5,854 research outputs found
Pulsation Period Change & Classical Cepheids: Probing the Details of Stellar Evolution
Measurements of secular period change probe real-time stellar evolution of
classical Cepheids making these measurements powerful constraints for stellar
evolution models, especially when coupled with interferometric measurements. In
this work, we present stellar evolution models and measured rates of period
change for two Galactic Cepheids: Polaris and l Carinae, both important
Cepheids for anchoring the Cepheid Leavitt law (period-luminosity relation).
The combination of previously-measured parallaxes, interferometric angular
diameters and rates of period change allows for predictions of Cepheid mass
loss and stellar mass. Using the stellar evolution models, We find that l Car
has a mass of about 9 consistent with stellar pulsation models, but
is not undergoing enhanced stellar mass loss. Conversely, the rate of period
change for Polaris requires including enhanced mass-loss rates. We discuss what
these different results imply for Cepheid evolution and the mass-loss mechanism
on the Cepheid instability strip.Comment: 2 pages, 1 figure, Poster presented at IAU307: New windows on massive
stars: asteroseismology, interferometry, and spectropolarimetry, Editors: G.
Meynet, C. Georgy, J.H. Groh & Ph. Ste
Coherent states, constraint classes, and area operators in the new spin-foam models
Recently, two new spin-foam models have appeared in the literature, both
motivated by a desire to modify the Barrett-Crane model in such a way that the
imposition of certain second class constraints, called cross-simplicity
constraints, are weakened. We refer to these two models as the FKLS model, and
the flipped model. Both of these models are based on a reformulation of the
cross-simplicity constraints. This paper has two main parts. First, we clarify
the structure of the reformulated cross-simplicity constraints and the nature
of their quantum imposition in the new models. In particular we show that in
the FKLS model, quantum cross-simplicity implies no restriction on states. The
deeper reason for this is that, with the symplectic structure relevant for
FKLS, the reformulated cross-simplicity constraints, in a certain relevant
sense, are now \emph{first class}, and this causes the coherent state method of
imposing the constraints, key in the FKLS model, to fail to give any
restriction on states. Nevertheless, the cross-simplicity can still be seen as
implemented via suppression of intertwiner degrees of freedom in the dynamical
propagation. In the second part of the paper, we investigate area spectra in
the models. The results of these two investigations will highlight how, in the
flipped model, the Hilbert space of states, as well as the spectra of area
operators exactly match those of loop quantum gravity, whereas in the FKLS (and
Barrett-Crane) models, the boundary Hilbert spaces and area spectra are
different.Comment: 21 pages; statements about gamma limits made more precise, and minor
phrasing change
Physical boundary state for the quantum tetrahedron
We consider stability under evolution as a criterion to select a physical
boundary state for the spinfoam formalism. As an example, we apply it to the
simplest spinfoam defined by a single quantum tetrahedron and solve the
associated eigenvalue problem at leading order in the large spin limit. We show
that this fixes uniquely the free parameters entering the boundary state.
Remarkably, the state obtained this way gives a correlation between edges which
runs at leading order with the inverse distance between the edges, in agreement
with the linearized continuum theory. Finally, we give an argument why this
correlator represents the propagation of a pure gauge, consistently with the
absence of physical degrees of freedom in 3d general relativity.Comment: 20 pages, 6 figure
A New Spin Foam Model for 4d Gravity
Starting from Plebanski formulation of gravity as a constrained BF theory we
propose a new spin foam model for 4d Riemannian quantum gravity that
generalises the well-known Barrett-Crane model and resolves the inherent to it
ultra-locality problem. The BF formulation of 4d gravity possesses two sectors:
gravitational and topological ones. The model presented here is shown to give a
quantization of the gravitational sector, and is dual to the recently proposed
spin foam model of Engle et al. which, we show, corresponds to the topological
sector. Our methods allow us to introduce the Immirzi parameter into the
framework of spin foam quantisation. We generalize some of our considerations
to the Lorentzian setting and obtain a new spin foam model in that context as
well.Comment: 40 pages; (v2) published versio
Optimum harvest time in Aquaculture: an application of economic principles to a Nile tilapia, Oreochromis niloticus (L.), growth model
A simple method is presented for determining the optimum time to harvest fish and the effect of fertilization type on optimum harvest time for Aquaculture. Optimum harvest time was similar for either maximizing fish yield or maximizing profit of fish harvested (price of fish times fish yield minus fish production cost), because the daily change in fish production cost was low for the low-input Nile tilapia, Oreochromis niloticus (L.), production system in Thailand. At a harvest time of 150 days for an organic fertilization treatment compared to an inorganic fertilization treatment fish yield increased from l-505 t/ha to 2-295 t/ha, and profit of fish harvested increased from 15657·1 baht/ha (US 948-2/ha). For the organic treatment, optimum harvest time occurred at 191 days, with a fish yield of 2·328 t/ha and a profit of 25520·5baht/ha (US 605·1/ha).Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/73931/1/j.1365-2109.1992.tb00807.x.pd
Crystalline cataract caused by a heterozygous missense mutation in γD-crystallin (CRYGD)
Purpose: To describe phenotypic characteristics of two pedigrees manifesting early onset crystalline cataract with mutations in the γD-crystallin gene (CRYGD). Methods: A detailed medical history was obtained from two Caucasian pedigrees manifesting autosomal dominant congenital cataracts. Genomic DNA was extracted from saliva (DNA Genotek). Single Nucleotide Polymorphism (SNP) based genome analysis of the larger pedigree revealed linkage to an 8.2 MB region on chromosome 2q33-q35 which encompassed the crystallin-gamma gene cluster (CRYG). Exons and flanking introns of CRYGA, CRYGB, CRYGC and CRYGD were amplified and sequenced to identify disease-causing mutations. Results: A morphologically unique cataract with extensive refractile “crystals ” scattered throughout the nucleus and perinuclear cortex was found in the probands from both pedigrees. A heterozygous C→A mutation was identified at position 109 of the coding sequence (R36S of the processed protein) in exon 2 of CRYGD and this missense mutation was found to cosegregate with the disease in the larger family; this mutation was then identified in affected individuals of pedigree 2 as well. Conclusions: The heterozygous 109C→A CRYGD missense mutation is associated with a distinct crystalline cataract in two US Caucasian pedigrees. This confirms crystalline cataract formation with this mutation, as previously reported in sporadic childhood case from the Czech Republic and in members of a Chinese family
Two are better than one: Volatility forecasting using multiplicative component GARCH‐MIDAS models
We examine the properties and forecast performance of multiplicative volatility
specifications that belong to the class of generalized autoregressive conditional
heteroskedasticity–mixed-data sampling (GARCH-MIDAS) models suggested in
Engle, Ghysels, and Sohn (Review of Economics and Statistics, 2013, 95, 776–797).
In those models volatility is decomposed into a short-term GARCH component
and a long-term component that is driven by an explanatory variable. We derive
the kurtosis of returns, the autocorrelation function of squared returns, and
the R2 of a Mincer–Zarnowitz regression and evaluate the QMLE and forecast
performance of these models in a Monte Carlo simulation. For S&P 500 data,
we compare the forecast performance of GARCH-MIDAS models with a wide
range of competitor models such as HAR (heterogeneous autoregression), realized GARCH, HEAVY (high-frequency-based volatility) and Markov-switching
GARCH. Our results show that the GARCH-MIDAS based on housing starts
as an explanatory variable significantly outperforms all competitor models at
forecast horizons of 2 and 3 months ahead
Numerical indications on the semiclassical limit of the flipped vertex
We introduce a technique for testing the semiclassical limit of a quantum
gravity vertex amplitude. The technique is based on the propagation of a
semiclassical wave packet. We apply this technique to the newly introduced
"flipped" vertex in loop quantum gravity, in order to test the intertwiner
dependence of the vertex. Under some drastic simplifications, we find very
preliminary, but surprisingly good numerical evidence for the correct classical
limit.Comment: 4 pages, 8 figure
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