3,870 research outputs found
On the effects of Cosmions upon the structure and evolution of very low mass stars
A number of recent studies have suggested that cosmions, or WIMPS, may play an important role in the energetics of the solar interior; in particular, it has been argued that these hypothetical particles may transport sufficient energy within the nuclear-burning solar core so as to depress the solar core temperature to the point of resolving the solar neutrino problem. Solutions to the solar neutrino problem have proven themselves to be quite nonunique, so that it is of some interest whether the cosmion solution can be tested in some independent manner. It is argued that if cosmions solve the solar neutrino problem, then they must also play an important role in the evolution of low mass main sequence stars; and, second, that if they do so, then a simple (long mean free path) model for the interaction of cosmions with baryons leads to changes in the structure of the nuclear-burning core which may be in principal observable. Such changes include suppression of a fully-convective core in very low mass main sequence stars; and a possible thermal runaway in the core of the nuclear burning region. Some of these changes may be directly observable, and hence may provide independent constraints on the properties of the cosmions required to solve the solar neutrino problem, perhaps even ruling them out
StemNet: An Evolving Service for Knowledge Networking in the Life Sciences
Up until now, crucial life science information resources, whether bibliographic or factual databases, are isolated from each other. Moreover, semantic metadata intended to structure their contents is supplied in a manual form only. In the StemNet project we aim at developing a framework for semantic interoperability for these resources. This will facilitate the extraction of relevant information from textual sources and the generation of semantic metadata in a fully automatic manner. In this way, (from a computational perspective) unstructured life science documents are linked to structured biological fact databases, in particular to the identifiers of genes, proteins, etc. Thus, life scientists will be able to seamlessly access information from a homogeneous platform, despite the fact that the original information was unlinked and scattered over the whole variety of heterogeneous life science information resources and, therefore, almost inaccessible for integrated systematic search by academic, clinical, or industrial users
The approach to thermalization in the classical phi^4 theory in 1+1 dimensions: energy cascades and universal scaling
We study the dynamics of thermalization and the approach to equilibrium in
the classical phi^4 theory in 1+1 spacetime dimensions. At thermal equilibrium
we exploit the equivalence between the classical canonical averages and
transfer matrix quantum traces of the anharmonic oscillator to obtain exact
results for the temperature dependence of several observables, which provide a
set of criteria for thermalization. We find that the Hartree approximation is
remarkably accurate in equilibrium. The non-equilibrium dynamics is studied by
numerically solving the equations of motion in light-cone coordinates for a
broad range of initial conditions and energy densities.The time evolution is
described by several stages with a cascade of energy towards the ultraviolet.
After a transient stage, the spatio-temporal gradient terms become larger than
the nonlinear term and a stage of universal cascade emerges.This cascade starts
at a time scale t_0 independent of the initial conditions (except for very low
energy density). Here the power spectra feature universal scaling behavior and
the front of the cascade k(t) grows as a power law k(t) sim t^alpha with alpha
lesssim 0.25. The wake behind the cascade is described as a state of Local
Thermodynamic Equilibrium (LTE) with all correlations being determined by the
equilibrium functional form with an effective time dependent temperatureTeff(t)
which slowly decreases as sim t^{-alpha}.Two well separated time scales emerge
while Teff(t) varies slowly, the wavectors in the wake with k < k(t) attain LTE
on much shorter time scales.This universal scaling stage ends when the front of
the cascade reaches the cutoff at a time t_1 sim a^{-1/alpha}. Virialization
starts to set much earlier than LTE. We find that strict thermalization is
achieved only for an infinite time scale.Comment: relevance for quantum field theory discussed providing validity
criteria. To appear in Phys. Rev.
Kinetic pinning and biological antifreezes
Biological antifreezes protect cold-water organisms from freezing. An example
are the antifreeze proteins (AFPs) that attach to the surface of ice crystals
and arrest growth. The mechanism for growth arrest has not been heretofore
understood in a quantitative way. We present a complete theory based on a
kinetic model. We use the `stones on a pillow' picture. Our theory of the
suppression of the freezing point as a function of the concentration of the AFP
is quantitatively accurate. It gives a correct description of the dependence of
the freezing point suppression on the geometry of the protein, and might lead
to advances in design of synthetic AFPs.Comment: 4 pages, 4 figure
Temperature and Emission-Measure Profiles Along Long-Lived Solar Coronal Loops Observed with TRACE
We report an initial study of temperature and emission measure distributions
along four steady loops observed with the Transition Region and Coronal
Explorer (TRACE) at the limb of the Sun. The temperature diagnostic is the
filter ratio of the extreme-ultraviolet 171-angstrom and 195-angstrom
passbands. The emission measure diagnostic is the count rate in the
171-angstrom passband. We find essentially no temperature variation along the
loops. We compare the observed loop structure with theoretical isothermal and
nonisothermal static loop structure.Comment: 10 pages, 3 postscript figures (LaTeX, uses aaspp4.sty). Accepted by
ApJ Letter
Characterization of the Inner Knot of the Crab: The Site of the Gamma-ray Flares?
One of the most intriguing results from the gamma-ray instruments in orbit
has been the detection of powerful flares from the Crab Nebula. These flares
challenge our understanding of pulsar wind nebulae and models for particle
acceleration. We report on the portion of a multiwavelength campaign using
Keck, HST, and Chandra concentrating on a small emitting region, the Crab's
inner knot, located a fraction of an arcsecond from the pulsar.
We find that the knot's radial size, tangential size, peak flux, and the
ratio of the flux to that of the pulsar are correlated with the projected
distance of the knot from the pulsar. A new approach, using singular value
decomposition for analyzing time series of images, was introduced yielding
results consistent with the more traditional methods while some uncertainties
were substantially reduced.
We exploit the characterization of the knot to discuss constraints on
standard shock-model parameters that may be inferred from our observations
assuming the inner knot lies near to the shocked surface. These include
inferences as to wind magnetization, shock shape parameters such as incident
angle and poloidal radius of curvature, as well as the IR/optical emitting
particle enthalpy fraction. We find that while the standard shock model gives
good agreement with observation in many respects, there remain two puzzles: (a)
The observed angular size of the knot relative to the pulsar--knot separation
is much smaller than expected; (b) The variable, yet high degree of
polarization reported is difficult to reconcile with a highly relativistic
downstream flow.Comment: 46 pages, 14 figures, submitted to the Astrophysical Journa
Universality of rain event size distributions
We compare rain event size distributions derived from measurements in
climatically different regions, which we find to be well approximated by power
laws of similar exponents over broad ranges. Differences can be seen in the
large-scale cutoffs of the distributions. Event duration distributions suggest
that the scale-free aspects are related to the absence of characteristic scales
in the meteorological mesoscale.Comment: 16 pages, 10 figure
Contemplative Science: An Insider's Prospectus
This chapter describes the potential far‐reaching consequences of contemplative higher education for the fields of science and medicine
The Vela Pulsar and its Synchrotron Nebula
(Abridged) We present high-resolution Chandra X-ray observations of PSR
B0833-45, the 89 ms pulsar associated with the Vela supernova remnant. We have
acquired two observations separated by one month to search for changes in the
pulsar and its environment following an extreme glitch in its rotation
frequency. We find a well-resolved nebula with a toroidal morphology remarkably
similar to that observed in the Crab Nebula, along with an axial Crab-like jet.
Between the two observations the flux from the pulsar is found to be steady to
within 0.75%; the 3 sigma limit on the fractional increase in the pulsar's
X-ray flux is < ~10^-5 of the inferred glitch energy. We use this limit to
constrain parameters of glitch models and neutron star structure. We do find a
significant increase in the flux of the nebula's outer arc; if associated with
the glitch, the inferred propagation velocity is > 0.7c, similar to that seen
in the brightening of the Crab Nebula wisps. We propose an explanation for the
X-ray structure of the Vela synchrotron nebula based on a model originally
developed for the Crab Nebula. In a departure from the Crab model, the
magnetization parameter "sigma" of the Vela pulsar wind is allowed to be of
order unity; this is consistent with the simplest MHD transport of magnetic
field from the pulsar to the nebula, where B < 4 X 10^-4 G. We review effects
that may enhance the probability of alignment between the spin axis and space
velocity of a pulsar, and speculate that short-period, slowly moving pulsars
are just the ones best-suited to producing synchrotron nebulae with such
aligned structures.Comment: 16 pages with 8 figures, uses LaTex, emulateapj.sty. Refereed
version. To appear in The Astrophysical Journa
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