8,093 research outputs found
Non-equilibrium mechanics and dynamics of motor activated gels
The mechanics of cells is strongly affected by molecular motors that generate
forces in the cellular cytoskeleton. We develop a model for cytoskeletal
networks driven out of equilibrium by molecular motors exerting transient
contractile stresses. Using this model we show how motor activity can
dramatically increase the network's bulk elastic moduli. We also show how motor
binding kinetics naturally leads to enhanced low-frequency stress fluctuations
that result in non-equilibrium diffusive motion within an elastic network, as
seen in recent \emph{in vitro} and \emph{in vivo} experiments.Comment: 21 pages, 8 figure
Low-Mass X-ray Binaries and Globular Clusters in Early-Type Galaxies. I. Chandra Observations
We present a Chandra survey of LMXBs in 24 early-type galaxies. Correcting
for detection incompleteness, the X-ray luminosity function (XLF) of each
galaxy is consistent with a powerlaw with negative logarithmic differential
slope, beta~2.0. However, beta strongly correlates with incompleteness,
indicating the XLF flattens at low-Lx. The composite XLF is well-fitted by a
powerlaw with a break at 2.21(+0.65,-0.56)E38 erg/s and beta=1.40(+0.10,-0.13)
and 2.84(+0.39,-0.30) below and above it, respectively. The break is close to
the Eddington limit for a 1.4Msun neutron-star, but the XLF shape rules out its
representing the division between neutron-star and black-hole systems. Although
the XLFs are similar, we find evidence of some variation between galaxies. The
high-Lx XLF slope does not correlate with age, but may correlate with
[alpha/Fe]. Considering only LMXBs with Lx>1E37 erg/s, matching the LMXBs with
globular clusters (GCs) identified in HST observations of 19 of the galaxies,
we find the probability a GC hosts an LMXB is proportional to LGC^alpha
ZFe^gamma} where alpha=1.01+/-0.19 and gamma=0.33+/-0.11. Correcting for GC
luminosity and colour effects, and detection incompleteness, we find no
evidence that the fraction of LMXBs with Lx>1e37 erg/s in GCs (40%), or the
fraction of GCs hosting LMXBs (~6.5%) varies between galaxies. The spatial
distribution of LMXBs resembles that of GCs, and the specific frequency of
LMXBs is proportional to the GC specific luminosity, consistent with the
hypothesis that all LMXBs form in GCs. If the LMXB lifetime is tau and the duty
cycle is Fd, our results imply ~1.5 (tau/1E8 yr)^-1 /Fd LMXBs are formed per
Gyr per GC and we place an upper limit of 1 active LMXB in the field per
3.4E9Lsun of V-band luminosity.Comment: 24 pages, 17 figures and 6 tables. Accepted for publication in the
Astrophysical Journal. Expanded discussion and various minor revisions to
improve robustness of results. Conclusions unchange
Nematic and Polar order in Active Filament Solutions
Using a microscopic model of interacting polar biofilaments and motor
proteins, we characterize the phase diagram of both homogeneous and
inhomogeneous states in terms of experimental parameters. The polarity of motor
clusters is key in determining the organization of the filaments in homogeneous
isotropic, polarized and nematic states, while motor-induced bundling yields
spatially inhomogeneous structures.Comment: 4 pages. 3 figure
Increasing concentrations of dichloromethane, CH2Cl2, inferred from CARIBIC air samples collected 1998–2012
Atmospheric concentrations of dichloromethane, CH2Cl2, a regulated toxic air pollutant and minor contributor to stratospheric ozone depletion, were reported to have peaked around 1990 and to be declining in the early part of the 21st century. Recent observations suggest this trend has reversed and that CH2Cl2 is once again increasing in the atmosphere. Despite the importance of ongoing monitoring and reporting of atmospheric CH2Cl2, no time series has been discussed in detail since 2006. The CARIBIC project (Civil Aircraft for the Regular Investigation of the atmosphere Based on an Instrument Container) has analysed the halocarbon content of whole-air samples collected at altitudes of between ~10–12 km via a custom-built container installed on commercial passenger aircraft since 1998, providing a long-term record of CH2Cl2 observations. In this paper we present this unique CH2Cl2 time series, discussing key flight routes which have been used at various times over the past 15 years. Between 1998 and 2012 increases were seen in all northern hemispheric regions and at different altitudes, ranging from ~7–10 ppt in background air to ~13–15 ppt in regions with stronger emissions (equating to a 38–69% increase). Of particular interest is the rising importance of India as a source of atmospheric CH2Cl2: based on CARIBIC data we provide regional emission estimates for the Indian subcontinent and show that regional emissions have increased from 3–14 Gg yr^-1 (1998–2000) to 16–25 Gg yr^-1 (2008). Potential causes of the increasing atmospheric burden of CH2Cl2 are discussed. One possible source is the increased use of CH2Cl2 as a feedstock for the production of HFC-32, a chemical used predominantly as a replacement for ozone-depleting substances in a variety of applications including air conditioners and refrigeration
Study of an advanced General Aviation Turbine Engine (GATE)
The best technology program for a small, economically viable gas turbine engine applicable to the general aviation helicopter and aircraft market for 1985-1990 was studied. Turboshaft and turboprop engines in the 112 to 746 kW (150 to 1000 hp) range and turbofan engines up to 6672 N (1500 lbf) thrust were considered. A good market for new turbine engines was predicted for 1988 providing aircraft are designed to capitalize on the advantages of the turbine engine. Parametric engine families were defined in terms of design and off-design performance, mass, and cost. These were evaluated in aircraft design missions selected to represent important market segments for fixed and rotary-wing applications. Payoff parameters influenced by engine cycle and configuration changes were aircraft gross mass, acquisition cost, total cost of ownership, and cash flow. Significant advantage over a current technology, small gas turbine engines was found especially in cost of ownership and fuel economy for airframes incorporating an air-cooled high-pressure ratio engine. A power class of 373 kW (500 hp) was recommended as the next frontier for technology advance where large improvements in fuel economy and engine mass appear possible through component research and development
Structural motifs of biomolecules
Biomolecular structures are assemblies of emergent anisotropic building
modules such as uniaxial helices or biaxial strands. We provide an approach to
understanding a marginally compact phase of matter that is occupied by proteins
and DNA. This phase, which is in some respects analogous to the liquid crystal
phase for chain molecules, stabilizes a range of shapes that can be obtained by
sequence-independent interactions occurring intra- and intermolecularly between
polymeric molecules. We present a singularityfree self-interaction for a tube
in the continuum limit and show that this results in the tube being positioned
in the marginally compact phase. Our work provides a unified framework for
understanding the building blocks of biomolecules.Comment: 13 pages, 5 figure
Delphi study to identify key features of community-based child and adolescent mental health services in the East of England.
OBJECTIVE: To identify priorities for the delivery of community-based Child and Adolescent Mental health Services (CAMHS). DESIGN: (1) Qualitative methods to gather public and professional opinions regarding the key principles and components of effective service delivery. (2) Two-round, two-panel adapted Delphi study. The Delphi method was adapted so professionals received additional feedback about the public panel scores. Descriptive statistics were computed. Items rated 8-10 on a scale of importance by ≥80% of both panels were identified as shared priorities. SETTING: Eastern region of England. PARTICIPANTS: (1) 53 members of the public; 95 professionals from the children's workforce. (2) Two panels. Public panel: round 1,n=23; round 2,n=16. Professional panel: round 1,n=44; round 2,n=33. RESULTS: 51 items met the criterion for between group consensus. Thematic grouping of these items revealed three key findings: the perceived importance of schools in mental health promotion and prevention of mental illness; an emphasis on how specialist mental health services are delivered rather than what is delivered (ie, specific treatments/programmes), and the need to monitor and evaluate service impact against shared outcomes that reflect well-being and function, in addition to the mere absence of mental health symptoms or disorders. CONCLUSIONS: Areas of consensus represent shared priorities for service provision in the East of England. These findings help to operationalise high level plans for service transformation in line with the goals and needs of those using and working in the local system and may be particularly useful for identifying gaps in ongoing transformation efforts. More broadly, the method used here offers a blueprint that could be replicated by other areas to support the ongoing transformation of CAMHS
Structure of a model TiO2 photocatalytic interface
The interaction of water with TiO2 is crucial to many of its practical
applications, including photocatalytic water splitting. Following the first
demonstration of this phenomenon 40 years ago there have been numerous studies
of the rutile single-crystal TiO2(110) interface with water. This has provided
an atomic-level understanding of the water-TiO2 interaction. However, nearly
all of the previous studies of water/TiO2 interfaces involve water in the
vapour phase. Here, we explore the interfacial structure between liquid water
and a rutile TiO2(110) surface pre-characterized at the atomic level. Scanning
tunnelling microscopy and surface X-ray diffraction are used to determine the
structure, which is comprised of an ordered array of hydroxyl molecules with
molecular water in the second layer. Static and dynamic density functional
theory calculations suggest that a possible mechanism for formation of the
hydroxyl overlayer involves the mixed adsorption of O2 and H2O on a partially
defected surface. The quantitative structural properties derived here provide a
basis with which to explore the atomistic properties and hence mechanisms
involved in TiO2 photocatalysis
Extremely Metal-Poor Galaxies: The HI Content
Extremely metal-poor (XMP) galaxies are chemically, and possibly dynamically,
primordial objects in the local Universe. Our objective is to characterize the
HI content of the XMP galaxies as a class, using as a reference the list of 140
known local XMPs compiled by Morales-Luis et al. (2011). We have observed 29
XMPs, which had not been observed before at 21 cm, using the Effelsberg radio
telescope. This information was complemented with HI data published in
literature for a further 53 XMPs. In addition, optical data from the literature
provided morphologies, stellar masses, star-formation rates and metallicities.
Effelsberg HI integrated flux densities are between 1 and 15 Jy km/s, while
line widths are between 20 and 120 km/s. HI integrated flux densities and line
widths from literature are in the range 0.1 - 200 Jy km/s and 15 - 150 km/s,
respectively. Of the 10 new Effelsberg detections, two sources show an
asymmetric double-horn profile, while the remaining sources show either
asymmetric (7 sources) or symmetric (1 source) single-peak 21 cm line profiles.
An asymmetry in the HI line profile is systematically accompanied by an
asymmetry in the optical morphology. Typically, the g-band stellar
mass-to-light ratios are ~0.1, whereas the HI gas mass-to-light ratios may be
up to 2 orders of magnitude larger. Moreover, HI gas-to-stellar mass ratios
fall typically between 10 and 20, denoting that XMPs are extremely gas-rich. We
find an anti-correlation between the HI gas mass-to-light ratio and the
luminosity, whereby fainter XMPs are more gas-rich than brighter XMPs,
suggesting that brighter sources have converted a larger fraction of their HI
gas into stars. The dynamical masses inferred from the HI line widths imply
that the stellar mass does not exceed 5% of the dynamical mass, while the
\ion{H}{i} mass constitutes between 20 and 60% of the dynamical mass.
(abridged)Comment: 30 pages, accepted for A&
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