40,390 research outputs found
Substructure in Tidal Streams; Tributaries in the Anticenter Ring
We report on the detection in Sloan Digital Sky Survey data of at least
three, roughly parallel components in a 65 degree-long stellar stream complex
previously identified with the Anticenter or Monoceros Ring. The three-stream
complex varies in width from 4 to 6 degrees along its length and appears to be
made up of two or more narrow substreams as well as a broader, diffuse
component. The width and complexity of the stream indicate that the progenitor
was likely a dwarf galaxy of significant size and mass. The stream is 8.9 kpc
distant and is oriented almost perpendicularly to our line of sight. The
visible portion of the stream does not pass near any known dwarf galaxies and a
preliminary orbit does not point to any viable progenitor candidates. Orbits
for the narrower substreams can be modeled with velocity offsets from the broad
component of about 8 km/s. We suggest that the broad component is likely to be
the remains of a dwarf galaxy, while the narrower streams constitute the
remnants of dynamically distinct components which may have included a native
population of globular clusters. While the color of the main sequence turn-off
is not unlike that for the Monoceros Ring, neither the visible stream nor any
reasonable projection of its orbit passes through Monoceros or Canis Major, and
we conclude that this stream is probably unrelated to the overdensities found
in these regions.Comment: 11 pages, 4 figures, accepted for publication in ApJ Letter
Evolution: Complexity, uncertainty and innovation
Complexity science provides a general mathematical basis for evolutionary thinking. It makes us face the inherent, irreducible nature of uncertainty and the limits to knowledge and prediction. Complex, evolutionary systems work on the basis of on-going, continuous internal processes of exploration, experimentation and innovation at their underlying levels. This is acted upon by the level above, leading to a selection process on the lower levels and a probing of the stability of the level above. This could either be an organizational level above, or the potential market place. Models aimed at predicting system behaviour therefore consist of assumptions of constraints on the micro-level – and because of inertia or conformity may be approximately true for some unspecified time. However, systems without strong mechanisms of repression and conformity will evolve, innovate and change, creating new emergent structures, capabilities and characteristics. Systems with no individual freedom at their lower levels will have predictable behaviour in the short term – but will not survive in the long term. Creative, innovative, evolving systems, on the other hand, will more probably survive over longer times, but will not have predictable characteristics or behaviour. These minimal mechanisms are all that are required to explain (though not predict) the co-evolutionary processes occurring in markets, organizations, and indeed in emergent, evolutionary communities of practice. Some examples will be presented briefly
Information Flow in an R and D Laboratory
Statistical analysis of hypotheses concerning roles of technological gatekeeper and primary groups in flow of information in small research and development laborator
The Production of HI in Photodissociation Regions and A Comparison with CO(1-0) Emission
The gas at the surfaces of molecular clouds in galaxies is heated and
dissociated by photons from young stars both near and far. HI resulting from
the dissociation of molecular hydrogen H2 emits hyperfine line emission at 21
cm, and warmed CO emits dipole rotational lines such as the 2.6 mm line of
CO(1-0). We use previously developed models for photodissociation regions
(PDRs) to compute the intensities of these HI and CO(1-0) lines as a function
of the total volume density n in the cloud and the far ultraviolet flux G0
incident upon it and present the results in units familiar to observers. The
intensities of these two lines behave differently with changing physical
conditions in the PDR, and, taken together, the two lines can provide a
ground-based radio astronomy diagnostic for determining n and G0 separately in
distant molecular clouds. This diagnostic is particularly useful in the range
Gzero <~ 100, 10 cm^{-3} <~ n <~ 10^5 cm^{-3}, which applies to a large
fraction of the volume of the interstellar medium in galaxies. If the molecular
cloud is located near discrete sources of far-UV (FUV) emission, the
PDR-generated HI and CO(1-0) emission on the cloud surface can be more easily
identified, appearing as layered ``blankets'' or ``blisters'' on the side of
the cloud nearest to the FUV source. As an illustration, we consider the
Galactic object G216 -2.5, i.e. ``Maddalena's Cloud'', which has been
previously identified as a large PDR in the Galaxy. We determine that this
cloud has n ~ 200 cm^{-3}, G0 ~ 0.8, consistent with other data.Comment: 13 Pages, 3 Figures. Accepted for publication in the Astrophysical
Journa
Spinor Parallel Propagator and Green's Function in Maximally Symmetric Spaces
We introduce the spinor parallel propagator for maximally symmetric spaces in
any dimension. Then, the Dirac spinor Green's functions in the maximally
symmetric spaces R^n, S^n and H^n are calculated in terms of intrinsic
geometric objects. The results are covariant and coordinate-independent.Comment: 7 page
Optical pumping of the electron spin polarization in bulk CuCl
In CuCl bulk crystal negatively charged excitons (trions ) can be
induced by the resonant optical excitation of extra electrons in conduction
band minimum. In the case of light polarization and due to the top valence band
structure of CuCl only the electrons with spin antiparallel to the direction of
the light propagation contribute to the formation of , while the emerging
can recombine into both possible electron states, with spin parallel and
antiparallel to the direction of light propagation. We propose to use this
mechanism for optical electronic spin pumping. We describe the dynamics of
pumping in terms of density matrix formalism. The coherent pumping laser pulse
propagating through the sample is described by Maxwell wave equation coupled to
the density matrix evolution equations. The results of our approximate simple
model calculations suggest that spin polarization close to 100% can be achieved
in time shorter than 100ps.Comment: new extended version, 7 pages, 4 figure
Influence of an Internal Magnetar on Supernova Remnant Expansion
Most of the proposed associations between magnetars and supernova remnant
suffer from age problems. Usually, supernova remnants ages are determined from
an approximation of the Sedov-Taylor phase relation between radius and age, for
a fixed energy of the explosion ~ 10^{51} erg. Those ages do not generally
agree with the characteristic ages of the (proposed) associated magnetars. We
show quantitatively that, by taking into account the energy injected on the
supernova remnant by magnetar spin-down, a faster expansion results, improving
matches between characteristic ages and supernova remnants ages. However, the
magnetar velocities inferred from observations would inviabilize some
associations. Since characteristic ages may not be good age estimators, their
influence on the likelihood of the association may not be as important.
In this work we present simple numerical simulations of supernova remnants
expansion with internal magnetars, and apply it to the observed objects. A
short initial spin period, thought to be important for the very generation of
the magnetic field, is also relevant for the modified expansion of the remnant.
We next analyze all proposed associations case-by-case, addressing the
likelyhood of each one, according to this perspective. We consider a larger
explosion energy and reasses the characteristic age issue, and conclude that
about 50% of the associations can be true ones, provided SGRs and AXPs are
magnetars.Comment: 30 pages, AAStex, 5 figures, format fixe
Forward Flux Sampling-type schemes for simulating rare events: Efficiency analysis
We analyse the efficiency of several simulation methods which we have
recently proposed for calculating rate constants for rare events in stochastic
dynamical systems, in or out of equilibrium. We derive analytical expressions
for the computational cost of using these methods, and for the statistical
error in the final estimate of the rate constant, for a given computational
cost. These expressions can be used to determine which method to use for a
given problem, to optimize the choice of parameters, and to evaluate the
significance of the results obtained. We apply the expressions to the
two-dimensional non-equilibrium rare event problem proposed by Maier and Stein.
For this problem, our analysis gives accurate quantitative predictions for the
computational efficiency of the three methods.Comment: 19 pages, 13 figure
Young mothers in care, contributing to the contemporary debate
Teenage pregnancy has become a broad issue in contemporary society and has become a focus for concern for young women in or exiting the care system. The article draws on interviews with twenty-four young mothers in, or on the fringes of the care system. It highlights the thoughts and feelings of these young women, specifically looking at the relationships that they have with their mothers, the father of their baby and their social workers. The mothers' sources of support and their perceptions of these are discussed. Implications for practice for working with this discrete group of young mothers are explored
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