7,517 research outputs found
Towards optical intensity interferometry for high angular resolution stellar astrophysics
Most neighboring stars are still detected as point sources and are beyond the
angular resolution reach of current observatories. Methods to improve our
understanding of stars at high angular resolution are investigated. Air
Cherenkov telescopes (ACTs), primarily used for Gamma-ray astronomy, enable us
to increase our understanding of the circumstellar environment of a particular
system. When used as optical intensity interferometers, future ACT arrays will
allow us to detect stars as extended objects and image their surfaces at high
angular resolution.
Optical stellar intensity interferometry (SII) with ACT arrays, composed of
nearly 100 telescopes, will provide means to measure fundamental stellar
parameters and also open the possibility of model-independent imaging. A data
analysis algorithm is developed and permits the reconstruction of high angular
resolution images from simulated SII data. The capabilities and limitations of
future ACT arrays used for high angular resolution imaging are investigated via
Monte-Carlo simulations. Simple stellar objects as well as stellar surfaces
with localized hot or cool regions can be accurately imaged.
Finally, experimental efforts to measure intensity correlations are
expounded. The functionality of analog and digital correlators is demonstrated.
Intensity correlations have been measured for a simulated star emitting
pseudo-thermal light, resulting in angular diameter measurements. The StarBase
observatory, consisting of a pair of 3 m telescopes separated by 23 m, is
described.Comment: PhD dissertatio
Supersymmetry and Bosonization in Three Dimensions
We discuss on the possible existence of a supersymmetric invariance in purely
fermionic planar systems and its relation to the fermion-boson mapping in
three-dimensional quantum field theory. We consider, as a very simple example,
the bosonization of free massive fermions and show that, under certain
conditions on the masses, this model displays a supersymmetric-like invariance
in the low energy regime. We construct the purely fermionic expression for the
supercurrent and the non-linear supersymmetry transformation laws. We argue
that the supersymmetry is absent in the limit of massless fermions where the
bosonized theory is non-local.Comment: 12 pages, LaTeX, no figures, to appear in Physics Letters
D6 branes and M theory geometrical transitions from gauged supergravity
We study the supergravity duals of supersymmetric theories arising in the
world-volume of D6 branes wrapping holomorphic two-cycles and special
Lagrangian three-cycles within the framework of eight dimensional gauged
supergravity. When uplifted to 11d, our solutions represent M-theory on the
background of, respectively, the small resolution of the conifold and a
manifold with G_2 holonomy. We further discuss on the flop and other possible
geometrical transitions and its implications.Comment: 16 pages, harvmac, one figure; references adde
Quantum Topological Excitations: from the Sawtooth Lattice to the Heisenberg Chain
The recently elucidated structure of the delafossite YCuO reveals a
Cu-O network with nearly independent chains having different
interactions between the spins. Motivated by this result, we study the
chain for various ratios of the base-base and
base-vertex interactions. By exact diagonalization and extrapolation, we show
that the elementary excitation spectrum, which (within numerical error) is the
same for total spins and 1, has a gap only in the interval
. The gap is dispersionless
for , but has increasing -dependence as moves away from unity, related to the instability of dimers in
the ground state.Comment: 4 pages, 6 figures (revtex twocolumn
Prioritized Garbage Collection: Explicit GC Support for Software Caches
Programmers routinely trade space for time to increase performance, often in
the form of caching or memoization. In managed languages like Java or
JavaScript, however, this space-time tradeoff is complex. Using more space
translates into higher garbage collection costs, especially at the limit of
available memory. Existing runtime systems provide limited support for
space-sensitive algorithms, forcing programmers into difficult and often
brittle choices about provisioning.
This paper presents prioritized garbage collection, a cooperative programming
language and runtime solution to this problem. Prioritized GC provides an
interface similar to soft references, called priority references, which
identify objects that the collector can reclaim eagerly if necessary. The key
difference is an API for defining the policy that governs when priority
references are cleared and in what order. Application code specifies a priority
value for each reference and a target memory bound. The collector reclaims
references, lowest priority first, until the total memory footprint of the
cache fits within the bound. We use this API to implement a space-aware
least-recently-used (LRU) cache, called a Sache, that is a drop-in replacement
for existing caches, such as Google's Guava library. The garbage collector
automatically grows and shrinks the Sache in response to available memory and
workload with minimal provisioning information from the programmer. Using a
Sache, it is almost impossible for an application to experience a memory leak,
memory pressure, or an out-of-memory crash caused by software caching.Comment: to appear in OOPSLA 201
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