3,168 research outputs found
Crystallization and characterization of Y2O3-SiO2 glasses
Glasses in the yttria-silica system with 20 to 40 mol pct Y2O3 were subjected to recrystallization studies after melting at 1900 to 2100 C in W crucibles in 1 and 50 atm N2. The TEM and XRD results obtained indicate the presence of the delta, gamma, gamma prime, and beta-Y2Si2O7 crystalline phases, depending on melting and quenching conditions. Heat treatment in air at 1100 to 1600 C increased the amount of crystallization, and led to the formation of Y2SiO5, cristabalite, and polymorphs of Y2Si2O7. Also investigated were the effects of 5 and 10 wt pct zirconia additions
IRMA via SDN: Intrusion Response and Monitoring Appliance via Software-Defined Networking
Recent approaches to network intrusion prevention systems (NIPSs) use software-defined networking (SDN) to take advantage of dynamic network reconfigurability and programmability, but issues remain with system component modularity, network size scalability, and response latency. We present IRMA, a novel SDN-based NIPS for enterprise networks, as a network appliance that captures data traffic, checks for intrusions, issues alerts, and responds to alerts by automatically reconfiguring network flows via the SDN control plane. With a composable, modular, and parallelizable service design, we show improved throughput and less than 100 ms average latency between alert detection and response.Roy J. Carver FellowshipOpe
Algorithms for Performance, Dependability, and Performability Evaluation using Stochastic Activity Networks
Modeling tools and technologies are important for aerospace development. At the University of Illinois, we have worked on advancing the state of the art in modeling by Markov reward models in two important areas: reducing the memory necessary to numerically solve systems represented as stochastic activity networks and other stochastic Petri net extensions while still obtaining solutions in a reasonable amount of time, and finding numerically stable and memory-efficient methods to solve for the reward accumulated during a finite mission time. A long standing problem when modeling with high level formalisms such as stochastic activity networks is the so-called state space explosion, where the number of states increases exponentially with size of the high level model. Thus, the corresponding Markov model becomes prohibitively large and solution is constrained by the the size of primary memory. To reduce the memory necessary to numerically solve complex systems, we propose new methods that can tolerate such large state spaces that do not require any special structure in the model (as many other techniques do). First, we develop methods that generate row and columns of the state transition-rate-matrix on-the-fly, eliminating the need to explicitly store the matrix at all. Next, we introduce a new iterative solution method, called modified adaptive Gauss-Seidel, that exhibits locality in its use of data from the state transition-rate-matrix, permitting us to cache portions of the matrix and hence reduce the solution time. Finally, we develop a new memory and computationally efficient technique for Gauss-Seidel based solvers that avoids the need for generating rows of A in order to solve Ax = b. This is a significant performance improvement for on-the-fly methods as well as other recent solution techniques based on Kronecker operators. Taken together, these new results show that one can solve very large models without any special structure
Dependency-Based Decomposition of Systems Involving Rare Events
Coordinated Science Laboratory was formerly known as Control Systems LaboratoryIBM Ph.D. Fellowshi
The MOSDEF Survey: Mass, Metallicity, and Star-formation Rate at z~2.3
We present results on the z~2.3 mass-metallicity relation (MZR) using early
observations from the MOSFIRE Deep Evolution Field (MOSDEF) survey. We use an
initial sample of 87 star-forming galaxies with spectroscopic coverage of
H\beta, [OIII]\lambda 5007, H\alpha, and [NII]\lambda 6584 rest-frame optical
emission lines, and estimate the gas-phase oxygen abundance based on the N2 and
O3N2 strong-line indicators. We find a positive correlation between stellar
mass and metallicity among individual z~2.3 galaxies using both the N2 and O3N2
indicators. We also measure the emission-line ratios and corresponding oxygen
abundances for composite spectra in bins of stellar mass. Among composite
spectra, we find a monotonic increase in metallicity with increasing stellar
mass, offset ~0.15-0.3 dex below the local MZR. When the sample is divided at
the median star-formation rate (SFR), we do not observe significant SFR
dependence of the z~2.3 MZR among either individual galaxies or composite
spectra. We furthermore find that z~2.3 galaxies have metallicities ~0.1 dex
lower at a given stellar mass and SFR than is observed locally. This offset
suggests that high-redshift galaxies do not fall on the local "fundamental
metallicity relation" among stellar mass, metallicity, and SFR, and may provide
evidence of a phase of galaxy growth in which the gas reservoir is built up due
to inflow rates that are higher than star-formation and outflow rates. However,
robust conclusions regarding the gas-phase oxygen abundances of high-redshift
galaxies await a systematic reappraisal of the application of locally
calibrated metallicity indicators at high redshift.Comment: 12 pages, 6 figures, accepted for publication in the Astrophysical
Journal (ApJ
The MOSDEF Survey: Detection of [OIII]4363 and the direct-method oxygen abundance of a star-forming galaxy at z=3.08
We present measurements of the electron-temperature based oxygen abundance
for a highly star-forming galaxy at z=3.08, COSMOS-1908. This is the highest
redshift at which [OIII]4363 has been detected, and the first time
that this line has been measured at z>2. We estimate an oxygen abundance of
12+log(O/H). This galaxy is a low-mass (
M), highly star-forming ( M yr) system that
hosts a young stellar population ( Myr). We investigate the physical
conditions of the ionized gas in COSMOS-1908 and find that this galaxy has a
high ionization parameter, little nebular reddening (),
and a high electron density ( cm). We compare the ratios of
strong oxygen, neon, and hydrogen lines to the direct-method oxygen abundance
for COSMOS-1908 and additional star-forming galaxies at z=0-1.8 with
[OIII]4363 measurements, and show that galaxies at z1-3 follow
the same strong-line correlations as galaxies in the local universe. This
agreement suggests that the relationship between ionization parameter and O/H
is similar for z0 and high-redshift galaxies. These results imply that
metallicity calibrations based on lines of oxygen, neon, and hydrogen do not
strongly evolve with redshift and can reliably estimate abundances out to
z3, paving the way for robust measurements of the evolution of the
mass-metallicity relation to high redshift.Comment: 7 pages, 3 figures, 1 table, accepted to ApJ Letter
The MOSDEF Survey: Electron Density and Ionization Parameter at
Using observations from the MOSFIRE Deep Evolution Field (MOSDEF) survey, we
investigate the physical conditions of star-forming regions in
galaxies, specifically the electron density and ionization state. From
measurements of the [O II]3726,3729 and [S
II]6716,6731 doublets, we find a median electron density of
cm at , an increase of an order of magnitude
compared to measurements of galaxies at . While galaxies are
offset towards significantly higher O values relative to local galaxies
at fixed stellar mass, we find that the high-redshift sample follows a similar
distribution to the low-metallicity tail of the local distribution in the
O vs. R and O3N2 diagrams. Based on these results, we propose
that star-forming galaxies have the same ionization parameter as
local galaxies at fixed metallicity. In combination with simple photoionization
models, the position of local and galaxies in excitation diagrams
suggests that there is no significant change in the hardness of the ionizing
spectrum at fixed metallicity from to . We find that
galaxies show no offset compared to low-metallicity local galaxies
in emission line ratio diagrams involving only lines of hydrogen, oxygen, and
sulfur, but show a systematic offset in diagrams involving [N II]6584.
We conclude that the offset of galaxies from the local star-forming
sequence in the [N II] BPT diagram is primarily driven by elevated N/O at fixed
O/H compared to local galaxies. These results suggest that the local gas-phase
and stellar metallicity sets the ionization state of star-forming regions at
and .Comment: 26 pages, 14 figures, accepted to Ap
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