50,480 research outputs found
Properties of Planetary Caustics in Gravitational Microlensing
Although some of the properties of the caustics in planetary microlensing
have been known, our understanding of them is mostly from scattered information
based on numerical approaches. In this paper, we conduct a comprehensive and
analytic analysis of the properties of the planetary caustics, which are one of
the two sets of caustics in planetary microlensing, those located away from the
central star. Under the perturbative approximation, we derive analytic
expressions for the location, size, and shape of the planetary caustic as a
function of the star-planet separation and the planet/star mass ratio. Based on
these expressions combined with those for the central caustic, which is the
other set of caustics located close to the central star, we compare the
similarities and differences between the planetary and central caustics. We
also present the expressions for the size ratio between the two types of
caustics and for the condition of the merging of the two types of caustics.
These analytic expressions will be useful in understanding the dependence of
the planetary lensing behavior on the planet parameters and thus in
interpreting the planetary lensing signalsComment: total 6 pages, including 6 figures, ApJ, submitte
Analysis of Microlensing Light Curves Induced by Multiple-Planet Systems
To maximize the number of planet detections by increasing efficiency, current
microlensing follow-up observation experiments are focusing on
high-magnification events to search for planet-induced perturbations near the
peak of lensing light curves. It was known that by monitoring
high-magnification events, it is possible to detect multiplicity signatures of
planetary systems. However, it was believed that the interpretation of the
signals and the characterization of the detected multiple-planet systems would
be difficult due to the complexity of the magnification pattern in the central
region combined with the large number of lensing parameters required to model
multiple-planet systems. In this paper, we demonstrate that in many cases the
central planetary perturbations induced by multiple planets can be well
approximated by the superposition of the single planetary perturbations where
the individual planet-primary pairs act as independent binary lens systems
(binary superposition). The validity of the binary-superposition approximation
implies that the analysis of perturbations induced by multiple planets can be
greatly simplified because the anomalies produced by the individual planet
components can be investigated separately by using relatively much simpler
single-planetary analysis, and thus enables better characterization of these
systems.Comment: Manuscript with high-resolution figures are available at
http://astroph.chungbuk.ac.kr/~cheongho/preprint.htm
Astrometric Method to Break the Photometric Degeneracy between Binary-source and Planetary Microlensing Perturbations
An extra-solar planet can be detected by microlensing because the planet can
perturb the smooth lensing light curve created by the primary lens. However, it
was shown by Gaudi that a subset of binary-source events can produce light
curves that closely resemble those produced by a significant fraction of
planet/star lens systems, causing serious contamination of a sample of
suspected planetary systems detected via microlensing. In this paper, we show
that if a lensing event is observed astrometrically, one can unambiguously
break the photometric degeneracy between binary-source and planetary lensing
perturbations. This is possible because while the planet-induced perturbation
in the trajectory of the lensed source image centroid shifts points away from
the opening of the unperturbed elliptical trajectory, while the perturbation
induced by the binary source companion points always towards the opening.
Therefore, astrometric microlensing observations by using future high-precision
interferometers will be important for solid confirmation of microlensing planet
detections.Comment: total 5 pages, including 1 figure and no table, ApJ, submitted,
better quality pdf file is avalilable at
http://astroph.chungbuk.ac.kr/~cheongho/publication.htm
Boundary conditions in the Dirac approach to graphene devices
We study a family of local boundary conditions for the Dirac problem
corresponding to the continuum limit of graphene, both for nanoribbons and
nanodots. We show that, among the members of such family, MIT bag boundary
conditions are the ones which are in closest agreement with available
experiments. For nanotubes of arbitrary chirality satisfying these last
boundary conditions, we evaluate the Casimir energy via zeta function
regularization, in such a way that the limit of nanoribbons is clearly
determined.Comment: 10 pages, no figure. Section on Casimir energy adde
From Group Recommendations to Group Formation
There has been significant recent interest in the area of group
recommendations, where, given groups of users of a recommender system, one
wants to recommend top-k items to a group that maximize the satisfaction of the
group members, according to a chosen semantics of group satisfaction. Examples
semantics of satisfaction of a recommended itemset to a group include the
so-called least misery (LM) and aggregate voting (AV). We consider the
complementary problem of how to form groups such that the users in the formed
groups are most satisfied with the suggested top-k recommendations. We assume
that the recommendations will be generated according to one of the two group
recommendation semantics - LM or AV. Rather than assuming groups are given, or
rely on ad hoc group formation dynamics, our framework allows a strategic
approach for forming groups of users in order to maximize satisfaction. We show
that the problem is NP-hard to solve optimally under both semantics.
Furthermore, we develop two efficient algorithms for group formation under LM
and show that they achieve bounded absolute error. We develop efficient
heuristic algorithms for group formation under AV. We validate our results and
demonstrate the scalability and effectiveness of our group formation algorithms
on two large real data sets.Comment: 14 pages, 22 figure
Finding the Leptonic Decay Mode of a Heavy Higgs Boson
We reanalyze the extraction of the heavy Higgs boson signal from the
Standard Model background at hadron supercolliders, taking into account revised
estimates of the top quark background. With new acceptance criteria the
detection of the signal remains viable. Requiring a forward jet-tag, a central
jet-veto, and a large relative transverse momentum of the two charged leptons
yields for one year of running at the SSC or LHC.Comment: LaTex(Revtex), 9 pages, 6 figures (available upon request),
MAD/PH/75
A Characteristic Planetary Feature in Double-Peaked, High-Magnification Microlensing Events
A significant fraction of microlensing planets have been discovered in
high-magnification events, and a significant fraction of these events exhibit a
double-peak structure at their peak. However, very wide or very close binaries
can also produce double-peaked high-magnification events, with the same gross
properties as those produced by planets. Traditionally, distinguishing between
these two interpretations has relied upon detailed modeling, which is both
time-consuming and generally does not provide insight into the observable
properties that allow discrimination between these two classes of models. We
study the morphologies of these two classes of double-peaked high-magnification
events, and identify a simple diagnostic that can be used to immediately
distinguish between perturbations caused by planetary and binary companions,
without detailed modeling. This diagnostic is based on the difference in the
shape of the intra-peak region of the light curves. The shape is smooth and
concave for binary lensing, while it tends to be either boxy or convex for
planetary lensing. In planetary lensing this intra-peak morphology is due to
the small, weak cusp of the planetary central caustic located between the two
stronger cusps. We apply this diagnostic to five observed double-peaked
high-magnification events to infer their underlying nature. A corollary of our
study is that good coverage of the intra-peak region of double-peaked
high-magnification events is likely to be important for their unique
interpretation.Comment: 6 pages, 3 figure
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