484 research outputs found
Planetary Collisions: Electromagnetic Signals
We investigate the electromagnetic signals accompanied with planetary
collisions and their event rate, and explore the possibility of directly
detecting such events. A typical Earth--Jupiter collision would give rise to a
prompt EUV-soft-X-ray flash lasting for hours and a bright IR afterglow lasting
for thousands of years. With the current and forthcoming observational
technology and facilities, some of these collisional flashes or the
post-collision remnants could be discovered.Comment: 4 pages, to appear in Proceedings of 14th Annual October Astrophysics
Conference in Maryland, "The Search for Other Worlds". A concise version of
ApJ, 596, L95 (2003), astro-ph/030821
Testing gravity in Large Extra Dimensions using Bose-Einstein Condensates
Recent conjectures that there are mesoscopically ``large'' extra dimensions,
through which gravity propagates have interesting implications for much of
physics. The scenario implies gross departures from Newton's law of gravity at
small length scales. Testing departures from Coulomb's law on sub-millimetre
scales is hard. It is now possible to routinely create Bose-Einstein
condensates with de Broglie wavelengths of order a and total size of
order . BEC condensates move coherently under gravitational
acceleration, and I propose that the transverse fringe shift due to the
acceleration of pair of interfering BECs passing a dense linear mass may be
measurable, and provide direct evidence for anomalous gravitational
acceleration. Ideally such experiments are best carried out in free fall to
maximise the time spent by a BEC in the non-Newtonian regime.Comment: 2 pages, double column, revtex, no figures Int J. Mod Phys D.
(Special issue of GRF essays 2002). Int. J. Mod. Phys. D. in pres
Dynamics and Interactions of Binaries and Neutron Stars in Globular Clusters
We model the dynamics of test binaries in isotropic, multi-mass models of
galactic globular clusters. The evolution of binary orbits through the cluster
potentials is modeled, including second order diffusion terms, and
probabilities for close encounters with field stars are calculated. We carry
out Monte Carlo simulations of the effects of the binary--single star
encounters on the binary population and distribution in the cluster, and
estimate the collision rate for different stellar populations in globular
clusters with different structural parameters. Assuming a Salpeter IMF, for low
concentration clusters the core encounter rate is dominated by turnoff mass
main--sequence stars and medium mass white dwarfs. For high concentration, high
density clusters the encounter probabilities are increasingly dominated by
neutron stars and heavy white dwarfs. Hence we predict a smaller ratio of blue
stragglers and cataclysmic variables to pulsars in high concentration clusters.
The total number of millisecond pulsars, and the ratio of single to binary
pulsars, is broadly consistent with the observed population, suggesting the
binary--single star encounters contribute significantly to the pulsar formation
rate in globular clusters, for the whole range of globular cluster types. The
number of millisecond pulsars and the ratio of pulsars in different globular
clusters is best explained by a total binary fraction comparable to that of the
galaxy, and a modest number of primordial neutron stars in the globular
clusters.Comment: 59 pages, uuencoded compressed postscript, including 18 figures.
Astrophysical Journal Supplements, in pres
Experimental hints of Gravity in Large Extra Dimensions?
Recent conjectures suggest the universe may have large extra dimensions,
through which gravity propagates. This implies gross departures from Newton's
law of gravity at small length scales. Here I consider some implications for
particle dynamics on scales comparable to the compactification radius, R_c
\ltorder 1 mm. During planet formation, coalescence of micron sized dust
grains to planetesimals is a rate critical step. Blum et al (2000) found dust
grain aggregates form low fractal dimension structures in microgravity,
consistent with high angular momentum coalescence. I consider the effects of
non-Newtonian gravity on dust aggregation on scales less than and show
they naturally coalesce into low dimensional structures with high specific
angular momentum. We infer microns.Comment: 5 pages, revtex, Int. Jour. Mod. Phys. D (Special issue of GRF essays
2001), single spaced versio
Language design for a personal learning environment design language
Approaching technology-enhanced learning from the perspective of a learner, we foster the idea of learning environment design, learner interactions, and tool interoperability. In this paper, we shortly summarize the motivation for our personal learning environment approach and describe the development of a domain-specific language for this purpose as well as its realization in practice. Consequently, we examine our learning environment design language according to its lexis and syntax, the semantics behind it, and pragmatical aspects within a first prototypic implementation. Finally, we discuss strengths, problematic aspects, and open issues of our approach
The Role of Primordial Kicks on Black Hole Merger Rates
Primordial stars are likely to be very massive \geq30\Msun, form in
isolation, and will likely leave black holes as remnants in the centers of
their host dark matter halos in the mass range
10^{6}-10^{10}\Ms. Such early black holes, at redshifts z\gtsim10, could
be the seed black holes for the many supermassive black holes found in galaxies
in the local universe. If they exist, their mergers with nearby supermassive
black holes may be a prime signal for long wavelength gravitational wave
detectors. We simulate formation of black holes in the center of high redshift
dark matter halos and explore implications of initial natal kick velocities
conjectured by some formation models. The central concentration of early black
holes in present day galaxies is reduced if they are born even with moderate
kicks of tens of km/s. The modest kicks allow the black holes to leave their
parent halo, which consequently leads to dynamical friction being less
effective on the lower mass black holes as compared to those still embedded in
their parent halos. Therefore, merger rates may be reduced by more than an
order of magnitude. Using analytical and illustrative cosmological N--body
simulations we quantify the role of natal kicks of black holes formed from
massive metal free stars on their merger rates with supermassive black holes in
present day galaxies. Our results also apply to black holes ejected by the
gravitational slingshot mechanism.Comment: 12 pages, 9 figure
Recommended from our members
Designing for change: mash-up personal learning environments
Institutions for formal education and most work places are equipped today with at least some kind of tools that bring together people and content artefacts in learning activities to support them in constructing and processing information and knowledge. For almost half a century, science and practice have been discussing models on how to bring personalisation through digital means to these environments. Learning environments and their construction as well as maintenance makes up the most crucial part of the learning process and the desired learning outcomes and theories should take this into account. Instruction itself as the predominant paradigm has to step down.
The learning environment is an (if not 'the�) important outcome of a learning process, not just a stage to perform a 'learning play'. For these good reasons, we therefore consider instructional design theories to be flawed.
In this article we first clarify key concepts and assumptions for personalised learning environments. Afterwards, we summarise our critique on the contemporary models for personalised adaptive learning. Subsequently, we propose our alternative, i.e. the concept of a mash-up personal learning environment that provides adaptation mechanisms for learning environment construction and maintenance. The web application mash-up solution allows learners to reuse existing (web-based) tools plus services.
Our alternative, LISL is a design language model for creating, managing, maintaining, and learning about learning environment design; it is complemented by a proof of concept, the MUPPLE platform. We demonstrate this approach with a prototypical implementation and a – we think – comprehensible example. Finally, we round up the article with a discussion on possible extensions of this new model and open problems
The Vela pulsar `jet': a companion-punctured bubble of fallback material
Markwardt and Oegelman (1995) used ROSAT to reveal a 12 by 45 arcmin
structure in 1 keV X rays around the Vela pulsar, which they interpret as a jet
emanating from the pulsar. We here present an alternative view of the nature of
this feature, namely that it consists of material from very deep inside the
exploding star, close to the mass cut between material that became part of the
neutron star and ejected material. The initial radial velocity of the inner
material was lower than the bulk of the ejecta, and formed a bubble of slow
material that started expanding again due to heating by the young pulsar's
spindown energy. The expansion is mainly in one direction, and to explain this
we speculate that the pre-supernova system was a binary. The explosion caused
the binary to unbind, and the pulsar's former companion carved a lower-density
channel into the main ejecta. The resulting puncture of the bubble's edge
greatly facilitated expansion along its path relative to other directions. If
this is the case, we can estimate the current speed of the former binary
companion and from this reconstruct the presupernova binary orbit. It follows
that the exploding star was a helium star, hence that the supernova was of type
Ib. Since the most likely binary companion is another neutron star, the
evolution of the Vela remnant and its surroundings has been rather more
complicated than the simple expansion of one supernova blast wave into
unperturbed interstellar material.Comment: submitted to MNRAS; 6 pages laTeX, 3 figures (1 postscript, 2 gif
files of images
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