5,706 research outputs found
The stochastic dynamics of micron and nanoscale elastic cantilevers in fluid: fluctuations from dissipation
The stochastic dynamics of micron and nanoscale cantilevers immersed in a
viscous fluid are quantified. Analytical results are presented for long slender
cantilevers driven by Brownian noise. The spectral density of the noise force
is not assumed to be white and the frequency dependence is determined from the
fluctuation-dissipation theorem. The analytical results are shown to be useful
for the micron scale cantilevers that are commonly used in atomic force
microscopy. A general thermodynamic approach is developed that is valid for
cantilevers of arbitrary geometry as well as for arrays of multiple cantilevers
whose stochastic motion is coupled through the fluid. It is shown that the
fluctuation-dissipation theorem permits the calculation of stochastic
quantities via straightforward deterministic methods. The thermodynamic
approach is used with deterministic finite element numerical simulations to
quantify the autocorrelation and noise spectrum of cantilever fluctuations for
a single micron scale cantilever and the cross-correlations and noise spectra
of fluctuations for an array of two experimentally motivated nanoscale
cantilevers as a function of cantilever separation. The results are used to
quantify the noise reduction possible using correlated measurements with two
closely spaced nanoscale cantilevers.Comment: Submitted to Nanotechnology April 26, 200
Transgender adults, gender-affirming hormone therapy and blood pressure: a systematic review
Objectives:
Gender-affirming hormone therapy (GHT) is utilized by people who are transgender to align their secondary sex characteristics with their sex identity. Data relating to cardiovascular outcomes in this population are limited. We aimed to review the impact of GHT on the blood pressure (BP) of transgender individuals.
Methods:
We searched PubMed/MEDLINE, SCOPUS and Cochrane Library databases for articles published relating to the BP of transgender adults commencing GHT. Methodological quality was assessed via the âQuality Assessment Tool for BeforeâAfter (PreâPost) Studies with No Control Groupâ.
Results:
Six hundred articles were screened, of which 14 studies were included in this systematic review encompassing 1309 individuals (âŒ50% transgender men and women) treated with GHT between 1989 and 2019. These articles were all preâpost observational studies without control groups. Mean ages ranged between 23.0â36.7 years (transgender men) and 25.2â34.8 years (transgender women). Interventions were diverse and included oral, transdermal and injectable hormonal preparations with 4 months to 5 years follow-up. Most studies in transgender men did not demonstrate a change in BP, whereas transgender women on GHT demonstrated an increase in SBP but not DBP. These studies were heterogenous with significant methodological limitations and only two were determined to have a good quality rating.
Conclusion:
There is currently insufficient data to advise the impact of GHT on BP in transgender individuals. Better quality research is essential to elucidate whether exogenous sex hormones modulate BP in transgender people and whether this putative alteration infers poorer cardiovascular outcomes
The role of cosmic ray pressure in accelerating galactic outflows
We study the formation of galactic outflows from supernova explosions (SNe)
with the moving-mesh code AREPO in a stratified column of gas with a surface
density similar to the Milky Way disk at the solar circle. We compare different
simulation models for SNe placement and energy feedback, including cosmic rays
(CR), and find that models that place SNe in dense gas and account for CR
diffusion are able to drive outflows with similar mass loading as obtained from
a random placement of SNe with no CRs. Despite this similarity, CR-driven
outflows differ in several other key properties including their overall
clumpiness and velocity. Moreover, the forces driving these outflows originate
in different sources of pressure, with the CR diffusion model relying on
non-thermal pressure gradients to create an outflow driven by internal pressure
and the random-placement model depending on kinetic pressure gradients to
propel a ballistic outflow. CRs therefore appear to be non-negligible physics
in the formation of outflows from the interstellar medium.Comment: 8 pages, 4 figures, accepted for publication in ApJL; movie of
simulated gas densities can be found here:
http://www.h-its.org/tap-images/galactic-outflows
Dinosaur tracks from the Kilmaluag Formation (Bathonian, Middle Jurassic) of Score Bay, Isle of Skye, Scotland, UK
Tracks of a juvenile theropod dinosaur with footprint lengths of between 2 and 9 cm as well as adults of the same ichnospecies with footprints of about 15â25 cm in length were found in the Bathonian (Middle Jurassic) Kilmaluag Formation of Score Bay, northwestern Trotternish Peninsula, Isle of Skye, Scotland, UK. Two footprint sizes occur together on the same bedding plane in the central portion of Score Bay, both in situ and on loose blocks. Another horizon containing footprints above this was also identified. The footprints from the lowest horizon were produced in a desiccated silty mud that was covered with sand. A close association of both adults and juveniles with similar travel direction indicated by the footprints may suggest post-hatching care in theropod dinosaurs. Other footprints, produced on a rippled sandy substrate, have been found on the slightly higher bedding plane at this locality. Loose blocks found 130 m to the northeast in the central part of Score Bay have not been correlated with any in situ sediments, but were preserved in a similar manner to those from the higher bedding plane. These tracks represent the youngest dinosaur remains yet found in Scotland
Structural basis for complement factor H-linked age-related macular degeneration
This is the final version of the article. Available from the publisher via the DOI in this record.Nearly 50 million people worldwide suffer from age-related macular degeneration (AMD), which causes severe loss of central vision. A single-nucleotide polymorphism in the gene for the complement regulator factor H (FH), which causes a Tyr-to-His substitution at position 402, is linked to approximately 50% of attributable risks for AMD. We present the crystal structure of the region of FH containing the polymorphic amino acid His402 in complex with an analogue of the glycosaminoglycans (GAGs) that localize the complement regulator on the cell surface. The structure demonstrates direct coordination of ligand by the disease-associated polymorphic residue, providing a molecular explanation of the genetic observation. This glycan-binding site occupies the center of an extended interaction groove on the regulator's surface, implying multivalent binding of sulfated GAGs. This finding is confirmed by structure-based site-directed mutagenesis, nuclear magnetic resonance-monitored binding experiments performed for both H402 and Y402 variants with this and another model GAG, and analysis of an extended GAG-FH complex.B. Prosser is funded by the Wellcome Trust Structural Biology Training Program
(075415/Z/04/Z). S. Johnson and P. Roversi were funded by grants to S.M. Lea from
the Medical Research Council (MRC) of the United Kingdom (grants G0400389 and
G0400775). D. Uhrin and P.N. Barlow were funded by the Wellcome Trust (078780/
Z/05/Z). S.J. Clark was funded by an MRC Doctoral Training Account (G78/7925),
and R.B. Sim and A.J. Day were funded by MRC core funding to the MRC
Immunochemistry Unit
Spacetime Virasoro algebra from strings on zero radius AdS_3
We study bosonic string theory in the light-cone gauge on AdS_3 spacetime
with zero radius of curvature (in string units) R/\sqrt{\alpha^\prime}=0. We
find that the worldsheet theory admits an infinite number of conserved
quantities which are naturally interpreted as spacetime charges and which form
a representation of (two commuting copies of) a Virasoro algebra. Near the
boundary of AdS_3 these charges are found to be isomorphic to the infinite set
of asymptotic Killing vectors of AdS_3 found originally by Brown and Henneaux.
In addition to the spacetime Virasoro algebra, there is a worldsheet Virasoro
algebra that generates diffeomorphisms of the spatial coordinate of the string
worldsheet. We find that if the worldsheet Virasoro algebra has a central
extension then the spacetime Virasoro algebra acquires a central extension via
a mechanism similar to that encountered in the context of the SL(2,R) WZW
model.Our observations are consistent with a recently proposed duality between
bosonic strings on zero radius AdS_d+1 and free field theory in d dimensions.Comment: 23 pages, uses JHEP.cls. References adde
Formation sites of Population III star formation: The effects of different levels of rotation and turbulence on the fragmentation behavior of primordial gas
We use the moving-mesh code arepo to investigate the effects of different levels of rotation and turbulence on the fragmentation of primordial gas and the formation of Population III stars. We consider 9 different combinations of turbulence and rotation and carry out 5 different realizations of each setup, yielding one of the largest sets of simulations of Population III star formation ever performed. We find that fragmentation in Population III star-forming systems is a highly chaotic process and show that the outcomes of individual realizations of the same initial conditions often vary significantly. However, some general trends are apparent. Increasing the turbulent energy promotes fragmentation, while increasing the rotational energy inhibits fragmentation. Within the âŒ1000 yr period that we simulate, runs including turbulence yield flat protostellar mass functions while purely rotational runs show a more top-heavy distribution. The masses of the individual protostars are distributed over a wide range from a few 10â3 Mâ to several tens of Mâ. The total mass growth rate of the stellar systems remains high throughout the simulations and depends only weakly on the degree of rotation and turbulence. Mergers between protostars are common, but predictions of the merger fraction are highly sensitive to the criterion used to decide whether two protostars should merge. Previous studies of Population III star formation have often considered only one realization per set of initial conditions. However, our results demonstrate that robust trends can only be reliably identified by considering averages over a larger sample of runs
Laser ablation loading of a surface-electrode ion trap
We demonstrate loading by laser ablation of Sr ions into a
mm-scale surface-electrode ion trap. The laser used for ablation is a pulsed,
frequency-tripled Nd:YAG with pulse energies of 1-10 mJ and durations of 3-5
ns. An additional laser is not required to photoionize the ablated material.
The efficiency and lifetime of several candidate materials for the laser
ablation target are characterized by measuring the trapped ion fluorescence
signal for a number of consecutive loads. Additionally, laser ablation is used
to load traps with a trap depth (40 meV) below where electron impact ionization
loading is typically successful ( 500 meV).Comment: 4 pages, 4 figure
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