25,079 research outputs found
Atmosphere, Interior, and Evolution of the Metal-Rich Transiting Planet HD 149026b
We investigate the atmosphere and interior of the new transiting planet HD
149026b, which appears to be very rich in heavy elements. We first compute
model atmospheres at metallicities ranging from solar to ten times solar, and
show how for cases with high metallicity or inefficient redistribution of
energy from the day side, the planet may develop a hot stratosphere due to
absorption of stellar flux by TiO and VO. The spectra predicted by these models
are very different than cooler atmosphere models without stratospheres. The
spectral effects are potentially detectable with the Spitzer Space Telescope.
In addition the models with hot stratospheres lead to a large limb brightening,
rather than darkening. We compare the atmosphere of HD 149026b to other
well-known transiting planets, including the recently discovered HD 189733b,
which we show have planet-to-star flux ratios twice that of HD 209458 and
TrES-1. The methane abundance in the atmosphere of HD 189733b is a sensitive
indicator of atmospheric temperature and metallicity and can be constrained
with Spitzer IRAC observations. We then turn to interior studies of HD 149026b
and use a grid of self-consistent model atmospheres and high-pressure equations
of state for all components to compute thermal evolution models of the planet.
We estimate that the mass of heavy elements within the planet is in the range
of 60 to 93 M_earth. Finally, we discuss trends in the radii of transiting
planets with metallicity in light of this new member of the class.Comment: Accepted to the Astrophysical Journal. 18 pages, including 10
figures. New section on the atmosphere of planet HD 189733b. Enhanced
discussion of atmospheric Ti chemistry and core mass for HD 149026
Force-extension relation of cross-linked anisotropic polymer networks
Cross-linked polymer networks with orientational order constitute a wide
class of soft materials and are relevant to biological systems (e.g., F-actin
bundles). We analytically study the nonlinear force-extension relation of an
array of parallel-aligned, strongly stretched semiflexible polymers with random
cross-links. In the strong stretching limit, the effect of the cross-links is
purely entropic, independent of the bending rigidity of the chains. Cross-links
enhance the differential stretching stiffness of the bundle. For hard
cross-links, the cross-link contribution to the force-extension relation scales
inversely proportional to the force. Its dependence on the cross-link density,
close to the gelation transition, is the same as that of the shear modulus. The
qualitative behavior is captured by a toy model of two chains with a single
cross-link in the middle.Comment: 7 pages, 4 figure
Renormalization of Poincare Transformations in Hamiltonian Semiclassical Field Theory
Semiclassical Hamiltonian field theory is investigated from the axiomatic
point of view. A notion of a semiclassical state is introduced. An "elementary"
semiclassical state is specified by a set of classical field configuration and
quantum state in this external field. "Composed" semiclassical states viewed as
formal superpositions of "elementary" states are nontrivial only if the Maslov
isotropic condition is satisfied; the inner product of "composed" semiclassical
states is degenerate. The mathematical proof of Poincare invariance of
semiclassical field theory is obtained for "elementary" and "composed"
semiclassical states. The notion of semiclassical field is introduced; its
Poincare invariance is also mathematically proved.Comment: LaTeX, 40 pages; short version of hep-th/010307
A scattering rate approach to the understanding of absorption line broadening in near-infrared AlGaN/GaN quantum wells
There has been much interest in the advancement of III-Nitride growth technology to fabricate AlGaN/GaN heterostructures for intersubband transitions (ISBTs). The large conduction band offset in these structures (up to 2 eV) allows transition energies in the near- to the far-infrared region, which have applications from telecommunications, such as in all-optical switches, to infra-red detectors for sensing and imaging. To date, ISBT electroluminescence has been elusive and absorption measurements remain an important method to verify band structure calculations. The growth quality can be inferred from the absorption spectrum, which will have line broadening with contributions that are both inhomogeneous (large-scale interface roughness, and non-parabolicity) and homogeneous (electron scattering related lifetime broadening). In the present work we calculated the contributions of various homogeneous broadening mechanisms (electron interaction with longitudinal-optical (LO) phonons, acoustic phonons, impurities and alloy disorder) to the full linewidth, and also the contribution of band non-parabolicity, which contributes to the inhomogeneous broadening. Calculations are then compared to the measured absorption spectra of several samples
Time extended production of neutrons during a solar flare
The most energetic neutral emissions expected from solar flares are gamma rays (10 MeV) from relativistic primary and secondary electron bremsstrahlung,from approx 0 meson decay, and from neutrons ( 50 MeV). Bremsstrahlung photon energies extend to that of the highest energy electron present, but the shape of the pi sup 0 gamma ray spectrum, peaking at 69 MeV, does not depend strongly on the proton spectrum above threshold, which is approx. 292 MeV for meson production on protons. The highest energy neutrons observed indicate directly the highest energy ions which interact at the Sun, and the presence or absence of anergy cutoff in the acceleration process. The high-energy proton spectrum shape can be determined from the neutron spectrum
Undergraduate views of the structure and value of the Clemson general education ePortfolio
We gathered structural information about the Clemson general education eportfolio as well as students\u27 views of the ePortfolio\u27s educational, professional, and assessment values. Fifty-one students who had completed the ePortfolio were interviewed, and they reported minimum structural content (artifacts and rationales) in their ePortfolios. Participants rated the usefulness of the ePortfolio as very low. However, they rated the ePortfolios as accurately describing their capabilities significantly higher. Participants with a major program ePortfolio rated the values higher than those without. We are currently interviewing undergraduates who have not completed the ePortfolio to determine if their plans and expectations are more positive than students who have actually completed the ePortfolio
Coherence, incoherence and scaling along the c axis of YBa_2Cu_3O_{6+x}
The optical properties of single crystals of YBa_2Cu_3O_{6+x} have been
examined along the c axis above and below the critical temperature (T_c) for a
wide range of oxygen dopings. The temperature dependence of the
optically-determined value of the dc conductivity (\sigma_{dc}) in the normal
state suggests a crossover from incoherent (hopping-type) transport at lower
oxygen dopings (x \lesssim 0.9) to more coherent anisotropic three-dimensional
behavior in the overdoped (x \approx 0.99) material at temperatures close to
T_c. The assumption that superconductivity occurs along the c axis through the
Josephson effect yields a scaling relation between the strength of the
superconducting condensate (\rho_{s,c}, a measure of the number of
superconducting carriers), the critical temperature, and the normal-state
c-axis value for \sigma_{dc} just above T_c; \rho_{s,c} \propto \sigma_{dc}
T_c. This scaling relation is observed along the c axis for all oxygen dopings,
as well as several other cuprate materials. However, the agreement with the
Josephson coupling model does not necessarily imply incoherent transport,
suggesting that these materials may indeed be tending towards coherent behavior
at the higher oxygen dopings.Comment: Six pages with four figures and one tabl
Quantum Dynamics of the Slow Rollover Transition in the Linear Delta Expansion
We apply the linear delta expansion to the quantum mechanical version of the
slow rollover transition which is an important feature of inflationary models
of the early universe. The method, which goes beyond the Gaussian
approximation, gives results which stay close to the exact solution for longer
than previous methods. It provides a promising basis for extension to a full
field theoretic treatment.Comment: 12 pages, including 4 figure
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