16,608 research outputs found
Reliability of systems with dependent components based on lattice polynomial description
Reliability of a system is considered where the components' random lifetimes
may be dependent. The structure of the system is described by an associated
"lattice polynomial" function. Based on that descriptor, general framework
formulas are developed and used to obtain direct results for the cases where a)
the lifetimes are "Bayes-dependent", that is, their interdependence is due to
external factors (in particular, where the factor is the "preliminary phase"
duration) and b) where the lifetimes' dependence is implied by upper or lower
bounds on lifetimes of components in some subsets of the system. (The bounds
may be imposed externally based, say, on the connections environment.) Several
special cases are investigated in detail
Distribution of Time-Averaged Observables for Weak Ergodicity Breaking
We find a general formula for the distribution of time-averaged observables
for systems modeled according to the sub-diffusive continuous time random walk.
For Gaussian random walks coupled to a thermal bath we recover ergodicity and
Boltzmann's statistics, while for the anomalous subdiffusive case a weakly
non-ergodic statistical mechanical framework is constructed, which is based on
L\'evy's generalized central limit theorem. As an example we calculate the
distribution of : the time average of the position of the particle,
for unbiased and uniformly biased particles, and show that exhibits
large fluctuations compared with the ensemble average .Comment: 5 pages, 2 figure
SwSt 1: an O-rich planetary nebula around a C-rich central star
The hydrogen-deficient carbon-rich [WCL] type central star HD167362 and its
oxygen-rich planetary nebula (PN) SwSt~1 are investigated. The nebular
chemistry might indicate a recent origin for the carbon-rich stellar spectrum.
Its stellar and nebular properties might therefore provide further
understanding of the origin of the [WCL] central star class. The UV-IR stellar
spectra are modelled with state of the codes and show ~40kK central star with a
wind and a C/O~3, indicative of efficient third dredge-up. The synthetic
stellar flux distribution is used to model the high density, compact PN, which
has a solar C/O ratio, is still enshrouded by 1200K and 230K dust shells and,
reported here for the first time, in molecular hydrogen. Although it appears
that the change in C/O ratio has been recent, the published spectroscopy since
1895 has been re-examined and no clear spectral change is seen. If an event
occurred that has turned it into a hydrogen-deficient central star, it did not
happen in the last 100 years.Comment: 31 pages, 19 figures (some are gif files), MNRAS in pres
Quenched invariance principle for random walks in balanced random environment
We consider random walks in a balanced random environment in ,
. We first prove an invariance principle (for ) and the
transience of the random walks when (recurrence when ) in an
ergodic environment which is not uniformly elliptic but satisfies certain
moment condition. Then, using percolation arguments, we show that under mere
ellipticity, the above results hold for random walks in i.i.d. balanced
environments.Comment: Published online in Probab. Theory Relat. Fields, 05 Oct 2010. Typo
(in journal version) corrected in (26
The integrated density of states of the random graph Laplacian
We analyse the density of states of the random graph Laplacian in the
percolating regime. A symmetry argument and knowledge of the density of states
in the nonpercolating regime allows us to isolate the density of states of the
percolating cluster (DSPC) alone, thereby eliminating trivially localised
states due to finite subgraphs. We derive a nonlinear integral equation for the
integrated DSPC and solve it with a population dynamics algorithm. We discuss
the possible existence of a mobility edge and give strong evidence for the
existence of discrete eigenvalues in the whole range of the spectrum.Comment: 4 pages, 1 figure. Supplementary material available at
http://www.theorie.physik.uni-goettingen.de/~aspel/data/spectrum_supplement.pd
Unravelling the chemical inhomogeneity of PNe with VLT FLAMES integral-field unit spectroscopy
Recent weak emission-line long-slit surveys and modelling studies of PNe have
convincingly argued in favour of the existence of an unknown component in the
planetary nebula plasma consisting of cold, hydrogen-deficient gas, as an
explanation for the long-standing recombination-line versus forbidden-line
temperature and abundance discrepancy problems. Here we describe the rationale
and initial results from a detailed spectroscopic study of three Galactic PNe
undertaken with the VLT FLAMES integral-field unit spectrograph, which advances
our knowledge about the small-scale physical properties, chemical abundances
and velocity structure of these objects across a two-dimensional field of view,
and opens up for exploration an uncharted territory in the study and modelling
of PNe and photoionized nebulae in general.Comment: 4 pages; 3 figures; invited paper to appear in proceedings of IAU
Symp. No. 234, 2006, Planetary Nebulae in our Galaxy and Beyond (held in
Hawaii, April 2006
Atomic and molecular interstellar absorption lines toward the high galactic latitude stars HD~141569 and HD~157841 at ultra-high resolution
We present ultra-high resolution (0.32 km/s) spectra obtained with the 3.9m
Anglo-Australian Telescope (AAT) and Ultra-High-Resolution Facility (UHRF), of
interstellar NaI D1, D2, Ca II K, K I and CH absorption toward two high
galactic latitude stars HD141569 and HD157841. We have compared our data with
21-cm observations obtained from the Leiden/Dwingeloo HI survey. We derive the
velocity structure, column densities of the clouds represented by the various
components and identify the clouds with ISM structures seen in the region at
other wavelengths. We further derive abundances, linear depletions and H2
fractional abundances for these clouds, wherever possible. Toward HD141569, we
detect two components in our UHRF spectra : a weak, broad component at - 15
km/s, seen only in CaII K absorption and another component at 0 km/s, seen in
NaI D1, D2, Ca II K, KI and CH absorption. In the case of the HD157841
sightline, a total of 6 components are seen on our UHRF spectra in NaI D1, D2
Ca II K, K I and CH absorption. 2 of these 6 components are seen only in a
single species.Comment: 16 pages, Latex, 4 figures, ps files Astrophysical Journal (in press
The Alexander-Orbach conjecture holds in high dimensions
We examine the incipient infinite cluster (IIC) of critical percolation in
regimes where mean-field behavior has been established, namely when the
dimension d is large enough or when d>6 and the lattice is sufficiently spread
out. We find that random walk on the IIC exhibits anomalous diffusion with the
spectral dimension d_s=4/3, that is, p_t(x,x)= t^{-2/3+o(1)}. This establishes
a conjecture of Alexander and Orbach. En route we calculate the one-arm
exponent with respect to the intrinsic distance.Comment: 25 pages, 2 figures. To appear in Inventiones Mathematica
A decade of ejecta dust formation in the Type IIn SN 2005ip
In order to understand the contribution of core-collapse supernovae to the
dust budget of the early universe, it is important to understand not only the
mass of dust that can form in core-collapse supernovae but also the location
and rate of dust formation. SN 2005ip is of particular interest since dust has
been inferred to have formed in both the ejecta and the post-shock region
behind the radiative reverse shock. We have collated eight optical archival
spectra that span the lifetime of SN 2005ip and we additionally present a new
X-shooter optical-near-IR spectrum of SN 2005ip at 4075d post-discovery. Using
the Monte Carlo line transfer code DAMOCLES, we have modelled the blueshifted
broad and intermediate width H, H and He I lines from 48d to
4075d post-discovery using an ejecta dust model. We find that dust in the
ejecta can account for the asymmetries observed in the broad and intermediate
width H, H and He I line profiles at all epochs and that it is
not necessary to invoke post-shock dust formation to explain the blueshifting
observed in the intermediate width post-shock lines. Using a Bayesian approach,
we have determined the evolution of the ejecta dust mass in SN 2005ip over 10
years presuming an ejecta dust model, with an increasing dust mass from
~10 M at 48d to a current dust mass of 0.1 M.Comment: Accepted by MNRAS, 17 pages, 11 figures. Author accepted manuscript.
Accepted on 04/03/19. Deposited on 07/03/1
Variations in the onset diameter for Martian layered ejecta morphologies and their implications for subsurface volatile reservoirs
We investigated regional variations in the onset diameter of craters displaying a single layer ejecta morphology within +/- 30 degrees latitude using Viking imagery. Our results generally agree with those of previous studies which show onset diameters of 5 to 6 km in the equatorial region, but we have identified localized regions with unusually small onset diameters. The largest region is located in Solis and Thaumasia Planae. The 3-5 km onset diameter range in this area indicates a near-surface ice-rich reservoir (depth similar to 110 m). This unusual concentration of near-surface ice may have resulted from magmatic-driven uplifts associated with the Tharsis rise, which modified parts of a regional aquifer/drainage basin system and resulted in the transfer and concentration of subsurface volatiles in this region
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