1,351 research outputs found
NASA's capability evolution toward commercial space
We discuss how the space industry moved from a government dominated field to a commercially driven field. In the context of this industry shift, we explore how NASA's capabilities developed from its early hierarchical model to the intergovernmental and then commercial network models. We refer in particular to NASA's organizational, cultural, relational, and technological capabilities. These developments over time suggest that these are dynamic capabilities that respond to the demands of the external environment and to mission imperatives
Smeared versus localised sources in flux compactifications
We investigate whether vacuum solutions in flux compactifications that are
obtained with smeared sources (orientifolds or D-branes) still survive when the
sources are localised. This seems to rely on whether the solutions are BPS or
not. First we consider two sets of BPS solutions that both relate to the GKP
solution through T-dualities: (p+1)-dimensional solutions from
spacetime-filling Op-planes with a conformally Ricci-flat internal space, and
p-dimensional solutions with Op-planes that wrap a 1-cycle inside an everywhere
negatively curved twisted torus. The relation between the solution with smeared
orientifolds and the localised version is worked out in detail. We then
demonstrate that a class of non-BPS AdS_4 solutions that exist for IASD fluxes
and with smeared D3-branes (or analogously for ISD fluxes with anti-D3-branes)
does not survive the localisation of the (anti) D3-branes. This casts doubts on
the stringy consistency of non-BPS solutions that are obtained in the limit of
smeared sources.Comment: 23 pages; v2: minor corrections, added references, version published
in JHE
PP Wave Limit and Enhanced Supersymmetry in Gauge Theories
We observe that the pp wave limit of compactifications of
type IIB string theory is universal, and maximally supersymmetric, as long as
is smooth and preserves some supersymmetry. We investigate a specific
case, . The dual SCFT, describing D3-branes at a
conifold singularity, has operators that we identify with the oscillators of
the light-cone string in the universal pp-wave background. The correspondence
is remarkable in that it relies on the exact spectrum of anomalous dimensions
in this CFT, along with the existence of certain exceptional series of
operators whose dimensions are protected only in the limit of large `t Hooft
coupling. We also briefly examine the singular case , for which
the pp wave background becomes a orbifold of the maximally supersymmetric
background by reflection of 4 transverse coordinates. We find operators in the
corresponding SCFT with the right properties to describe both the
untwisted and the twisted sectors of the closed string.Comment: 15 pages, LaTeX; v2: added more detail to a derivation, and a
preprint number; v3: minor corrections, some remarks and references adde
Decompactifications and Massless D-Branes in Hybrid Models
A method of determining the mass spectrum of BPS D-branes in any phase limit
of a gauged linear sigma model is introduced. A ring associated to monodromy is
defined and one considers K-theory to be a module over this ring. A simple but
interesting class of hybrid models with Landau-Ginzburg fibres over CPn are
analyzed using special Kaehler geometry and D-brane probes. In some cases the
hybrid limit is an infinite distance in moduli space and corresponds to a
decompactification. In other cases the hybrid limit is at a finite distance and
acquires massless D-branes. An example studied appears to correspond to a novel
theory of supergravity with an SU(2) gauge symmetry where the gauge and
gravitational couplings are necessarily tied to each other.Comment: PDF-LaTeX, 34 pages, 2 mps figure
Yours ever (well, maybe): Studies and signposts in letter writing
Electronic mail and other digital communications technologies seemingly threaten to end the era of handwritten and typed letters, now affectionately seen as part of snail mail. In this essay, I analyze a group of popular and scholarly studies about letter writing-including examples of pundits critiquing the use of e-mail, etiquette manuals advising why the handwritten letter still possesses value, historians and literary scholars studying the role of letters in the past and what it tells us about our present attitudes about digital communications technologies, and futurists predicting how we will function as personal archivists maintaining every document including e-mail. These are useful guideposts for archivists, providing both a sense of the present and the past in the role, value and nature of letters and their successors. They also provide insights into how such documents should be studied, expanding our gaze beyond the particular letters, to the tools used to create them and the traditions dictating their form and function. We also can discern a role for archivists, both for contributing to the literature about documents and in using these studies and commentaries, suggesting not a new disciplinary realm but opportunities for new interdisciplinary work. Examining a documentary form makes us more sensitive to both the innovations and traditions as it shifts from the analog to the digital; we can learn not to be caught up in hysteria or nostalgia about one form over another and archivists can learn about what they might expect in their labors to document society and its institutions. At one time, paper was part of an innovative technology, with roles very similar to the Internet and e-mail today. It may be that the shifts are far less revolutionary than is often assumed. Reading such works also suggests, finally, that archivists ought to rethink how they view their own knowledge and how it is constructed and used. © 2010 Springer Science+Business Media B.V
Hypermoduli Stabilization, Flux Attractors, and Generating Functions
We study stabilization of hypermoduli with emphasis on the effects of
generalized fluxes. We find a class of no-scale vacua described by ISD
conditions even in the presence of geometric flux. The associated flux
attractor equations can be integrated by a generating function with the
property that the hypermoduli are determined by a simple extremization
principle. We work out several orbifold examples where all vector moduli and
many hypermoduli are stabilized, with VEVs given explicitly in terms of fluxes.Comment: 45 pages, no figures; Version submitted to JHE
A peculiar HI cloud near the distant globular cluster Pal 4
We present 21-cm observations of four Galactic globular clusters, as part of
the on-going GALFA-HI Survey at Arecibo. We discovered a peculiar HI cloud in
the vicinity of the distant (109 kpc) cluster Pal 4, and discuss its properties
and likelihood of association with the cluster. We conclude that an association
of the HI cloud and Pal 4 is possible, but that a chance coincidence between
Pal 4 and a nearby compact high-velocity cloud cannot be ruled out altogether.
New, more stringent upper limits were derived for the other three clusters: M
3, NGC 5466, and Pal 13. We briefly discuss the fate of globular cluster gas
and the interaction of compact clouds with the Galactic Halo gas.Comment: Accepted for publication in MNRA
Recommended from our members
Localisation and origin of the bacteriochlorophyll-derived photosensitizer in the retina of the deep-sea dragon fish Malacosteus niger
Most deep-sea fish have a single visual pigment maximally sensitive at short wavelengths, approximately matching the spectrum of both downwelling sunlight and bioluminescence. However, Malcosteus niger produces far-red bioluminescence and its longwave retinal sensitivity is enhanced by red-shifted visual pigments, a longwave reflecting tapetum and, uniquely, a bacteriochlorophyllderived photosensitizer. The origin of the photosensitizer, however, remains unclear. We investigated whether the bacteriochlorophyll was produced by endosymbiotic bacteria within unusual structures adjacent to the photoreceptors that had previously been described in this species. However, microscopy, elemental analysis and SYTOX green staining provided no evidence for such localised retinal bacteria, instead the photosensitizer was shown to be distributed throughout the retina. Furthermore, comparison of mRNA from the retina of Malacosteus to that of the closely related Pachystomias microdon (which does not contain a bacterichlorophyll-derived photosensitzer) revealed no genes of bacterial origin that were specifically up-regulated in Malacosteus. Instead up-regulated Malacosteus genes were associated with photosensitivity and may relate to its unique visual ecology and the chlorophyll-based visual system. We also suggest that the unusual longwave-reflecting, astaxanthin-based, tapetum of Malacosteus may protect the retina from the potential cytotoxicity of such a system
U-dual fluxes and Generalized Geometry
We perform a systematic analysis of generic string flux compactifications,
making use of Exceptional Generalized Geometry (EGG) as an organizing
principle. In particular, we establish the precise map between fluxes, gaugings
of maximal 4d supergravity and EGG, identifying the complete set of gaugings
that admit an uplift to 10d heterotic or type IIB supegravity backgrounds. Our
results reveal a rich structure, involving new deformations of 10d supergravity
backgrounds, such as the RR counterparts of the -deformation. These new
deformations are expected to provide the natural extension of the
-deformation to full-fledged F-theory backgrounds. Our analysis also
provides some clues on the 10d origin of some of the particularly less
understood gaugings of 4d supergravity. Finally, we derive the explicit
expression for the effective superpotential in arbitrary N = 1 heterotic or
type IIB orientifold compactifications, for all the allowed fluxes.Comment: 58 pages, 6 table
GASKAP -- The Galactic ASKAP Survey
A survey of the Milky Way disk and the Magellanic System at the wavelengths
of the 21-cm atomic hydrogen (HI) line and three 18-cm lines of the OH molecule
will be carried out with the Australian Square Kilometre Array Pathfinder
telescope. The survey will study the distribution of HI emission and absorption
with unprecedented angular and velocity resolution, as well as molecular line
thermal emission, absorption, and maser lines. The area to be covered includes
the Galactic plane (|b|< 10deg) at all declinations south of delta = +40deg,
spanning longitudes 167deg through 360deg to 79deg at b=0deg, plus the entire
area of the Magellanic Stream and Clouds, a total of 13,020 square degrees. The
brightness temperature sensitivity will be very good, typically sigma_T ~ 1 K
at resolution 30arcsec and 1 km/s. The survey has a wide spectrum of scientific
goals, from studies of galaxy evolution to star formation, with particular
contributions to understanding stellar wind kinematics, the thermal phases of
the interstellar medium, the interaction between gas in the disk and halo, and
the dynamical and thermal states of gas at various positions along the
Magellanic Stream.Comment: 45 pages, 8 figures, Pub. Astron. Soc. Australia (in press
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