6,256 research outputs found
A spin-foam vertex amplitude with the correct semiclassical limit
Spin-foam models are hoped to provide a dynamics for loop quantum gravity.
These start from the Plebanski formulation of gravity, in which gravity is
obtained from a topological field theory, BF theory, through constraints,
which, however, select more than one gravitational sector, as well as an
unphysical degenerate sector. We show this is why terms beyond the needed
Feynman-prescribed one appear in the semiclassical limit of the EPRL spin-foam
amplitude. By quantum mechanically isolating a single gravitational sector, we
modify this amplitude, yielding a spin-foam amplitude for loop quantum gravity
with the correct semiclassical limit.Comment: 5 pages; sign error corrected, further clarification regarding the
semi-classical limit added, as well as other minor change
The Sun in Time: Age, Rotation, and Magnetic Activity of the Sun and Solar-type Stars and Effects on Hosted Planets
Multi-wavelength studies of solar analogs (G0-5 V stars) with ages from ~50
Myr to 9 Gyr have been carried out as part of the "Sun in Time" program for
nearly 20 yrs. From these studies it is inferred that the young (ZAMS) Sun was
rotating more than 10x faster than today. As a consequence, young solar-type
stars and the early Sun have vigorous magnetohydrodynamic (MHD) dynamos and
correspondingly strong coronal X-ray and transition region / chromospheric
FUV-UV emissions. To ensure continuity and homogeneity for this program, we use
a restricted sample of G0-5 V stars with masses, radii, T(eff), and internal
structure (i.e. outer convective zones) closely matching those of the Sun. From
these analogs we have determined reliable rotation-age-activity relations and
X-ray - UV (XUV) spectral irradiances for the Sun (or any solar-type star) over
time. These XUV irradiance measures serve as input data for investigating the
photo-ionization and photo-chemical effects of the young, active Sun on the
paleo-planetary atmospheres and environments of solar system planets. These
measures are also important to study the effects of these high energy emissions
on the numerous exoplanets hosted by solar-type stars of different ages.
Recently we have extended the study to include lower mass, main-sequence
(dwarf) dK and dM stars to determine relationships among their rotation
spin-down rates and coronal and chromospheric emissions as a function of mass
and age. From rotation-age-activity relations we can determine reliable ages
for main-sequence G, K, M field stars and, subsequently, their hosted planets.
Also inferred are the present and the past XUV irradiance and plasma flux
exposures that these planets have endured and the suitability of the hosted
planets to develop and sustain life.Comment: 12 pages, 6 figures; to appear in the proceedings of IAU 258: The
Ages of Star
The Secret Lives of Cepheids: A Multi-Wavelength Study of the Atmospheres and Real-Time Evolution of Classical Cepheids
The primary goal of this study is to observe how complex the behaviors of
Cepheids can be, and to show how the continued monitoring of Cepheids at
multiple wavelengths can begin to reveal their "secret lives."
We aim to achieve this through optical photometry, UV spectroscopy and X-ray
imaging. Through Villanova's guaranteed access to ground-based telescopes, we
have secured well-covered light curves as regularly as possible. Amplitudes and
times of max brightness were obtained and compared to previous literature
results. At UV wavelengths, we have secured hi-res spectra of 2 nearby Cepheids
- delta Cep and beta Dor - with HST-COS. Also, we have obtained X-ray images of
5 Cepheids with XMM-Newton and the Chandra X-ray Observatory, and further
observations with both satellites have been proposed for (XMM) and approved
(Chandra).
Optical photometry has shown that 8 of the 10 observed Cepheids have
amplitude variability, or hints thereof, and all 10 show period variability
(recent, long-term or possibly periodic). The UV spectra reveal emission lines
from heated atmospheric plasmas of 10^4 - 10^5 K that vary in phase with the
Cepheid pulsations. The X-ray images have detected the three nearest Cepheids
observed (Polaris, delta Cep and beta Dor), while the distances of the other
two place their fluxes likely at or below detector background levels. The X-ray
fluxes for delta Cep show possible phased variability, but anti-correlated with
the UV emission lines (i.e. high X-ray flux during low UV flux, and vice
versa).
Further data are required to ultimately confirm Blazhko-like cycles in
Cepheids, X-ray variability with phase and the particulars of the high-energy
variability such as phase-lags between atmospheric plasma emissions of
different temperature and the exact contributions of the possible heating
mechanism.Comment: Thesis (Ph.D.) -- James Cook University, 201
Classical Cepheids Require Enhanced Mass Loss
Measurements of rates of period change of Classical Cepheids probe stellar
physics and evolution. Additionally, better understanding of Cepheid structure
and evolution provides greater insight into their use as standard candles and
tools for measuring the Hubble constant. Our recent study of the period change
of the nearest Cepheid, Polaris, suggested that it is undergoing enhanced mass
loss when compared to canonical stellar evolution model predictions. In this
work, we expand the analysis to rates of period change measured for about 200
Galactic Cepheids and compare them to population synthesis models of Cepheids
including convective core overshooting and enhanced mass loss. Rates of period
change predicted from stellar evolution models without mass loss do not agree
with observed rates whereas including enhanced mass loss yields predicted rates
in better agreement with observations. This is the first evidence that enhanced
mass loss as suggested previously for Polaris and delta Cephei must be a
ubiquitous property of Classical Cepheids.Comment: 6 pages, 4 figures, Accepted for publication in ApJ Letter
Investigating ICAPM with Dynamic Conditional Correlations
This paper examines the intertemporal relation between expected return and risk for 30 stocks in the Dow Jones Industrial Average. The mean-reverting dynamic conditional correlation model of Engle (2002) is used to estimate a stock’s conditional covariance with the market and test whether
the conditional covariance predicts time-variation in the stock’s expected return. The risk-aversion coefficient, restricted to be the same across stocks in panel regression, is estimated to be between
two and four and highly significant. This result is robust across different market portfolios, different sample periods, alternative specifications of the conditional mean and covariance processes, and including a wide variety of state variables that proxy for the intertemporal hedging demand component of the ICAPM. Risk premium induced by the conditional covariation of individual stocks with the market portfolio remains economically and statistically significant after controlling for risk premiums induced by conditional covariation with macroeconomic variables (federal funds rate, default spread, and term spread), financial factors (size, book-to-market, and momentum), and volatility measures (implied, GARCH, and range volatility)
Low-energy electron transport with the method of discrete ordinates
The one-dimensional discrete ordinates code ANISN was adapted to transport low energy (a few MeV) electrons. Calculated results obtained with ANISN were compared with experimental data for transmitted electron energy and angular distribution data for electrons normally incident on aluminum slabs of various thicknesses. The calculated and experimental results are in good agreement for a thin slab (0.2 of the electron range), but not for the thicker slabs (0.6 of the electron range). Calculated results obtained with ANISN were also compared with results obtained using Monte Carlo methods
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