5,281 research outputs found
Star Formation and Gas Dynamics in Galactic Disks: Physical Processes and Numerical Models
Star formation depends on the available gaseous "fuel" as well as galactic
environment, with higher specific star formation rates where gas is
predominantly molecular and where stellar (and dark matter) densities are
higher. The partition of gas into different thermal components must itself
depend on the star formation rate, since a steady state distribution requires a
balance between heating (largely from stellar UV for the atomic component) and
cooling. In this presentation, I discuss a simple thermal and dynamical
equilibrium model for the star formation rate in disk galaxies, where the basic
inputs are the total surface density of gas and the volume density of stars and
dark matter, averaged over ~kpc scales. Galactic environment is important
because the vertical gravity of the stars and dark matter compress gas toward
the midplane, helping to establish the pressure, and hence the cooling rate. In
equilibrium, the star formation rate must evolve until the gas heating rate is
high enough to balance this cooling rate and maintain the pressure imposed by
the local gravitational field. In addition to discussing the formulation of
this equilibrium model, I review the current status of numerical simulations of
multiphase disks, focusing on measurements of quantities that characterize the
mean properties of the diffuse ISM. Based on simulations, turbulence levels in
the diffuse ISM appear relatively insensitive to local disk conditions and
energetic driving rates, consistent with observations. It remains to be
determined, both from observations and simulations, how mass exchange processes
control the ratio of cold-to-warm gas in the atomic ISM.Comment: 8 pages, 1 figure; to appear in "IAU Symposium 270: Computational
Star formation", Eds. J. Alves, B. Elmegreen, J. Girart, V. Trimbl
A New Halo Finding Method for N-Body Simulations
We have developed a new halo finding method, Physically Self-Bound (PSB)
group finding algorithm, which can efficiently identify halos located even at
crowded regions. This method combines two physical criteria such as the tidal
radius of a halo and the total energy of each particle to find member
particles. Two hierarchical meshes are used to increase the speed and the power
of halo identification in the parallel computing environments. First, a coarse
mesh with cell size equal to the mean particle separation is
used to obtain the density field over the whole simulation box. Mesh cells
having density contrast higher than a local cutoff threshold
are extracted and linked together for those adjacent to each other. This
produces local-cell groups. Second, a finer mesh is used to obtain density
field within each local-cell group and to identify halos. If a density shell
contains only one density peak, its particles are assigned to the density peak.
But in the case of a density shell surrounding at least two density peaks, we
use both the tidal radii of halo candidates enclosed by the shell and the total
energy criterion to find physically bound particles with respect to each halo.
Similar to DENMAX and HOP, the \hfind method can efficiently identify small
halos embedded in a large halo, while the FoF and the SO do not resolve such
small halos. We apply our new halo finding method to a 1-Giga particle
simulation of the CDM model and compare the resulting mass function
with those of previous studies. The abundance of physically self-bound halos is
larger at the low mass scale and smaller at the high mass scale than proposed
by the Jenkins et al. (2001) who used the FoF and SO methods. (abridged)Comment: 10 pages, 8 figs, submitted to Ap
From dictatorship to a reluctant democracy: Stroke therapists talking about self-management
This article is made available through the Brunel Open Access Publishing Fund.Purpose: Self-management is being increasingly promoted within chronic conditions including
stroke. Concerns have been raised regarding professional ownership of some programmes,
yet little is known of the professionalâs experience. This paper aims to present the views of
trained therapists about the utility of a specific self-management approach in stroke
rehabilitation. Method: Eleven stroke therapists trained in the self-management approach
participated in semi-structured interviews. These were audio recorded, transcribed verbatim
and analysed thematically. Results: Two overriding themes emerged. The first was the sense
that in normal practice therapists act as ââbenign dictatorsââ, committed to help their patients,
but most comfortable when they, the professional, are in control. Following the adoption of
the self-management approach therapists challenged themselves to empower stroke
survivors to take control of their own recovery. However, therapists had to confront many
internal and external challenges in this transition of power resulting in the promotion of a
somewhat ââreluctant democracyââ. Conclusions: This study illustrates that stroke therapists
desire a more participatory approach to rehabilitation. However, obstacles challenged the
successful delivery of this goal. If self-management is an appropriate model to develop in
post stroke pathways, then serious consideration must be given to how and if these
obstacles can be overcome
On Local Approximations to the Nonlinear Evolution of Large-Scale Structure
We present a comparative analysis of several methods, known as local
Lagrangian approximations, which are aimed to the description of the nonlinear
evolution of large-scale structure. We have investigated various aspects of
these approximations, such as the evolution of a homogeneous ellipsoid,
collapse time as a function of initial conditions, and asymptotic behavior. As
one of the common features of the local approximations, we found that the
calculated collapse time decreases asymptotically with the inverse of the
initial shear. Using these approximations, we have computed the cosmological
mass function, finding reasonable agreement with N-body simulations and the
Press-Schechter formula.Comment: revised version with color figures, minor changes, accepted for
publication in the Astrophysical Journal, 30 pages, 13 figure
Fiscal decentralisation and local government efficiency: Does relative deprivation matter?
Fiscal decentralisation arguably improves government efficiency because it enhances responsiveness to local policy issues and incentivises fiscal discipline. However, critics suggest that central control over local spending is necessary to equalise fiscal outcomes between prosperous and deprived areas. Using a two-stage analysis, we investigate the validity of these arguments by analysing the separate and combined effects of fiscal decentralisation and socio-economic deprivation on the productive efficiency of English local governments during 2002?2008. The results suggest that decentralisation is positively related to productive efficiency and that there is a negative relationship between socio-economic deprivation and efficiency. Further analysis reveals that deprivation weakens the positive decentralisation-efficiency relationship, calling into question simplistic proposals for fiscal decentralisation
Analytical theory for the initial mass function: CO clumps and prestellar cores
We derive an analytical theory of the prestellar core initial mass function
based on an extension of the Press-Schechter statistical formalism. With the
same formalism, we also obtain the mass spectrum for the non self-gravitating
clumps produced in supersonic flows. The mass spectrum of the self-gravitating
cores reproduces very well the observed initial mass function and identifies
the different mechanisms responsible for its behaviour. The theory predicts
that the shape of the IMF results from two competing contributions, namely a
power-law at large scales and an exponential cut-off (lognormal form) centered
around the characteristic mass for gravitational collapse. The cut-off exists
already in the case of pure thermal collapse, provided that the underlying
density field has a lognormal distribution. Whereas pure thermal collapse
produces a power-law tail steeper than the Salpeter value, dN/dlog M\propto
M^{-x}, with x=1.35, this latter is recovered exactly for the (3D) value of the
spectral index of the velocity power spectrum, n\simeq 3.8, found in
observations and in numerical simulations of isothermal supersonic turbulence.
Indeed, the theory predicts that x=(n+1)/(2n-4) for self-gravitating structures
and x=2-n'/3 for non self-gravitating structures, where n' is the power
spectrum index of log(rho). We show that, whereas supersonic turbulence
promotes the formation of both massive stars and brown dwarfs, it has an
overall negative impact on star formation, decreasing the star formation
efficiency. This theory provides a novel theoretical foundation to understand
the origin of the IMF and to infer its behaviour in different environments. It
also provides a complementary approach and useful guidance to numerical
simulations exploring star formation, while making testable predictions.Comment: To appear in Ap
The outsourcing of social care in Britain : what does it mean for voluntary sector workers?
While recent decades have witnessed a growth in the outsourcing of public services in Britain, the post-1997 UK Labour governments have sought to put in place mechanisms aimed at encouraging long-term collaborative contracting relationships marked by less reliance on cost-based competition. This article explores empirically how far these mechanisms have achieved their aims and thereby acted to protect the employment conditions of staff, and links this exploration to debates concerning the employment implications of organizational reforms within public sectors internationally. It concludes that in terms of bringing income security to the voluntary sector and stability to employment terms and conditions these efforts have been unsuccessful, and consequently casts doubts on more optimistic interpretations of the employment effects of organizational restructuring in the British public sector
Hospital food service: a comparative analysis of systems and introducing the âSteamplicityâ concept
Background Patient meals are an integral part of treatment hence the provision and consumption of a balanced diet, essential to aid recovery. A number of food service systems are used to provide meals and the Steamplicity concept has recently been introduced. This seeks, through the application of a static, extended choice menu, revised patient ordering procedures, new cooking processes and individual patient food heated/cooked at ward level, to address some of the current hospital food service concerns. The aim of this small-scale study, therefore, was to compare a cook-chill food service operation against Steamplicity. Specifically, the goals were to measure food intake and wastage at ward level; âstakeholdersâ (i.e. patients, staff, etc.) satisfaction with both systems; and patientsâ acceptability of the food provided.
Method The study used both quantitative (self-completed patient questionnaires, n = 52) and qualitative methods (semi-structured interviews, n = 16) with appropriate stakeholders including medical and food service staff, patients and their visitors.
Results Patients preferred the Steamplicity system overall and in particular in terms of food choice, ordering, delivery and food quality. Wastage was considerably less with the Steamplicity system, although care must be taken to ensure that poor operating procedures do not negate this advantage. When the total weight of food consumed in the ward at each meal is divided by the number of main courses served, at lunch, the mean intake with the cook-chill system was 202 g whilst that for the Steamplicity system was 282 g and for the evening meal, 226 g compared with 310 g.
Conclusions The results of this small study suggest that Steamplicity is more acceptable to patients and encourages the consumption of larger portions. Further evaluation of the Steamplicity system is warranted.
The purpose of this study was to directly compare selected aspects (food wastage at ward level; satisfaction with systems and food provided) of a traditional cook-chill food service operation against âSteamplicityâ. Results indicate that patients preferred the âSteamplictyâ system in all areas: food choice, ordering, delivery, food quality and overall. Wastage was considerably less with the âSteamplicityâ system; although care must be taken to ensure that poor operating procedures do not negate this advantage. When the total weight of food consumed in the ward at each meal is divided by the number of main courses served, results show that at lunch, mean intake with the cook-chill system was 202g whilst that for the âSteamplicityâ system was 282g and for the evening meal, 226g compared with 310g
The performance of local authority sports facilities in England during a period of recession and austerity
This paper examines how public sport facilities in England, which are dependent on subsidy from local authorities, have performed during a period of recession and austerity. Using data derived from Sport England's National Benchmarking Service (NBS), we track the changes in a series of key operational performance measures to investigate how local authority sports centres have fared between 2005/06 and 2015/16. Four dimensions of performance are analysed, namely: access; finance; utilisation; and, customer satisfaction. The study includes 1,116 sports centres in the time frame under review. Our results show that the overall financial efficiency of English public sport facilities has improved significantly in the face of a reduction in local authority expenditure on sport and leisure services. There appears to be a business model in the sector that includes outsourcing management and raising activity charges, which has been accompanied by higher levels of customer satisfaction with price-related service attributes and with the overall experience of using a facility. A possible explanation for these findings might be an improved quality of provision and greater customer orientation. At the same time, there appears to be a diminished focus on social inclusion objectives. We therefore conclude that promoting access to public sport facilities for hard-to-reach or disadvantaged groups has been compromised in exchange for the pursuit of financial stability. A key challenge is how to achieve market development whereby new and targeted customers are attracted to these existing facilities
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