11,319 research outputs found
Radiative Bulk Viscosity
Viscous resistance to changes in the volume of a gas arises when different
degrees of freedom have different relaxation times. Collisions tend to oppose
the resulting departures from equilibrium and, in so doing, generate entropy.
Even for a classical gas of hard spheres, when the mean free paths or mean
flight times of constituent particles are long, we find a nonvanishing bulk
viscosity. Here we apply a method recently used to uncover this result for a
classical rarefied gas to radiative transfer theory and derive an expression
for the radiative stress tensor for a gray medium with absorption and Thomson
scattering. We determine the transport coefficients through the calculation of
the comoving entropy generation. When scattering dominates absorption, the bulk
viscosity becomes much larger than either the shear viscosity or the thermal
conductivity.Comment: 17 pages. Latex with referee style file of MNRAS (mn.sty). MNRAS, in
pres
A prospectus for a theory of variable variability
It is proposed that the kind of stellar variability exhibited by the Sun in its magnetic activity cycle should be considered as a prototype of a class of stellar variability. The signature includes long 'periods' (compared to that of the radial fundamental model), erratic behavior, and intermittency. As other phenomena in the same variability class we nominate the liminosity fluctuations of ZZ Ceti stars and the solar 160 m oscillation. We discuss the possibility that analogous physical mechanisms are at work in all these cases, namely instabilities driven in a thin layer. These instabilities should be favorable to grave modes (in angle) and should arise in conditions that may allow more than one kind of instability to occur at once. The interaction of these competing instabilities produces complicated temporal variations. Given suitable idealizations, it is shown how to begin to compute solutions of small, but finite, amplitude
Destabilizing Taylor-Couette flow with suction
We consider the effect of radial fluid injection and suction on
Taylor-Couette flow. Injection at the outer cylinder and suction at the inner
cylinder generally results in a linearly unstable steady spiralling flow, even
for cylindrical shears that are linearly stable in the absence of a radial
flux. We study nonlinear aspects of the unstable motions with the energy
stability method. Our results, though specialized, may have implications for
drag reduction by suction, accretion in astrophysical disks, and perhaps even
in the flow in the earth's polar vortex.Comment: 34 pages, 9 figure
Foreign entry into underwriting services: evidence from Japan's "Big Bang" deregulation
We examine the impact of foreign underwriting activity on bond markets using issue level data in the Japanese "Samurai" and euro-yen bond markets. Firms choosing Japanese underwriters tend to be Japanese, riskier, and smaller. We find that Japanese underwriting fees, while higher overall on average, are actually lower after conditioning for issuer characteristics. Moreover, firms tend to sort properly in their choice of underwriter, in the sense that a switch in underwriter nationality would be predicted to result in an increase in underwriting fees. Finally, we conduct a matching exercise to examine the 1995 liberalization of foreign access to the "Samurai" bond market, using yen-denominated issues in the euro-yen market as a control. Foreign entry led to a statistically and economically significant decrease in underwriting fees in the Samurai bond market, as spreads fell by an average of 23 basis points. Overall, our results suggest that the market for underwriting services is partially segmented by nationality, as issuers appear to have preferred habitats, but entry increases market competition.Japan
Habitable Climates: The Influence of Eccentricity
In the outer regions of the habitable zone, the risk of transitioning into a
globally frozen "snowball" state poses a threat to the habitability of planets
with the capacity to host water-based life. We use a one-dimensional energy
balance climate model (EBM) to examine how obliquity, spin rate, orbital
eccentricity, and ocean coverage might influence the onset of such a snowball
state. For an exoplanet, these parameters may be strikingly different from the
values observed for Earth. Since, for constant semimajor axis, the annual mean
stellar irradiation scales with (1-e^2)^(-1/2), one might expect the greatest
habitable semimajor axis (for fixed atmospheric composition) to scale as
(1-e^2)^(-1/4). We find that this standard ansatz provides a reasonable lower
bound on the outer boundary of the habitable zone, but the influence of
obliquity and ocean fraction can be profound in the context of planets on
eccentric orbits. For planets with eccentricity 0.5, our EBM suggests that the
greatest habitable semimajor axis can vary by more than 0.8 AU (78%!) depending
on obliquity, with higher obliquity worlds generally more stable against
snowball transitions. One might also expect that the long winter at an
eccentric planet's apoastron would render it more susceptible to global
freezing. Our models suggest that this is not a significant risk for Earth-like
planets around Sun-like stars since such planets are buffered by the thermal
inertia provided by oceans covering at least 10% of their surface. Since
planets on eccentric orbits spend much of their year particularly far from the
star, such worlds might turn out to be especially good targets for direct
observations with missions such as TPF-Darwin. Nevertheless, the extreme
temperature variations achieved on highly eccentric exo-Earths raise questions
about the adaptability of life to marginally or transiently habitable
conditions.Comment: References added, text and figures updated, accepted by Ap
Polygyny as myth: towards understanding extra-marital relations in Lesotho
Paper presented at the Wits History Workshop: Structure and Experience in the Making of Apartheid, 6-10 February, 199
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