal fibers filled with nanoparticle-doped liquid crystals
We infiltrate liquid crystals doped with BaTiO3 nanoparticles in a photonic crystal fiber and compare the measured transmission spectrum with the one achieved without dopant. New interesting features, such as frequency modulation response of the device and a transmission spectrum with tunable attenuation on the short wavelength side of the widest bandgap, suggest a potential application of this device as a tunable all-in-fiber gain equalization filter with an adjustable slope. The tunability of the device is achieved by varying the amplitude and the frequency of the applied external electric field. The threshold voltage for doped and undoped liquid crystals in a silica capillary and in a glass cell are also measured as a function of the frequency of the external electric field and the achieved results are compared
Optimal Coherent Filtering for Single Noisy Photons
We introduce a filter using a noise-free quantum buffer with large optical
bandwidth that can both filter temporal-spectral modes, as well as
inter-convert them and change their frequency. We show that such quantum
buffers optimally filter out temporal-spectral noise; producing identical
single-photons from many distinguishable noisy single-photon sources with the
minimum required reduction in brightness. We then experimentally demonstrate a
noise-free quantum buffer in a warm atomic system that is well matched to
quantum dots and can outperform all intensity (incoherent) filtering schemes
for increasing indistinguishability.Comment: 5 pages, 4 Figure
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