5,495 research outputs found
On reaction-subdiffusion equations
To analyze possible generalizations of reaction-diffusion schemes for the
case of subdiffusion we discuss a simple monomolecular conversion A --> B. We
derive the corresponding kinetic equations for local A and B concentrations.
Their form is rather unusual: The parameters of reaction influence the
diffusion term in the equation for a component A, a consequence of the
nonmarkovian nature of subdiffusion. The equation for a product contains a term
which depends on the concentration of A at all previous times. Our discussion
shows that reaction-subdiffusion equations may not resemble the corresponding
reaction-diffusion ones and are not obtained by a trivial change of the
diffusion operator for a subdiffusion one
Stationary components of HeI in strong magnetic fields - a tool to identify magnetic DB white dwarfs
In only three of the 61 known magnetic white dwarfs helium has been
identified unambiguously while about 20% of all non-magnetic stars of this
class are known to contain HeI or HeII. Until recently, data for HeI data were
available only for magnetic fields below 20MG. This changed with the
publication of extensive data by the group in Heidelberg. The corresponding
calculations have now been completed for the energetically lowest five states
of singlet and triplet symmetry for the subspaces with |m| <= 3; selected
calculations have been performed for even higher excitations. In strongly
magnetized white dwarfs only line components are visible whose wavelengths vary
slowly with respect to the magnetic field, particularly stationary components
which have a wavelength minimum or maximum in the range of the magnetic fields
strengths on the stellar surface. In view of the many ongoing surveys finding
white dwarfs we want to provide the astronomical community with a tool to
identify helium in white dwarfs for fields up to 5.3GG. To this end we present
all calculated helium line components whose wavelengths in the UV, optical, and
near IR vary slowly enough with respect to the field strength to produce
visible absorption features. We also list all stationary line components in
this spectral range. Finally, we find series of minima and maxima which occur
as a result of series of extremal transitions to increasingly higher
excitations. We estimated the limits for 8 series which can possibly give rise
to additional absorption in white dwarf spectra; one strong absorption feature
in GD229 which is yet unexplained by stationary components is very close to two
estimated series limits.Comment: 8 pages, 2 figures, accepted for publication by Astronomy and
Astrophysic
Mesoscopic description of reactions under anomalous diffusion: A case study
Reaction-diffusion equations deliver a versatile tool for the description of
reactions in inhomogeneous systems under the assumption that the characteristic
reaction scales and the scales of the inhomogeneities in the reactant
concentrations separate. In the present work we discuss the possibilities of a
generalization of reaction-diffusion equations to the case of anomalous
diffusion described by continuous-time random walks with decoupled step length
and waiting time probability densities, the first being Gaussian or Levy, the
second one being an exponential or a power-law lacking the first moment. We
consider a special case of an irreversible or reversible A ->B conversion and
show that only in the Markovian case of an exponential waiting time
distribution the diffusion- and the reaction-term can be decoupled. In all
other cases, the properties of the reaction affect the transport operator, so
that the form of the corresponding reaction-anomalous diffusion equations does
not closely follow the form of the usual reaction-diffusion equations
Spectrum of Neutral Helium in Strong Magnetic Fields
We present extensive and accurate calculations for the excited state spectrum
of spin-polarized neutral helium in a range of magnetic field strengths up to
G. Of considerable interest to models of magnetic white dwarf stellar
atmospheres, we also present results for the dipole strengths of the low lying
transitions among these states. Our methods rely on a systematically saturated
basis set approach to solving the Hartree--Fock self-consistent field
equations, combined with an ``exact'' stochastic method to estimate the
residual basis set truncation error and electron correlation effects. We also
discuss the applicability of the adiabatic approximation to strongly magnetized
multi-electron atoms.Comment: 19 pages, 7 figures, 10 table
Propagation of squeezed radiation through amplifying or absorbing random media
We analyse how nonclassical features of squeezed radiation (in particular the
sub-Poissonian noise) are degraded when it is transmitted through an amplifying
or absorbing medium with randomly located scattering centra. Both the cases of
direct photodetection and of homodyne detection are considered. Explicit
results are obtained for the dependence of the Fano factor (the ratio of the
noise power and the mean current) on the degree of squeezing of the incident
state, on the length and the mean free path of the medium, the temperature, and
on the absorption or amplification rate.Comment: 8 pages, 4 figure
The 160 Square Degree ROSAT Survey: the Revised Catalog of 201 Clusters with Spectroscopic Redshifts
We present the revised catalog of galaxy clusters detected as extended X-ray
sources in the 160 Square Degree ROSAT Survey, including spectroscopic
redshifts and X-ray luminosities for 200 of the 201 members. The median
redshift is z~0.25 and the median X-ray luminosity is 4.2e+43 erg/s/h50^2
(0.5-2.0 keV). This is the largest high-redshift sample of X-ray selected
clusters published to date. There are 73 objects at z>0.3 and 22 objects at
z>0.5 drawn from a statistically complete flux-limited survey with a median
object flux of 1.4d-13 erg/cm^2/s. We describe the optical follow-up of these
clusters with an emphasis on our spectroscopy which has yielded 155 cluster
redshifts, 110 of which are presented here for the first time. These
measurements combined with 45 from the literature and other sources provide
near-complete spectroscopic coverage for our survey. We discuss the final
optical identifications for the extended X-ray sources in the survey region and
compare our results to similar X-ray cluster searches.Comment: 17 pages, 7 figs, accepted for publication in ApJ, a version with
full resolution images is available at
http://www.eso.org/~cmullis/papers/160sd-catalog.ps.gz, machine-readable
versions of the catalog are available at
http://www.eso.org/~cmullis/research/160sd-catalog.htm
Muted change in Atlantic overturning circulation over some glacial-aged Heinrich events
Heinrich eventsâsurges of icebergs into the North Atlantic Oceanâpunctuated the last glacial period. The events are associated with millennial-scale cooling in the Northern Hemisphere. Fresh water from the melting icebergs is thought to have interrupted the Atlantic meridional overturning circulation, thus minimizing heat transport into the northern North Atlantic. The northward flow of warm water passes through the Florida Straits and is reflected in the distribution of seawater properties in this region. Here we investigate the northward flow through this region over the past 40,000 years using oxygen isotope measurements of benthic foraminifera from two cores on either side of the Florida Straits. These measurements allow us to estimate water density, which is related to flow through the thermal wind balance. We infer a substantial reduction of flow during Heinrich Event 1 and the Younger Dryas cooling, but little change during Heinrich Events 2 and 3, which occurred during an especially cold phase of the last glacial period. We speculate that because glacial circulation was already weakened before the onset of Heinrich Events 2 and 3, freshwater forcing had little additional effect. However, low-latitude climate perturbations were observed during all events. We therefore suggest that these perturbations may not have been directly caused by changes in heat transport associated with Atlantic overturning circulation as commonly assumed
Gas hydrate dissociation off Svalbard induced by isostatic rebound rather than global warming
Methane seepage from the upper continental slopes of Western Svalbard has previously been attributed to gas hydrate dissociation induced by anthropogenic warming of ambient bottom waters. Here we show that sediment cores drilled off Prins Karls Foreland contain freshwater from dissociating hydrates. However, our modeling indicates that the observed pore water freshening began around 8 ka BP when the rate of isostatic uplift outpaced eustatic sea-level rise. The resultant local shallowing and lowering of hydrostatic pressure forced gas hydrate dissociation and dissolved chloride depletions consistent with our geochemical analysis. Hence, we propose that hydrate dissociation was triggered by postglacial isostatic rebound rather than anthropogenic warming. Furthermore, we show that methane fluxes from dissociating hydrates were considerably smaller than present methane seepage rates implying that gas hydrates were not a major source of methane to the oceans, but rather acted as a dynamic seal, regulating methane release from deep geological reservoirspublishersversionPeer reviewe
The Observed Growth of Massive Galaxy Clusters I: Statistical Methods and Cosmological Constraints
(Abridged) This is the first of a series of papers in which we derive
simultaneous constraints on cosmological parameters and X-ray scaling relations
using observations of the growth of massive, X-ray flux-selected galaxy
clusters. Our data set consists of 238 clusters drawn from the ROSAT All-Sky
Survey, and incorporates extensive follow-up observations using the Chandra
X-ray Observatory. Here we describe and implement a new statistical framework
required to self-consistently produce simultaneous constraints on cosmology and
scaling relations from such data, and present results on models of dark energy.
In spatially flat models with a constant dark energy equation of state, w, the
cluster data yield Omega_m=0.23 +- 0.04, sigma_8=0.82 +- 0.05, and w=-1.01 +-
0.20, marginalizing over conservative allowances for systematic uncertainties.
These constraints agree well and are competitive with independent data in the
form of cosmic microwave background (CMB) anisotropies, type Ia supernovae
(SNIa), cluster gas mass fractions (fgas), baryon acoustic oscillations (BAO),
galaxy redshift surveys, and cosmic shear. The combination of our data with
current CMB, SNIa, fgas, and BAO data yields Omega_m=0.27 +- 0.02, sigma_8=0.79
+- 0.03, and w=-0.96 +- 0.06 for flat, constant w models. For evolving w
models, marginalizing over transition redshifts in the range 0.05-1, we
constrain the equation of state at late and early times to be respectively
w_0=-0.88 +- 0.21 and w_et=-1.05 +0.20 -0.36. The combined data provide
constraints equivalent to a DETF FoM of 15.5. Our results highlight the power
of X-ray studies to constrain cosmology. However, the new statistical framework
we apply to this task is equally applicable to cluster studies at other
wavelengths.Comment: 16 pages, 7 figures. v4: final version (typographic corrections).
Results can be downloaded at
https://www.stanford.edu/group/xoc/papers/xlf2009.htm
Conductionâdominated heat transport of the annual temperature signal in soil
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95182/1/jgrb13727.pd
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