8,477 research outputs found
The origin of very wide binary systems
The majority of stars in the Galactic field and halo are part of binary or
multiple systems. A significant fraction of these systems have orbital
separations in excess of thousands of astronomical units, and systems wider
than a parsec have been identified in the Galactic halo. These binary systems
cannot have formed through the 'normal' star-formation process, nor by capture
processes in the Galactic field. We propose that these wide systems were formed
during the dissolution phase of young star clusters. We test this hypothesis
using N-body simulations of evolving star clusters and find wide binary
fractions of 1-30%, depending on initial conditions. Moreover, given that most
stars form as part of a binary system, our theory predicts that a large
fraction of the known wide 'binaries' are, in fact, multiple systems.Comment: 4 pages, 1 figure, to appear in the proceedings of IAU Symposium 266,
eds. R. de Grijs & J.R.D. Lepin
Why do starless cores appear more flattened than protostellar cores?
We evaluate the intrinsic three dimensional shapes of molecular cores, by
analysing their projected shapes. We use the recent catalogue of molecular line
observations of Jijina et al. and model the data by the method originally
devised for elliptical galaxies. Our analysis broadly supports the conclusion
of Jones et al. that molecular cores are better represented by triaxial
intrinsic shapes (ellipsoids) than biaxial intrinsic shapes (spheroids).
However, we find that the best fit to all of the data is obtained with more
extreme axial ratios () than those derived by Jones et al.
More surprisingly, we find that starless cores have more extreme axial ratios
than protostellar cores -- starless cores appear more `flattened'. This is the
opposite of what would be expected from modeling the freefall collapse of
triaxial ellipsoids. The collapse of starless cores would be expected to
proceed most swiftly along the shortest axis - as has been predicted by every
modeller since Zel'dovich - which should produce more flattened cores around
protostars, the opposite of what is seen.Comment: 7 pages, 3 figure
Globalising assessment: an ethnography of literacy assessment, camels and fast food in the Mongolian Gobi
What happens when standardised literacy assessments travel globally? The paper presents an ethnographic account of adult literacy assessment events in rural Mongolia. It examines the dynamics of literacy assessment in terms of the movement and re-contextualisation of test items as they travel globally and are received locally by Mongolian respondents. The analysis of literacy assessment events is informed by Goodwin’s ‘participation framework’ on language as embodied and situated interactive phenomena and by Actor Network Theory. Actor Network Theory (ANT) is applied to examine literacy assessment events as processes of translation shaped by an ‘assemblage’ of human and non-human actors (including the assessment texts)
The shape of the initial cluster mass function: what it tells us about the local star formation efficiency
We explore how the expulsion of gas from star-cluster forming cloud-cores due
to supernova explosions affects the shape of the initial cluster mass function,
that is, the mass function of star clusters when effects of gas expulsion are
over. We demonstrate that if the radii of cluster-forming gas cores are roughly
constant over the core mass range, as supported by observations, then more
massive cores undergo slower gas expulsion. Therefore, for a given star
formation efficiency, more massive cores retain a larger fraction of stars
after gas expulsion. The initial cluster mass function may thus differ from the
core mass function substantially, with the final shape depending on the star
formation efficiency. A mass-independent star formation efficiency of about 20
per cent turns a power-law core mass function into a bell-shaped initial
cluster mass function, while mass-independent efficiencies of order 40 per cent
preserve the shape of the core mass function.Comment: accepted in Ap
What does a universal IMF imply about star formation?
We show that the same initial mass function (IMF) can result from very
different modes of star formation from very similar underlying core and/or
system mass functions. In particular, we show that the canonical IMF can be
recovered from very similar system mass functions, but with very different mass
ratio distributions within those systems. This is a consequence of the
basically log-normal shapes of all of the distributions. We also show that the
relationships between the shapes of the core, system, and stellar mass
functions may not be trivial. Therefore, different star formation in different
regions could still result in the same IMF.Comment: 6 pages, 4 figures. MNRAS, in pres
The Dynamical Implications of Multiple Stellar Formation Events in Galactic Globular Clusters
Various galactic globular clusters display abundance anomalies that affect
the morphology of their colour-magnitude diagrams. In this paper we consider
the possibility of helium enhancement in the anomalous horizontal branch of NGC
2808. We examine the dynamics of a self-enrichment scenario in which an initial
generation of stars with a top-heavy initial mass function enriches the
interstellar medium with helium via the low-velocity ejecta of its asymptotic
giant branch stars. This enriched medium then produces a second generation of
stars which are themselves helium-enriched. We use a direct N-body approach to
perform five simulations and conclude that such two-generation clusters are
both possible and would not differ significantly from their single-generation
counterparts on the basis of dynamics. We find, however, that the stellar
populations of such clusters would differ from single-generation clusters with
a standard initial mass function and in particular would be enhanced in white
dwarf stars. We conclude, at least from the standpoint of dynamics, that
two-generation globular clusters are feasible.Comment: 24 pages, 7 figures, 3 tables. Accepted for publication in Ap
Catastrophic Failure Modes Assessment of the International Space Station Alpha
This report summarizes a series of analyses to quantify the hazardous effects of meteoroid/debris penetration of Space Station Alpha manned module protective structures. These analyses concentrate on determining (a) the critical crack length associated with six manned module pressure wall designs that, if exceeded, would lead to unstopped crack propagation and rupture of manned modules, and (b) the likelihood of crew or station loss following penetration of unsymmetrical di-methyl hydrazine tanks aboard the proposed Russian FGB ('Tug') propulsion module and critical elements aboard the control moment gyro module (SPP-1). Results from these quantified safety analyses are useful in improving specific design areas, thereby reducing the overall likelihood of crew or station loss following orbital debris penetration
Dynamical order, disorder and propagating defects in homogeneous system of relaxation oscillators
Reaction-diffusion (RD) mechanisms in chemical and biological systems can
yield a variety of patterns that may be functionally important. We show that
diffusive coupling through the inactivating component in a generic model of
coupled relaxation oscillators give rise to a wide range of spatio-temporal
phenomena. Apart from analytically explaining the genesis of anti-phase
synchronization and spatially patterned oscillatory death regimes in the model
system, we report the existence of a chimera state, characterized by spatial
co-occurrence of patches with distinct dynamics. We also observe propagating
phase defects in both one- and two-dimensional media resembling persistent
structures in cellular automata, whose interactions may be used for computation
in RD systems.Comment: 6 pages, 4 figure
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