4,280 research outputs found
Tropical polar cones, hypergraph transversals, and mean payoff games
We discuss the tropical analogues of several basic questions of convex
duality. In particular, the polar of a tropical polyhedral cone represents the
set of linear inequalities that its elements satisfy. We characterize the
extreme rays of the polar in terms of certain minimal set covers which may be
thought of as weighted generalizations of minimal transversals in hypergraphs.
We also give a tropical analogue of Farkas lemma, which allows one to check
whether a linear inequality is implied by a finite family of linear
inequalities. Here, the certificate is a strategy of a mean payoff game. We
discuss examples, showing that the number of extreme rays of the polar of the
tropical cyclic polyhedral cone is polynomially bounded, and that there is no
unique minimal system of inequalities defining a given tropical polyhedral
cone.Comment: 27 pages, 6 figures, revised versio
The Literacy of America's College Students
Measures the literacy of 1,827 graduating college and university students from eighty institutions. Looks at the ability to perform prose tasks such as read and use texts; search and comprehend forms; and conduct quantitative, computational tasks
Critical ingredients of supernova Ia radiative-transfer modeling
We explore the physics of SN Ia light curves and spectra using the 1-D
non-LTE time-dependent radiative-transfer code CMFGEN. Rather than adjusting
ejecta properties to match observations, we select as input one "standard" 1-D
Chandrasekhar-mass delayed-detonation hydrodynamical model, and then explore
the sensitivity of radiation and gas properties on radiative-transfer modeling
assumptions. The correct computation of SN Ia radiation is not exclusively a
solution to an "opacity problem", characterized by the treatment of a large
number of lines. It is also key to treat important atomic processes
consistently. Besides handling line blanketing in non-LTE, we show that
including forbidden line transitions of metals is increasingly important for
the temperature and ionization of the gas beyond maximum light. Non-thermal
ionization and excitation are also critical since they affect the color
evolution and the Delta-M15 of our model. While impacting little the bolometric
luminosity, a more complete treatment of decay routes leads to enhanced line
blanketing, e.g., associated with 48Ti in the U and B bands. Overall, we find
that SN Ia radiation properties are influenced in a complicated way by the
atomic data we employ, so that obtaining converged results is a challenge. We
nonetheless obtain a good match to the golden standard type Ia SN 2005cf in the
optical and near-IR, from 5 to 60d after explosion, suggesting that assuming
spherical symmetry is not detrimental to SN Ia radiative-transfer modeling at
these times. Multi-D effects no doubt matter, but they are perhaps less
important than accurately treating non-LTE processes [abridged].Comment: Accepted to MNRA
Constraints on the explosion mechanism and progenitors of type Ia supernovae
Observations of SN 2011fe at early times reveal an evolution analogous to a
fireball model of constant color. In contrast, our unmixed delayed detonations
of Chandrasekhar-mass white dwarfs (DDC series) exhibit a faster brightening
concomitant with a shift in color to the blue. In this paper, we study the
origin of these discrepancies. We find that strong chemical mixing largely
resolves the photometric mismatch at early times, but it leads to an enhanced
line broadening that contrasts, for example, with the markedly narrow SiII6355A
line of SN 2011fe. We also explore an alternative configuration with
pulsational-delayed detonations (PDDEL model series). Because of the pulsation,
PDDEL models retain more unburnt carbon, have little mass at high velocity, and
have a much hotter outer ejecta after the explosion. The pulsation does not
influence the inner ejecta, so PDDEL and DDC models exhibit similar radiative
properties beyond maximum. However, at early times, PDDEL models show bluer
optical colors and a higher luminosity, even for weak mixing. Their early-time
radiation is derived primarily from the initial shock-deposited energy in the
outer ejecta rather than radioactive decay heating. Furthermore, PDDEL models
show short-lived CII lines, reminiscent of SN 2013dy. They typically exhibit
lines that are weaker, narrower, and of near-constant width, reminiscent of SN
2011fe. In addition to multi-dimensional effects, varying configurations for
such ``pulsations" offer a source of spectral diversity amongst SNe Ia. PDDEL
and DDC models also provide one explanation for low- and high-velocity gradient
SNe Ia.Comment: Accepted to MNRA
Neutron-powered precursors of kilonovae
The merger of binary neutron stars (NSs) ejects a small quantity of neutron
rich matter, the radioactive decay of which powers a day to week long thermal
transient known as a kilonova. Most of the ejecta remains sufficiently dense
during its expansion that all neutrons are captured into nuclei during the
r-process. However, recent general relativistic merger simulations by Bauswein
and collaborators show that a small fraction of the ejected mass (a few per
cent, or ~1e-4 Msun) expands sufficiently rapidly for most neutrons to avoid
capture. This matter originates from the shocked-heated interface between the
merging NSs. Here we show that the beta-decay of these free neutrons in the
outermost ejecta powers a `precursor' to the main kilonova emission, which
peaks on a timescale of a few hours following merger at U-band magnitude ~22
(for an assumed distance of 200 Mpc). The high luminosity and blue colors of
the neutron precursor render it a potentially important counterpart to the
gravitational wave source, that may encode valuable information on the
properties of the merging binary (e.g. NS-NS versus NS-black hole) and the NS
equation of state. Future work is necessary to assess the robustness of the
fast moving ejecta and the survival of free neutrons in the face of neutrino
absorptions, although the precursor properties are robust to a moderate amount
of leptonization. Our results provide additional motivation for short latency
gravitational wave triggers and rapid follow-up searches with sensitive ground
based telescopes.Comment: 6 pages, 5 figures, accepted to MNRAS main journa
[CoIII] versus NaID in type Ia supernova spectra
The high metal content and fast expansion of supernova (SN) Ia ejecta lead to
considerable line overlap in their optical spectra. Uncertainties in
composition and ionization further complicate the process of line
identification. In this paper, we focus on the 5900A emission feature seen in
SN Ia spectra after bolometric maximum, a line which in the last two decades
has been associated with [CoIII]5888A or NaID. Using non-LTE time-dependent
radiative-transfer calculations based on Chandrasekhar-mass delayed-detonation
models, we find that NaID line emission is extremely weak at all post-maximum
epochs. Instead, we predict the presence of [CoIII]5888A after maximum in all
our SN Ia models, which cover a range from 0.12 to 0.87Msun of 56Ni. We also
find that the [CoIII]5888A forbidden line is present within days of bolometric
maximum, and strengthens steadily for weeks thereafter. Both predictions are
confirmed by observations. Rather than trivial taxonomy, these findings confirm
that it is necessary to include forbidden-line transitions in
radiative-transfer simulations of SNe Ia, both to obtain the correct ejecta
cooling rate and to match observed optical spectra.Comment: Accepted to MNRA
Sub-Inertial Gravity Modes in the B8V Star KIC 7760680 Reveal Moderate Core Overshooting and Low Vertical Diffusive Mixing
KIC 7760680 is so far the richest slowly pulsating B star, by exhibiting 36
consecutive dipole () gravity (g-) modes. The monotonically decreasing
period spacing of the series, in addition to the local dips in the pattern
confirm that KIC 7760680 is a moderate rotator, with clear mode trapping in
chemically inhomogeneous layers. We employ the traditional approximation of
rotation to incorporate rotational effects on g-mode frequencies. Our detailed
forward asteroseismic modelling of this g-mode series reveals that KIC 7760680
is a moderately rotating B star with mass M. By
simultaneously matching the slope of the period spacing, and the number of
modes in the observed frequency range, we deduce that the equatorial rotation
frequency of KIC 7760680 is 0.4805 day, which is 26\% of its Roche break
up frequency. The relative deviation of the model frequencies and those
observed is less than one percent. We succeed to tightly constrain the
exponentially-decaying convective core overshooting parameter to . This means that convective core overshooting can
coexist with moderate rotation. Moreover, models with exponentially-decaying
overshoot from the core outperform those with the classical step-function
overshoot. The best value for extra diffusive mixing in the radiatively stable
envelope is confined to (with in cm sec), which is notably smaller than theoretical
predictions.Comment: 12 Figures, 2 Tables, all data publicly available for download;
accepted for publication in Astrophysical Journa
Analysis of Model-Aided Navigation of Unmanned Aerial Vehicles
To overcome the rapid and unbounded error growth of low-cost Inertial Navigation Systems (INS), aircraft localization methods commonly compensate for Inertial Measurement Unit (IMU) sensor errors by integrating them with Global Positioning System (GPS) measurements via a Kalman Filter. However, over the past decade, the potential of GPS jamming or even spoofing GPS signals has forced the research community to focus on the development of GPS-denied navigation technologies. Among the GPS-denied techniques, one approach that has been considered is the use of a Vehicle Dynamic Models (VDM) to reduce the rate at which an INS becomes unusable. As such, this Master\u27s thesis considers the use of different aerodynamic modeling approaches to aid in compensation of IMU errors of a fixed-wing Unmanned Aerial Vehicle (UAV). The goals of this research are to evaluate the sensitivity of the performance of dynamic model aided navigation in the context of low-cost platforms where performance benefit must be weighed against the complexity that is required to develop the dynamic model. To do this, first, in simulation, the sensitivity to the required modeling accuracy is shown by perturbing the model coefficients with errors. In addition, different sensors and sensor grades are evaluated, and three different model-aided navigation architectures are discussed and evaluated. To conduct this work, a UAV simulation is developed within which a UAV trajectory is driven by ``truth\u27\u27 dynamic model and then IMU measurements are derived and errors are added to them using standard stochastic models for IMU sensors. Finally, the algorithm performance is then evaluated using actual UAV flight testing data from a low cost testbed equipped with GPS and IMU sensors. The testbed used and modeled is a 2.4 m span fixed wing UAV designed and instrumented at WVU
How can allocative inefficiency reveal risk preference? An empirical investigation on French wheat farms
We focus on a simple framework on wheat producer behaviour in a context of price output uncertainty. More precisely, we establish a relationship between ex post output price level and allocative inefficiency that allows to characterize farmers’ risk preferences. Given this analysis, the connection between risk aversion and other socioeconomic variables (such as degree of output specialisation, total asset, debts, farmer’s age…) can furthermore empirically be explored. This relationship is empirically tested on an unbalanced panel including about 650 wheat producers located in the French Department of Meuse over 1992- 2003.Producer behaviour, allocative inefficiency, risk aversion, Crop Production/Industries, Risk and Uncertainty,
Measurements of total scattering spectra from bocaccio (Sebastes paucispinis)
Marine sportfishing in southern California is a huge industry with annual revenues totaling many billions of dollars. However, the stocks of lingcod and six rockfish species have been declared overfished by the Pacific Fisheries Management Council. As part of a multifaceted fisheries management plan, marine conservation areas, covering many million square nautical miles, have been
mandated. To monitor the recovery of the rockfish stocks in these areas, scientists are faced with the following
challenges: 1) multiple species of rockfish exist in these areas; 2) the species reside near or on the bottom at depths of 80 to 300 m; and 3) they are low in numerical density. To meet these challenges, multifrequency
echosounders, multibeam sonar, and cameras mounted on remotely operated vehicles are frequently used (Reynolds et al., 2001). The accuracy and precision of these echosounder
results are largely dependent upon the accuracy of the species classification and target strength estimation
(MacLennan and Simmonds, 1992)
- …