1,476 research outputs found
The Effects of Clumping and Substructure on ICM Mass Measurements
We examine an ensemble of 48 simulated clusters to determine the effects of
small-scale density fluctuations and large-scale substructure on X-ray
measurements of the intracluster medium (ICM) mass. We measure RMS density
fluctuations in the ICM which can be characterized by a mean mass-weighted
clumping factor C = /^2 between 1.3 and 1.4 within a density
contrast of 500 times the critical density. These fluctuations arise from the
cluster history of accretion shocks and major mergers, and their presence
enhances the cluster's luminosity relative to the smooth case. We expect,
therefore, that ICM mass measurements utilizing models which assume uniform
density at a given radius carry a bias of order sqrt(C) = 1.16. We verify this
result by performing ICM mass measurements on X-ray images of the simulations
and finding the expected level of bias.
The varied cluster morphologies in our ensemble also allow us to investigate
the effects of departures from spherical symmetry on our measurements. We find
that the presence of large-scale substructure does not further bias the
resulting gas mass unless it is pronounced enough to produce a second peak in
the image of at least 1% the maximum surface brightness. We analyze the subset
of images with no secondary peaks and find a bias of 9% and a Gaussian random
error of 4% in the derived mass.Comment: To appear in ApJ
Heavy oil production with energy effective steam-assisted gravity drainage
In reservoirs with extra heavy oil and bitumen, thermal methods are used to reduce the viscosity, in order to extract the oil. Steam-assisted gravity drainage (SAGD) is a thermal method where continuous steam injection is used. In this method, two horizontal wells are placed in parallel. The upper well injects steam and the lower well produces oil and condensed water. The continuous steam injection creates a chamber with uniform temperature. Heavy oil and bitumen reserves in Western Canada, which exceed 175 billion barrels, are becoming increasingly important petroleum sources due to the technical success of the SAGD processes. This study includes Computational fluid dynamics (CFD) modelling and simulations of a horizontal oil well with SAGD. The simulations are performed with inflow control devices (ICD) and autonomous inflow control valves (AICV) completion. In the SAGD processes, it is important that the residence time for steam in the reservoir is high enough to ensure that all the injected steam condenses in the reservoir to reduce the amount of steam injection and thereby making the SAGD process more energy effective. The simulations are carried out with ICD completion to delay the steam breakthrough and with AICV completion to prevent breakthrough of steam and water to the well. The numerical results showed that a most of the steam was produced together with the oil when ICD completion was used. AICV was able to close for steam and water, and the steam was thereby forced to condense in the reservoir, resulting in better utilization of the condensation energy
Constraints on \Omega_0 and Cluster Evolution Using the ROSAT LogN-LogS
We examine the likelihoods of different cosmological models and cluster
evolutionary histories by comparing semi-analytical predictions of X-ray
cluster number counts to observational data from the ROSAT satellite. We model
cluster abundance as a function of mass and redshift using a Press-Schechter
distribution, and assume the temperature T(M,z) and bolometric luminosity
L_X(M,z) scale as power laws in mass and epoch, in order to construct expected
counts as a function of X-ray flux. The L_X-M scaling is fixed using the local
luminosity function while the degree of evolution in the X-ray luminosity with
redshift L_X \propto (1+z)^s is left open, with s an interesting free parameter
which we investigate. We examine open and flat cosmologies with initial,
scale-free fluctuation spectra having indices n = 0, -1 and -2. An independent
constraint arising from the slope of the luminosity-temperature relation
strongly favors the n = -2 spectrum.
The expected counts demonstrate a strong dependence on \Omega_0 and s, with
lesser dependence on \lambda_0 and n. Comparison with the observed counts
reveals a "ridge" of acceptable models in the \Omega_0 - s plane, roughly
following the relation s = 6 \Omega_0 and spanning low-density models with a
small degree of evolution to \Omega = 1 models with strong evolution. Models
with moderate evolution are revealed to have a strong lower limit of \Omega_0
\gtrsim 0.3, and low-evolution models imply that \Omega_0 < 1 at a very high
confidence level. We suggest observational tests for breaking the degeneracy
along this ridge, and discuss implications for evolutionary histories of the
intracluster medium.Comment: MNRAS LaTeX style format, submitted to MNRAS 3/26/97. Thirteen pages,
eleven postscript figures. Uses epsf macros to include figure
Roughening of Fracture Surfaces: the Role of Plastic Deformations
Post mortem analysis of fracture surfaces of ductile and brittle materials on
the m-mm and the nm scales respectively, reveal self affine graphs with an
anomalous scaling exponent . Attempts to use elasticity
theory to explain this result failed, yielding exponent up
to logarithms. We show that when the cracks propagate via plastic void
formations in front of the tip, followed by void coalescence, the voids
positions are positively correlated to yield exponents higher than 0.5.Comment: 4 pages, 6 figure
Scale Free Cluster Distributions from Conserving Merging-Fragmentation Processes
We propose a dynamical scheme for the combined processes of fragmentation and
merging as a model system for cluster dynamics in nature and society displaying
scale invariant properties. The clusters merge and fragment with rates
proportional to their sizes, conserving the total mass. The total number of
clusters grows continuously but the full time-dependent distribution can be
rescaled over at least 15 decades onto a universal curve which we derive
analytically. This curve includes a scale free solution with a scaling exponent
of -3/2 for the cluster sizes.Comment: 4 pages, 3 figure
Four Measures of the Intracluster Medium Temperature and Their Relation to a Cluster's Dynamical State
We employ an ensemble of hydrodynamic cluster simulations to create spatially
and spectrally resolved images of quality comparable to Chandra's expected
performance. Emission from simulation mass elements is represented using the
XSPEC mekal program assuming 0.3 solar metallicity, and the resulting spectra
are fit with a single-temperature model. Despite significant departures from
isothermality in the cluster gas, single-temperature models produce acceptable
fits to 20,000 source photon spectra. The spectral fit temperature T_s is
generally lower than the mass weighted average temperature T_m due to the
influence of soft line emission from cooler gas being accreted as part of the
hierarchical clustering process. In a Chandra-like bandpass of 0.5 to 9.5 keV
we find a nearly uniform fractional bias of (T_m-T_s)/T_s = 20% with occasional
large deviations in smaller clusters. In the more traditional 2.0 to 9.5 keV
bandpass, the fractional deviation is scale-dependent and on average follows
the relation (T_m-T_s)/T_s = 0.2 log(T_m). This bias results in a spectral
mass-temperature relationship with slope about 1.6, intermediate between the
virial relation M ~ T_m^{3/2} and the observed relation M_{ICM} ~ T^2. Imaging
each cluster in the ensemble at 16 epochs in its evolutionary history, we
catalogue merger events with mass ratios exceeding 10% in order to investigate
the relationship between spectral temperature and proximity to a major merger
event. Clusters that are very cool relative to the mean mass-temperature
relationship lie preferentially close to a merger, suggesting a viable
observational method to cull a subset of dynamically young clusters from the
general population.Comment: 34 pages, including 2 tables and 14 figures (one in color). Compiled
using LaTeX 2.09 with graphics package and aaspp4 style. The simulated
spectral data files used in this paper are available for public consumption
at http://redshift.stanford.edu/bfm
Comparing the temperatures of galaxy clusters from hydro-N-body simulations to Chandra and XMM-Newton observations
Theoretical studies of the physical processes guiding the formation and
evolution of galaxies and galaxy clusters in the X-ray are mainly based on the
results of numerical hydrodynamical N-body simulations, which in turn are often
directly compared to X-ray observations. Although trivial in principle, these
comparisons are not always simple. We demonstrate that the projected
spectroscopic temperature of thermally complex clusters obtained from X-ray
observations is always lower than the emission-weighed temperature, which is
widely used in the analysis of numerical simulations. We show that this
temperature bias is mainly related to the fact that the emission-weighted
temperature does not reflect the actual spectral properties of the observed
source. This has important implications for the study of thermal structures in
clusters, especially when strong temperature gradients, like shock fronts, are
present. Because of this bias, in real observations shock fronts appear much
weaker than what is predicted by emission-weighted temperature maps, and may
even not be detected. This may explain why, although numerical simulations
predict that shock fronts are a quite common feature in clusters of galaxies,
to date there are very few observations of objects in which they are clearly
seen. To fix this problem we propose a new formula, the spectroscopic-like
temperature function, and show that, for temperature larger than 3 keV, it
approximates the spectroscopic temperature better than few per cent, making
simulations more directly comparable to observations.Comment: Submitted for publication in MNRAS; 15 pages, 10 color figures and 13
BW figures,mn2e.cls. High resolution figures available here:
http://people.roma2.infn.it/~mazzotta/preprints/mazzotta.pd
Taxonomy and evolutionary relationships within species of section Rimosae (Inocybe) based on ITS, LSU and mtSSU sequence data
The present study aimed at elucidating the structure of Inocybe subg. Inosperma sect. Rimosae but included also representatives from subg. Mallocybe and the genus Auritella. Phylogenetic relationships were inferred using ITS, LSU and mtSSU sequence data. The analyses recovered the ingroup as a monophyletic, strongly supported clade. The results indicate that recognizing Auritella on the genus level renders Inocybe paraphyletic. The species traditionally placed in sect. Rimosae were found to be distributed over two strongly supported clades, Maculata and Rimosae s.s. The Maculata clade clusters with sect. Cervicolores and the two represent subg. Inosperma in a strict sense. Rimosae s.s. emerges as an independent, supported clade well separated from Inosperma s.s. Twenty-one terminal groups were correlated with morphologically distinct species. In addition several taxa on single branches and minor less supported clades were recovered. A key to the identified species of the Maculata and Rimosae s.s. clades which occur in Northwest Europe is provided
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