22,617 research outputs found
PACS and SPIRE photometer maps of M 33: First results of the HERschel M 33 Extended Survey (HERM33ES)
Context. Within the framework of the HERM33ES key program, we are studying the star forming interstellar medium in the nearby, metal-poor spiral galaxy M 33, exploiting the high resolution and sensitivity of Herschel.
Aims. We use PACS and SPIRE maps at 100, 160, 250, 350, and 500 μm wavelength, to study the variation of the spectral energy distributions (SEDs) with galacto-centric distance.
Methods. Detailed SED modeling is performed using azimuthally averaged fluxes in elliptical rings of 2 kpc width, out to 8 kpc galacto-centric distance. Simple isothermal and two-component grey body models, with fixed dust emissivity index, are fitted to the SEDs between 24 μm and 500 μm using also MIPS/Spitzer data, to derive first estimates of the dust physical conditions.
Results. The far-infrared and submillimeter maps reveal the branched, knotted spiral structure of M 33. An underlying diffuse disk is seen in all SPIRE maps (250–500 μm). Two component fits to the SEDs agree better than isothermal models with the observed, total and radially averaged flux densities. The two component model, with β fixed at 1.5, best fits the global and the radial SEDs. The cold dust component clearly dominates; the relative mass of the warm component is less than 0.3% for all the fits. The temperature of the warm component is not well constrained and is found to be about 60 K ± 10 K. The temperature of the cold component drops significantly from ~24 K in the inner 2 kpc radius to 13 K beyond 6 kpc radial distance, for the best fitting model. The gas-to-dust ratio for β = 1.5, averaged over the galaxy, is higher than the solar value by a factor of 1.5 and is roughly in agreement with the subsolar metallicity of M 33
Kaluza-Klein solitons reexamined
In (4 + 1) gravity the assumption that the five-dimensional metric is
independent of the fifth coordinate authorizes the extra dimension to be either
spacelike or timelike. As a consequence of this, the time coordinate and the
extra coordinate are interchangeable, which in turn allows the conception of
different scenarios in 4D from a single solution in 5D. In this paper, we make
a thorough investigation of all possible 4D scenarios, associated with this
interchange, for the well-known Kramer-Gross-Perry-Davidson-Owen set of
solutions. We show that there are {\it three} families of solutions with very
distinct geometrical and physical properties. They correspond to different sets
of values of the parameters which characterize the solutions in 5D. The
solutions of physical interest are identified on the basis of physical
requirements on the induced-matter in 4D. We find that only one family
satisfies these requirements; the other two violate the positivity of
mass-energy density. The "physical" solutions possess a lightlike singularity
which coincides with the horizon. The Schwarzschild black string solution as
well as the zero moment dipole solution of Gross and Perry are obtained in
different limits. These are analyzed in the context of Lake's geometrical
approach. We demonstrate that the parameters of the solutions in 5D are not
free, as previously considered. Instead, they are totally determined by
measurements in 4D. Namely, by the surface gravitational potential of the
astrophysical phenomena, like the Sun or other stars, modeled in Kaluza-Klein
theory. This is an important result which may help in observations for an
experimental/observational test of the theory.Comment: In V2 we include an Appendix, where we examine the conformal
approach. Minor changes at the beginning of section 2. In V3 more references
are added. Minor editorial changes in the Introduction and Conclusions
section
Young Black Men’s Risk to Firearm Homicide in Night Time Johannesburg, South Africa: a Retrospective Analysis based on the National Injury Mortality Surveillance System
Based on data from the South African National Injury Mortality Surveillance System (NIMSS), an epidemiological surveillance system of fatal injuries, this article reports on a retrospective analysis of the data on homicide in Johannesburg, South Africa. In South Africa, as is the case in other African countries, the collection of comprehensive, quality injury data, on which inferential analyses can be conducted, remains a challenge. As such, the analysis here was drawn from the NIMSS for homicides in Johannesburg for the years 2001 to 2005, as this period offered one the most complete datasets for homicide for the city. Focusing on the 5153 night time homicide victims, a binary logistic regression model was utilised to identify the likelihood of specific risk factors occurring in certain groups of people and contexts. The results illustrate that sex, race and time at night are particularly important risk factors for night time firearm homicide and the most at-risk population for night time homicide is urbanised young black men. The article concludes with a discussion of implications the results might have for preventative efforts, calling for programming targeted at young black men. Limitations of the investigation are noted.Keywords: firearm, homicide, Johannesburg, interpersonal violence, night time, South Africa, young black male
Singularity in Gravitational Collapse of Plane Symmetric Charged Vaidya Spacetime
We study the final outcome of gravitational collapse resulting from the plane
symmetric charged Vaidya spacetime. Using the field equations, we show that the
weak energy condition is always satisfied by collapsing fluid. It is found that
the singularity formed is naked. The strength of singularity is also
investigated by using Nolan's method. This turns out to be a strong curvature
singularity in Tipler's sense and hence provides a counter example to the
cosmic censorship hypothesis.Comment: 8 pages, accepted for publication in Mod. Phys. Lett
Pumps as turbines for efficient energy recovery in water supply networks
The present work aims to enhance the energy efficiency of water supply networks by investigating technical and economical feasibility of energy recovery plants at low installed capacities. A cost effective stainless steel machine is investigated in pump and turbine operation and established conversion methods are used for predicting the best efficiency point of the turbine. The obtained results show a reasonable agreement of the BEP, but experimental investigations are still indispensable for a determination of complete Q-H-characteristics. The hydraulic machine is implemented in field at the transfer shaft of a high-level tank and valuable data concerning investment costs and water hammer have been collected. A proposed cost classification scheme will facilitate the acquisition of investment costs for further applications. The economic profitability of the hydropower plant is evaluated by the net present value method and the obtained results give incentives to exploit unused energy recovery potential within water supply systems
Single Top Quark Production and Decay at Next-to-leading Order in Hadron Collision
We present a calculation of the next-to-leading order QCD corrections, with
one-scale phase space slicing method, to single top quark production and decay
process at hadron colliders.
Using the helicity amplitude method, the angular correlation of the final state
partons and the spin correlation of the top quark are preserved. The effect of
the top quark width is also examined.Comment: 47 pages, 9 figure
Necessary and sufficient condition for hydrostatic equilibrium in general relativity
We present explicit examples to show that the `compatibility criterion' is
capable of providing a {\em necessary} and {\em sufficient} condition for any
regular configuration to be compatible with the state of hydrostatic
equilibrium. This conclusion is drawn on the basis of the finding that the
relation gives the necessary and sufficient condition for dynamical
stability of equilibrium configurations only when the compatibility criterion
for these configurations is appropriately satisfied. In this regard, we
construct an appropriate sequence composed of core-envelope models on the basis
of compatibility criterion, such that each member of this sequence satisfies
the extreme case of causality condition at the centre. The maximum
stable value of (which occurs for the model corresponding to
the maximum value of mass in the mass-radius relation) and the corresponding
central value of the local adiabatic index, , of
this model are found fully consistent with those of the corresponding {\em
absolute} values, , and ,
which impose strong constraints on these parameters of such models. In addition
to this example, we also study dynamical stability of pure adiabatic polytropic
configurations on the basis of variational method for the choice of the `trial
function', , as well as the mass-central density relation,
since the compatibility criterion is appropriately satisfied for these models.
The results of this example provide additional proof in favour of the statement
regarding compatibility criterion mentioned above.Comment: 31 pages (double-spaced) revtex style, 1 figure in `ps' forma
Exact relativistic stellar models with liquid surface. I. Generalizing Buchdahl's polytrope
A family of exact relativistic stellar models is described. The family
generalizes Buchdahl's n=1 polytropic solution. The matter content is a perfect
fluid and, excluding Buchdahl's original model, it behaves as a liquid at low
pressures in the sense that the energy density is non-zero in the zero pressure
limit. The equation of state has two free parameters, a scaling and a stiffness
parameter. Depending on the value of the stiffness parameter the fluid
behaviour can be divided in four different types. Physical quantities such as
masses, radii and surface redshifts as well as density and pressure profiles
are calculated and displayed graphically. Leaving the details to a later
publication, it is noted that one of the equation of state types can quite
accurately approximate the equation of state of real cold matter in the outer
regions of neutron stars. Finally, it is observed that the given equation of
state does not admit models with a conical singularity at the center.Comment: 19 pages, 12 figures (16 eps files), LaTeX2e with the standard
packages amssymb, amsmath, graphicx, subfigure, psfra
Infrared Photometry of Starless Dense Cores
Deep JHKs photometry was obtained towards eight dense molecular cores and J-H
vs. H-Ks color-color plots are presented. Our photometry, sensitive to the
detection of a 1 solar mass, 1 X 10^6 year old star through approx. 35 - 50
magnitudes of visual extinction, shows no indication of the presence of
star/disk systems based on J-H vs. H-Ks colors of detected objects. The stars
detected towards the cores are generally spatially anti-correlated with core
centers suggesting a background origin, although we cannot preclude the
possibility that some stars detected at H and Ks alone, or Ks alone, are not
low mass stars or brown dwarfs (< 0.3 Solar Masses) behind substantial amounts
of visual extinction (e.g. 53 magnitudes for L183B). Lower limits to optical
extinctions are estimated for the detected background stars, with high
extinctions being encountered, in the extreme case ranging up to at least Av =
46, and probably higher. The extinction data are used to estimate cloud masses
and densities which are comparable to those determined from molecular line
studies. Variations in cloud extinctions are consistent with a systematic
nature to cloud density distributions and column density variations and
extinctions are found to be consistent with submillimeter wave continuum
studies of similar regions. The results suggest that some cores have achieved
significant column density contrasts (approx. 30) on sub-core scales (approx.
0.05 pc) without having formed known stars.Comment: 44 pages including tables and figures, accepted ApJ, March 24, 200
A wave driver theory for vortical waves propagating across junctions with application to those between rigid and compliant walls
A theory is described for propagation of vortical waves across alternate rigid and compliant panels. The structure in the fluid side at the junction of panels is a highly vortical narrow viscous structure which is idealized as a wave driver. The wave driver is modelled as a ‘half source cum half sink’. The incoming wave terminates into this structure and the outgoing wave emanates from it. The model is described by half Fourier–Laplace transforms respectively for the upstream and downstream sides of the junction. The cases below cutoff and above cutoff frequencies are studied. The theory completely reproduces the direct numerical simulation results of Davies & Carpenter (J. Fluid Mech., vol. 335, 1997, p. 361). Particularly, the jumps across the junction in the kinetic energy integral, the vorticity integral and other related quantities as obtained in the work of Davies & Carpenter are completely reproduced. Also, some important new concepts emerge, notable amongst which is the concept of the pseudo group velocity
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