2,594 research outputs found
Penrose Limits, the Colliding Plane Wave Problem and the Classical String Backgrounds
We show how the Szekeres form of the line element is naturally adapted to
study Penrose limits in classical string backgrounds. Relating the "old"
colliding wave problem to the Penrose limiting procedure as employed in string
theory we discuss how two orthogonal Penrose limits uniquely determine the
underlying target space when certain symmetry is imposed. We construct a
conformally deformed background with two distinct, yet exactly solvable in
terms of the string theory on R-R backgrounds, Penrose limits. Exploiting
further the similarities between the two problems we find that the Penrose
limit of the gauged WZW Nappi-Witten universe is itself a gauged WZW plane wave
solution of Sfetsos and Tseytlin. Finally, we discuss some issues related to
singularity, show the existence of a large class of non-Hausdorff solutions
with Killing Cauchy Horizons and indicate a possible resolution of the problem
of the definition of quantum vacuum in string theory on these time-dependent
backgrounds.Comment: Some misprints corrected. Matches the version in print. To appear in
Classical & Quantum Gravit
Avalanches and the Distribution of Reconnection Events in Magnetized Circumstellar Disks
Cosmic rays produced by young stellar objects can potentially alter the
ionization structure, heating budget, chemical composition, and accretion
activity in circumstellar disks. The inner edges of these disks are truncated
by strong magnetic fields, which can reconnect and produce flaring activity
that accelerates cosmic radiation. The resulting cosmic rays can provide a
source of ionization and produce spallation reactions that alter the
composition of planetesimals. This reconnection and particle acceleration are
analogous to the physical processes that produce flaring in and heating of
stellar coronae. Flaring events on the surface of the Sun exhibit a power-law
distribution of energy, reminiscent of those measured for Earthquakes and
avalanches. Numerical lattice-reconnection models are capable of reproducing
the observed power-law behavior of solar flares under the paradigm of
self-organized criticality. One interpretation of these experiments is that the
solar corona maintains a nonlinear attractor -- or ``critical'' -- state by
balancing energy input via braided magnetic fields and output via reconnection
events. Motivated by these results, we generalize the lattice-reconnection
formalism for applications in the truncation region of magnetized disks. Our
numerical experiments demonstrate that these nonlinear dynamical systems are
capable of both attaining and maintaining criticality in the presence of
Keplerian shear and other complications. The resulting power-law spectrum of
flare energies in the equilibrium attractor state is found to be nearly
universal in magnetized disks. This finding indicates that magnetic
reconnection and flaring in the inner regions of circumstellar disks occur in a
manner similar to activity on stellar surfaces
Cloud fragmentation and proplyd-like features in HII regions imaged by HST
We have analyzed HST ACS and WFPC2 new and archival images of eight HII
regions to look for new proto-planetary disks (proplyds) similar to those found
in the Orion Nebula. We find a wealth of features similar in size (though many
are larger) to the bright cusps around the Orion Nebula proplyds. None of them,
however, contains a definitive central star. From this, we deduce that the new
cusps may not be proplyds, but instead are fragments of molecular cloud
material. Out of all the features found in the eight HII regions examined, only
one, an apparent edge-on silhouette in M17, may have a central star. This
feature might join the small number of bona fide proplyds found outside the
Orion Nebula, in M8, M20 and possibly in M16. In line with the results found
recently by Smith et al. (2005), the paucity of proplyds outside the Orion
Nebula, may be explained by their transient nature as well as by the specific
environmental conditions under whichthey can be observed.Comment: 51 pages; 19 figures; 5 tables. Accepted by A
Massive open star clusters using the VVV survey II. Discovery of six clusters with Wolf-Rayet stars
Context: The ESO Public Survey "VISTA Variables in the V\'ia L\'actea" (VVV)
provides deep multi-epoch infrared observations for an unprecedented 562 sq.
degrees of the Galactic bulge, and adjacent regions of the disk. In this survey
nearly 150 new open clusters and cluster candidates have been discovered. Aims:
This is the second in a series of papers about young, massive open clusters
observed using the VVV survey. We present the first study of six recently
discovered clusters. These clusters contain at least one newly discovered
Wolf-Rayet (WR) star. Methods: Following the methodology presented in the first
paper of the series, wide-field, deep JHKs VVV observations, combined with new
infrared spectroscopy, are employed to constrain fundamental parameters for a
subset of clusters. Results: We affirm that the six studied stellar groups are
real young (2-7 Myr) and massive (between 0.8 and 2.2 10^3 Msol) clusters. They
are highly obscured (Av ~ 5-24 mag) and compact (1-2 pc). In addition to WR
stars, two of the six clusters also contain at least one red supergiant star.
We claim the discovery of 8 new WR stars, and 3 stars showing WR-like emission
lines which could be classified WR or OIf. Preliminary analysis provides
initial masses of ~30-50 Msol for the WR stars. Finally,we discuss the spiral
structure of the Galaxy using as tracers the six new clusters together with the
previously studied VVV clusters.Comment: 17 pages, 8 figurs, accepted in A&
Trapped surfaces, horizons and exact solutions in higher dimensions
A very simple criterion to ascertain if (D-2)-surfaces are trapped in
arbitrary D-dimensional Lorentzian manifolds is given. The result is purely
geometric, independent of the particular gravitational theory, of any field
equations or of any other conditions. Many physical applications arise, a few
shown here: a definition of general horizon, which reduces to the standard one
in black holes/rings and other known cases; the classification of solutions
with a (D-2)-dimensional abelian group of motions and the invariance of the
trapping under simple dimensional reductions of the
Kaluza-Klein/string/M-theory type. Finally, a stronger result involving closed
trapped surfaces is presented. It provides in particular a simple sufficient
condition for their absence.Comment: 7 pages, no figures, final version to appear in Class. Quantum Gra
Exact Einstein-scalar field solutions for formation of black holes in a cosmological setting
We consider self-interacting scalar fields coupled to gravity. Two classes of
exact solutions to Einstein's equations are obtained: the first class
corresponds to the minimal coupling, the second one to the conformal coupling.
One of the solutions is shown to describe a formation of a black hole in a
cosmological setting. Some properties of this solution are described. There are
two kinds of event horizons: a black hole horizon and cosmological horizons.
The cosmological horizons are not smooth. There is a mild curvature
singularity, which affects extended bodies but allows geodesics to be extended.
It is also shown that there is a critical value for a parameter on which the
solution depends. Above the critical point, the black hole singularity is
hidden within a global black hole event horizon. Below the critical point, the
singularity appears to be naked. The relevance to cosmic censorship is
discussed.Comment: 25 pages, 2 figure
The [O III] Veil: Astropause of Eta Carinae's Wind?
We present narrowband images of eta Carinae in the light of [O III] 5007
obtained with HST/WFPC2, as well as a ground-based image in the same emission
line with a larger field of view. These images show a thin veil of [O III]
emission around eta Car and its ejecta, confirming the existence of an
oxygen-bearing ``cocoon'' inferred from spectra. This [O III] veil may be the
remnant of the pre-outburst wind of eta Car, and its outer edge probably marks
the interface where eta Car's ejecta meet the stellar wind of the nearby O4
V((f)) star HD303308 or other ambient material -- i.e., it marks the
``astropause'' in eta Car's wind. This veil is part of a more extensive [O III]
shell that appears to be shaped and ionized by HD303308. A pair of HST images
with a 10 yr baseline shows no proper motion, limiting the expansion speed away
from eta Car to 12pm13 km/s, or an expansion age of a few times 10^4 yr. Thus,
this is probably the decelerated pre-outburst LBV wind of eta Car. The [O III]
morphology is very different from that seen in [N II], which traces young knots
of CNO-processed material; this represents a dramatic shift in the chemical
makeup of material recently ejected by eta Car. This change in the chemical
abundances may have resulted from the sudden removal of the star's outer
envelope during eta Car's 19th century outburst or an earlier but similar
event.Comment: 11 pages, 4 figs. Figs 1 and 3 in color. Accepted to AJ, October 200
Stellar Rotation in Young Clusters. II. Evolution of Stellar Rotation and Surface Helium Abundance
We derive the effective temperatures and gravities of 461 OB stars in 19
young clusters by fitting the H-gamma profile in their spectra. We use
synthetic model profiles for rotating stars to develop a method to estimate the
polar gravity for these stars, which we argue is a useful indicator of their
evolutionary status. We combine these results with projected rotational
velocity measurements obtained in a previous paper on these same open clusters.
We find that the more massive B-stars experience a spin down as predicted by
the theories for the evolution of rotating stars. Furthermore, we find that the
members of binary stars also experience a marked spin down with advanced
evolutionary state due to tidal interactions. We also derive non-LTE-corrected
helium abundances for most of the sample by fitting the He I 4026, 4387, 4471
lines. A large number of helium peculiar stars are found among cooler stars
with Teff < 23000 K. The analysis of the high mass stars (8.5 solar masses < M
< 16 solar masses) shows that the helium enrichment process progresses through
the main sequence (MS) phase and is greater among the faster rotators. This
discovery supports the theoretical claim that rotationally induced internal
mixing is the main cause of surface chemical anomalies that appear during the
MS phase. The lower mass stars appear to have slower rotation rates among the
low gravity objects, and they have a large proportion of helium peculiar stars.
We suggest that both properties are due to their youth. The low gravity stars
are probably pre-main sequence objects that will spin up as they contract.
These young objects very likely host a remnant magnetic field from their natal
cloud, and these strong fields sculpt out surface regions with unusual chemical
abundances.Comment: 50 pages 18 figures, accepted by Ap
Higher education, mature students and employment goals: policies and practices in the UK
This article considers recent policies of Higher Education in the UK, which are aimed at widening participation and meeting the needs of employers. The focus is on the growing population of part-time students, and the implications of policies for this group. The article takes a critical perspective on government policies, using data from a major study of mature part-time students, conducted in two specialist institutions in the UK, a London University college and a distance learning university. Findings from this study throw doubt on the feasibility of determining a priori what kind of study pathway is most conducive for the individual in terms of employment gains and opportunities for upward social mobility. In conclusion, doubts are raised as to whether policies such as those of the present UK government are likely to achieve its aims. Such policies are not unique to the UK, and lessons from this country are relevant to most of the developed world
On the Asymptotic Behaviour of Cosmological Models in Scalar-Tensor Theories of Gravity
We study the qualitative properties of cosmological models in scalar-tensor
theories of gravity by exploiting the formal equivalence of these theories with
general relativity minimally coupled to a scalar field under a conformal
transformation and field redefinition. In particular, we investigate the
asymptotic behaviour of spatially homogeneous cosmological models in a class of
scalar-tensor theories which are conformally equivalent to general relativistic
Bianchi cosmologies with a scalar field and an exponential potential whose
qualitative features have been studied previously. Particular attention is
focussed on those scalar-tensor theory cosmological models, which are shown to
be self-similar, that correspond to general relativistic models that play an
important r\^{o}le in describing the asymptotic behaviour of more general
models (e.g., those cosmological models that act as early-time and late-time
attractors).Comment: 22 pages, submitted to Phys Rev
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