25,559 research outputs found
Characterization of Microlensing Planets with Moderately Wide Separations
In future high-cadence microlensing surveys, planets can be detected through
a new channel of an independent event produced by the planet itself. The two
populations of planets to be detected through this channel are wide-separation
planets and free-floating planets. Although they appear as similar short
time-scale events, the two populations of planets are widely different in
nature and thus distinguishing them is important. In this paper, we investigate
the lensing properties of events produced by planets with moderately wide
separations from host stars. We find that the lensing behavior of these events
is well described by the Chang-Refsdal lensing and the shear caused by the
primary not only produces a caustic but also makes the magnification contour
elongated along the primary-planet axis. The elongated magnification contour
implies that the light curves of these planetary events are generally
asymmetric and thus the asymmetry can be used to distinguish the events from
those produced by free-floating planets. The asymmetry can be noticed from the
overall shape of the light curve and thus can hardly be missed unlike the very
short-duration central perturbation caused by the caustic. In addition, the
asymmetry occurs regardless of the event magnification and thus the bound
nature of the planet can be identified for majority of these events. The close
approximation of the lensing light curve to that of the Chang-Refsdal lensing
implies that the analysis of the light curve yields only the information about
the projected separation between the host star and the planet.Comment: 4 pages, 2 figure
Global Agrifood Value Chains and Local Poverty Reduction: What Happens to Those Who Don’t Plug In?
Structural changes in the global agrifood value chain have transformed food production in developing countries including Indonesia. One element of this is the spread of supermarket retailing. By increasing the demand for and returns to higher quality produce, this development has the potential to improve living standards in a sector where poverty has been persistent. Many studies have shown the magnitude of price premiums available to farmers who sell to supermarkets. However, little attention has been paid to how the introduction of a supermarket retailer affects those farmers who continue to sell to traditional market channels. Our data suggests that in regions where there are both modern and traditional buyers, competition effects result in the immiserization of farmers who continue to sell to traditional markets. This result underlines the fact that while sectorial transformation has desirable poverty reduction potential, actual impacts are lumpy. The distribution of farmer participation in a region may result in a case where the upgrading of agrifood supply chains can increase poverty in the absence of policy interventions
V2X Meets NOMA: Non-Orthogonal Multiple Access for 5G Enabled Vehicular Networks
Benefited from the widely deployed infrastructure, the LTE network has
recently been considered as a promising candidate to support the
vehicle-to-everything (V2X) services. However, with a massive number of devices
accessing the V2X network in the future, the conventional OFDM-based LTE
network faces the congestion issues due to its low efficiency of orthogonal
access, resulting in significant access delay and posing a great challenge
especially to safety-critical applications. The non-orthogonal multiple access
(NOMA) technique has been well recognized as an effective solution for the
future 5G cellular networks to provide broadband communications and massive
connectivity. In this article, we investigate the applicability of NOMA in
supporting cellular V2X services to achieve low latency and high reliability.
Starting with a basic V2X unicast system, a novel NOMA-based scheme is proposed
to tackle the technical hurdles in designing high spectral efficient scheduling
and resource allocation schemes in the ultra dense topology. We then extend it
to a more general V2X broadcasting system. Other NOMA-based extended V2X
applications and some open issues are also discussed.Comment: Accepted by IEEE Wireless Communications Magazin
Estimating Marginal Hazard Ratios by Simultaneously Using A Set of Propensity Score Models: A Multiply Robust Approach
The inverse probability weighted Cox model is frequently used to estimate marginal hazard ratios. Its validity requires a crucial condition that the propensity score model is correctly specified. To provide protection against misspecification of the propensity score model, we propose a weighted estimation method rooted in empirical likelihood theory. The proposed estimator is multiply robust in that it is guaranteed to be consistent when a set of postulated propensity score models contains a correctly specified model. Our simulation studies demonstrate satisfactory finite sample performance of the proposed method in terms of consistency and efficiency. We apply the proposed method to compare the risk of postoperative hospitalization between sleeve gastrectomy and Roux-en-Y gastric bypass using data from a large medical claims and billing database.We further extend the development to multi-site studies to enable each site to postulate multiple site-specific propensity score models
Short-duration lensing events: I. wide-orbit planets? free-floating low-mass objects? or high-velocity stars?
Short duration lensing events tend to be generated by low-mass lenses or by
lenses with high transverse velocities. Furthermore, for any given lens mass
and speed, events of short duration are preferentially caused by nearby lenses
(mesolenses) that can be studied in detail, or else by lenses so close to the
source star that finite-source-size effects may be detected, yielding
information about both the Einstein ring radius and the surface of the lensed
star. Planets causing short-duration events may be in orbits with any
orientation, and may have semimajor axes smaller than an AU, or they may reach
the outer limits of their planetary systems, in the region corresponding to the
Solar System's Oort Cloud. They can have masses larger than Jupiter's or
smaller than Pluto's. Lensing therefore has a unique potential to expand our
understanding of planetary systems. A particular advantage of lensing is that
it can provide precision measurements of system parameters, including the
masses of and projected separation between star and planet. We demonstrate how
the parameters can be extracted and show that a great deal can be learned. For
example, it is remarkable that the gravitational mass of nearby free-floating
planet-mass lenses can be measured by complementing observations of a
photometric event with deep images that detect the planet itself. A fraction of
short events may be caused by high-velocity stars located within a kpc. Many
high-velocity lenses are likely to be neutron stars that received large natal
kicks. Other high-speed stars may be members of the halo population. Still
others may be hypervelocity stars that have been ejected from the Galactic
Center, or runaway stars escaped from close binaries, possibly including the
progenitor binaries of Type Ia supernovae.Comment: 17 pages; 2 figures; submitted to ApJ 3 July 200
Short-duration lensing events: II. Expectations and Protocols
Ongoing microlensing observations by OGLE and MOA regularly identify and
conduct high-cadence sampling of lensing events with Einstein diameter crossing
time, tau_E, of 16 or fewer days. Events with estimated values of tau_E of one
to two days have been detected. Short duration events tend to be generated by
low-mass lenses or by lenses with high transverse velocities. We compute the
expected rates, demonstrate the expected ranges of parameters for lenses of
different mass, and develop a protocol for observing and modeling
short-duration events. Relatively minor additions to the procedures presently
used will increase the rate of planet discovery, and also discover or place
limits on the population of high-speed dim stars and stellar remnants in the
vicinity of the Sun.Comment: 17 pages; 3 figures; submitted to ApJ 3 July 200
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