88,706 research outputs found
Nonperturbative Solution of the Super-Virasoro Constraints
We present the solution of the discrete super-Virasoro constraints to all
orders of the genus expansion. Integrating over the fermionic variables we get
a representation of the partition function in terms of the one-matrix model. We
also obtain the nonperturbative solution of the super-Virasoro constraints in
the double scaling limit but do not find agreement between our flows and the
known supersymmetric extensions of KdV.Comment: 12 pages, CERN-TH.6761/9
Long baseline interferometry: a promising tool for multiplicity investigations of massive stars
Massive binaries are crucial laboratories that allow us to investigate
processes occurring in quite extreme conditions, such as particle acceleration,
high-energy emission, or even dust formation. All these processes are
intimately dependent on binarity. Our understanding of the underlying physics
strongly requires preliminary multiplicity studies likely to uncover still
undetected binaries, and determine their orbital parameters. However, classical
spectroscopic approaches sometimes fail to provide a solution to this issue.
Long baseline interferometry turns out to be a promising complementary
technique to address the question of the multiplicity of massive stars. A
campaign has been initiated with the VLTI to take benefit of this technique.Comment: 1 page, to appear in the proceedings of the ESO conference "The
Interferometric View on Hot Stars", held in Vina del Mar, Chile (2-6 March
2009), RevMexA
Tentative insight into the multiplicity of the persistent dust maker WR106 from X-ray observations
This paper presents the results of the analysis of the very first dedicated
X-ray observation with XMM-Newton of WR106. This carbon-rich WC9d Wolf-Rayet
star belongs to the category of persistent dust makers (WCd stars). The issue
of the multiplicity of these dust makers is pivotal to understand the dust
formation process, and in this context X-ray observations may allow to reveal
an X-ray emission attributable to colliding-winds in a binary system. The main
result of this analysis is the lack of detection of X-rays coming from WR106.
Upper limits on the X-ray flux are estimated, but the derived numbers are not
sufficient to provide compelling constraints on the existence or not of a
colliding-wind region. Detailed inspection of archive data bases reveals that
persistent dust makers have been poorly investigated by the most sensitive
X-ray observatories. Certainly, the combination of several approaches to
indirectly constrain their multiplicity should be applied to lift a part of the
veil on the nature of these persistent dust makers.Comment: 7 pages, 2 figures, 2 tables; accepted for publication in New
Astronomy (in press
Long-term XMM-Newton investigation of two particle-accelerating colliding-wind binaries in NGC6604: HD168112 and HD167971
The long-term (over more than one decade) X-ray emission from two massive
stellar systems known to be particle accelerators is investigated using
XMM-Newton. Their X-ray properties are interpreted taking into account recent
information about their multiplicity and orbital parameters. The two targets,
HD168112 and HD167971 appear to be overluminous in X-rays, lending additional
support to the idea that a significant contribution of the X-ray emission comes
from colliding-wind regions. The variability of the X-ray flux from HD168112 is
interpreted in terms of varying separation expected to follow the 1/D rule for
adiabatic shocked winds. For HD167971, marginal decrease of the X-ray flux in
September 2002 could tentatively be explained by a partial wind eclipse in the
close pair. No long-term variability could be demonstrated despite the
significant difference of separation between 2002 and 2014. This suggests the
colliding-wind region in the wide orbit does not contribute a lot to the total
X-ray emission, with a main contribution coming from the radiative shocked
winds in the eclipsing pair. The later result provides evidence that shocks in
a colliding-wind region may be efficient particle accelerators even in the
absence of bright X-ray emission, suggesting particle acceleration may operate
in a wide range of conditions. Finally, in hierarchical triple O-type systems,
thermal X-rays do not necessarily constitute an efficient tracer to detect the
wind-wind interaction in the long period orbit.Comment: 12 pages, 4 postscript figures, 6 table
Higher Order Graviton Scattering in M(atrix) Theory
In matrix theory the effective action for graviton-graviton scattering is a
double expansion in the relative velocity and inverse separation. We discuss
the systematics of this expansion and subject matrix theory to a new test. Low
energy supergravity predicts the coefficient of the term, a
two-loop effect, in agreement with explicit matrix model calculation.Comment: 15 pages, 1 epsf figure, LaTeX. Minor change
An efficient temporal logic for robotic task planning
Computations required for temporal reasoning can be prohibitively expensive if fully general representations are used. Overly simple representations, such as totally ordered sequence of time points, are inadequate for use in a nonlinear task planning system. A middle ground is identified which is general enough to support a capable nonlinear task planner, but specialized enough that the system can support online task planning in real time. A Temporal Logic System (TLS) was developed during the Intelligent Task Automation (ITA) project to support robotic task planning. TLS is also used within the ITA system to support plan execution, monitoring, and exception handling
Scanning tunneling spectroscopy of a dilute two-dimensional electron system exhibiting Rashba spin splitting
Using scanning tunneling spectroscopy (STS) at 5 K in B-fields up to 7 T, we
investigate the local density of states of a two-dimensional electron system
(2DES) created by Cs adsorption on p-type InSb(110). The 2DES, which in
contrast to previous STS studies exhibits a 2D Fermi level, shows standing
waves at B = 0 T with corrugations decreasing with energy and with wave numbers
in accordance with theory. In magnetic field percolating drift states are
observed within the disorder broadened Landau levels. Due to the large electric
field perpendicular to the surface, a beating pattern of the Landau levels is
found and explained quantitatively by Rashba spin splitting within the lowest
2DES subband. The Rashba splitting does not contribute significantly to the
standing wave patterns in accordance with theory.Comment: 9 pages, 9 figures, submitted to Phys. Rev.
A quasi-Newton proximal splitting method
A new result in convex analysis on the calculation of proximity operators in
certain scaled norms is derived. We describe efficient implementations of the
proximity calculation for a useful class of functions; the implementations
exploit the piece-wise linear nature of the dual problem. The second part of
the paper applies the previous result to acceleration of convex minimization
problems, and leads to an elegant quasi-Newton method. The optimization method
compares favorably against state-of-the-art alternatives. The algorithm has
extensive applications including signal processing, sparse recovery and machine
learning and classification
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