51,979 research outputs found
Fluctuations and correlations in population models with age structure
We study the population profile in a simple discrete time model of population
dynamics. Our model, which is closely related to certain ``bit-string'' models
of evolution, incorporates competition for resources via a population dependent
death probability, as well as a variable reproduction probability for each
individual as a function of age. We first solve for the steady-state of the
model in mean field theory, before developing analytic techniques to compute
Gaussian fluctuation corrections around the mean field fixed point. Our
computations are found to be in good agreement with Monte-Carlo simulations.
Finally we discuss how similar methods may be applied to fluctuations in
continuous time population models.Comment: 4 page
Limits on Clouds and Hazes for the TRAPPIST-1 Planets
The TRAPPIST-1 planetary system is an excellent candidate for study of the
evolution and habitability of M-dwarf planets. Transmission spectroscopy
observations performed with the Hubble Space Telescope (HST) suggest the
innermost five planets do not possess clear hydrogen atmospheres. Here we
reassess these conclusions with recently updated mass constraints and expand
the analysis to include limits on metallicity, cloud top pressure, and the
strength of haze scattering. We connect recent laboratory results of particle
size and production rate for exoplanet hazes to a one-dimensional atmospheric
model for TRAPPIST-1 transmission spectra. Doing so, we obtain a
physically-based estimate of haze scattering cross sections. We find haze
scattering cross sections on the order of 1e-26 to 1e-19 cm squared are needed
in hydrogen-rich atmospheres for TRAPPIST-1 d, e, and f to match the HST data.
For TRAPPIST-1 g, we cannot rule out a clear hydrogen-rich atmosphere. We also
modeled the effects an opaque cloud deck and substantial heavy element content
have on the transmission spectra. We determine that hydrogen-rich atmospheres
with high altitude clouds, at pressures of 12mbar and lower, are consistent
with the HST observations for TRAPPIST-1 d and e. For TRAPPIST-1 f and g, we
cannot rule out clear hydrogen-rich cases to high confidence. We demonstrate
that metallicities of at least 60xsolar with tropospheric (0.1 bar) clouds
agree with observations. Additionally, we provide estimates of the precision
necessary for future observations to disentangle degeneracies in cloud top
pressure and metallicity. Our results suggest secondary, volatile-rich
atmospheres for the outer TRAPPIST-1 planets d, e, and f.Comment: 15 pages, 3 figures, 2 tables, accepted in the Astronomical Journa
Functional characterization of synthetic leukotriene B and its stereochemical isomers.
Leukotriene B (LTB), a potent lipid chemotactic factor for neutrophils, is 5S,12R-dihydroxy-6,14-cis,8,10-trans-eicosatetraenoic acid (Fig 1), based upon direct comparison of natural LTB with synthetic 5S,12R-dihydroxy-6,8,10,14-eicosatetraenoic acid (5,12-di-HETE) stereoisomers in three biological assays. Of the six synthetic stereoisomers evaluated, only the 5S,12R,6,14-cis,8,10-trans compound had chemotactic potency for human neutrophils in vitro that was comparable to that of natural LTB, with a concentration of 3 X 10(9-9) M eliciting a one-half maximum response. In contrast, the racemic mixture of 5R,12R- and 5S,12S-6,10-trans,8,14-cis, the racemic mixture of 5S,12R- and 5R,12S-6,10-trans,8,14-cis, the 5S,12R-6,8-trans,10,14-cis, the 5S,12R-6,8,10-trans,14-cis, and the 5S,12S-6,8,10-trans,14-cis stereoisomers required concentrations of 3 X 10(-7) to 1 X 10(-6) M to elicit comparable responses. Only natural LTB and its synthetic counterpart elicited a local neutrophil infiltration when injected into the skin of the rhesus monkey at 10 ng and 100 ng per site. Natural and synthetic LTB at a concentration of 3 X 10(-8) M each provoked an EC25 contractile response of guinea pig pulmonary parenchymal strips in vitro, whereas the other four tested stereoisomers of 5,12-di-HETE were inactive at this concentration. Structure-function analyses suggest that the neutrophil chemotactic activity depends critically upon the C-1 to C-12 domain, including the stereochemistry of the 6-,8-,and 10-olefinic bonds and the presence of both hydroxyl groups
Knizhnik-Zamolodchikov-type equations for gauged WZNW models
We study correlation functions of coset constructions by utilizing the method
of gauge dressing. As an example we apply this method to the minimal models and
to the Witten 2D black hole. We exhibit a striking similarity between the
latter and the gravitational dressing. In particular, we look for logarithmic
operators in the 2D black hole.Comment: 24 pages, latex, no figures. More discussion of logarithmic operators
was adde
3-dimensional Gravity from the Turaev-Viro Invariant
We study the -deformed su(2) spin network as a 3-dimensional quantum
gravity model. We show that in the semiclassical continuum limit the
Turaev-Viro invariant obtained recently defines naturally regularized
path-integral la Ponzano-Regge, In which a contribution from
the cosmological term is effectively included. The regularization dependent
cosmological constant is found to be , where
. We also discuss the relation to the Euclidean Chern-Simons-Witten
gravity in 3-dimension.Comment: 11page
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