2,913 research outputs found
Evaporation and Accretion of Extrasolar Comets Following White Dwarf Kicks
Several lines of observational evidence suggest that white dwarfs receive
small birth kicks due to anisotropic mass loss. If other stars possess
extrasolar analogues to the Solar Oort cloud, the orbits of comets in such
clouds will be scrambled by white dwarf natal kicks. Although most comets will
be unbound, some will be placed on low angular momentum orbits vulnerable to
sublimation or tidal disruption. The dusty debris from these comets will
manifest itself as an IR excess temporarily visible around newborn white
dwarfs; examples of such disks may already have been seen in the Helix Nebula,
and around several other young white dwarfs. Future observations with the James
Webb Space Telescope may distinguish this hypothesis from alternatives such as
a dynamically excited Kuiper Belt analogue. Although competing hypotheses
exist, the observation that of young white dwarfs possess such
disks, if interpreted as indeed being cometary in origin, provides indirect
evidence that low mass gas giants (thought necessary to produce an Oort cloud)
are common in the outer regions of extrasolar planetary systems. Hydrogen
abundances in the atmospheres of older white dwarfs can, if sufficiently low,
also be used to place constraints on the joint parameter space of natal kicks
and exo-Oort cloud models.Comment: 22 pages, 13 figures, published in MNRAS. Changes made to match
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Judges as Medical Decision Makers: Is the Cure Worse than the Disease
I shall examine and criticize three of the many judicial decisions in the area of law and medicine. These cases are Doe v. Bolton, Superintendent of Belchertown State School v. Saikewicz, and Rogers v. Commissioner of the Department of Mental Health. Those of you who like to think of the law as reason and justice tempered by mercy will be offended by what I have to say; but I shall be evenhanded. Those of you who think of medicine as science and art tempered by compassion will also be offended. My justification for the critical and polemical thesis I shall present is my deep and growing conviction that in law, as often as in medicine, the cure can be worse than the disease. There is a word in medicine for cures that create diseases-the word is iatrogenic. Law needs a similar word; let me suggest juridicogenic
Three-dimensional flows in slowly-varying planar geometries
We consider laminar flow in channels constrained geometrically to remain
between two parallel planes; this geometry is typical of microchannels obtained
with a single step by current microfabrication techniques. For pressure-driven
Stokes flow in this geometry and assuming that the channel dimensions change
slowly in the streamwise direction, we show that the velocity component
perpendicular to the constraint plane cannot be zero unless the channel has
both constant curvature and constant cross-sectional width. This result implies
that it is, in principle, possible to design "planar mixers", i.e. passive
mixers for channels that are constrained to lie in a flat layer using only
streamwise variations of their in-plane dimensions. Numerical results are
presented for the case of a channel with sinusoidally varying width
Observing Lense-Thirring Precession in Tidal Disruption Flares
When a star is tidally disrupted by a supermassive black hole (SMBH), the
streams of liberated gas form an accretion disk after their return to
pericenter. We demonstrate that Lense-Thirring precession in the spacetime
around a rotating SMBH can produce significant time evolution of the disk
angular momentum vector, due to both the periodic precession of the disk and
the nonperiodic, differential precession of the bound debris streams. Jet
precession and periodic modulation of disk luminosity are possible
consequences. The persistence of the jetted X-ray emission in the Swift
J164449.3+573451 flare suggests that the jet axis was aligned with the spin
axis of the SMBH during this event.Comment: 4 pages, 4 figures. Accepted for publication in Physical Review
Letters. Minor changes made to match proof
Exploratory Analysis of Functional Data via Clustering and Optimal Segmentation
We propose in this paper an exploratory analysis algorithm for functional
data. The method partitions a set of functions into clusters and represents
each cluster by a simple prototype (e.g., piecewise constant). The total number
of segments in the prototypes, , is chosen by the user and optimally
distributed among the clusters via two dynamic programming algorithms. The
practical relevance of the method is shown on two real world datasets
Experimental and theoretical scaling laws for transverse diffusive broadening in two-phase laminar flows in microchannels
This letter quantifies both experimentally and theoretically the diffusion of low-molecular-weight species across the interface between two aqueous solutions in pressure-driven laminar flow in microchannels at high Peclet numbers. Confocal fluorescent microscopy was used to visualize a fluorescent product formed by reaction between chemical species carried separately by the two solutions. At steady state, the width of the reaction-diffusion zone at the interface adjacent to the wall of the channel and transverse to the direction of flow scales as the one-third power of both the axial distance down the channel (from the point where the two streams join) and the average velocity of the flow, instead of the more familiar one- half power scaling which was measured in the middle of the channel. A quantitative description of reaction-diffusion processes near the walls of the channel, such as described in this letter, is required for the rational use of laminar flows for performing spatially resolved surface chemistry and biology inside microchannels and for understanding three-dimensional features of mass transport in shearing flows near surfaces
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