2,096 research outputs found
Kinetics of the reaction of nitric oxide with hydrogen
Mixtures of NO and H2 diluted in argon or krypton were heated by incident shock waves, and the infrared emission from the fundamental vibration-rotation band of NO at 5.3 microns was used to monitor the time-varying NO concentration. The reaction kinetics were studied in the temperature range 2400-4500 K using a shock-tube technique. The decomposition of nitric oxide behind the shock was found to be modeled well by a fifteen-reaction system. A principle result of the study was the determination of the rate constant for the reaction H + NO yields N + OH, which may be the rate-limiting step for NO removal in some combustion systems. Experimental values of k sub 1 were obtained for each test through comparisons of measured and numerically predicted NO profiles
Decomposition of NO studied by infrared emission and CO laser absorption
A diagnostic technique for monitoring the concentration of NO using absorption of CO laser radiation was developed and applied in a study of the decomposition kinetics of NO. Simultaneous measurements of infrared emission by NO at 5.3 microns were also made to validate the laser absorption technique. The data were obtained behind incident shocks in NO-N2O-Ar (or Kr) mixtures, with temperatures in the range 2400-4100 K. Rate constants for dominant reactions were inferred from comparisons with computer simulations of the reactive flow
Teacher Knowledge and Selection of Evidence-Based Practices: A survey study
Federal legislation and state and local policies mandate the use of evidence-based practices (EBPs) and aim to improve the quality of education for all students. Federal mandates (No Child Left Behind (NCLB) Act of 2001) coupled with teacher training requirements and the need for identifying effective practices for use with students with and without disabilities, highlight the need for teachers to not only implement EBPs but to identify such practices for implementation. The passage of NCLB marked the first time in education that the use of scientific research to inform instructional decisions was mandated
Multifluid, Magnetohydrodynamic Shock Waves with Grain Dynamics II. Dust and the Critical Speed for C Shocks
This is the second in a series of papers on the effects of dust on
multifluid, MHD shock waves in weakly ionized molecular gas. We investigate the
influence of dust on the critical shock speed, v_crit, above which C shocks
cease to exist. Chernoff showed that v_crit cannot exceed the grain
magnetosound speed, v_gms, if dust grains are dynamically well coupled to the
magnetic field. We present numerical simulations of steady shocks where the
grains may be well- or poorly coupled to the field. We use a time-dependent,
multifluid MHD code that models the plasma as a system of interacting fluids:
neutral particles, ions, electrons, and various ``dust fluids'' comprised of
grains with different sizes and charges. Our simulations include grain inertia
and grain charge fluctuations but to highlight the essential physics we assume
adiabatic flow, single-size grains, and neglect the effects of chemistry. We
show that the existence of a phase speed v_phi does not necessarily mean that C
shocks will form for all shock speeds v_s less than v_phi. When the grains are
weakly coupled to the field, steady, adiabatic shocks resemble shocks with no
dust: the transition to J type flow occurs at v_crit = 2.76 v_nA, where v_nA is
the neutral Alfven speed, and steady shocks with v_s > 2.76 v_nA are J shocks
with magnetic precursors in the ion-electron fluid. When the grains are
strongly coupled to the field, v_crit = min(2.76 v_nA, v_gms). Shocks with
v_crit < v_s < v_gms have magnetic precursors in the ion-electron-dust fluid.
Shocks with v_s > v_gms have no magnetic precursor in any fluid. We present
time-dependent calculations to study the formation of steady multifluid shocks.
The dynamics differ qualitatively depending on whether or not the grains and
field are well coupled.Comment: 43 pages with 17 figures, aastex, accepted by The Astrophysical
Journa
An Automatic and Symbolic Parallelization System for Distributed Memory Parallel Computers
This paper describes ASPAR (Automatic and Symbolic PARallelization) which consists of a source-to-source parallelizer and a set of interactive graphic tools. While the issues of data dependency have already been explored and used in many parallel computer systems such as vector and shared memory machines, distributed memory parallel computers require, in addition, explicit data decomposition. New symbolic analysis and data-dependency analysis methods are used to determine an explicit data decomposition scheme. Automatic parallelization models using high level communications are also described in this paper. The target applications are of the “regular-mesh" type typical of many scientific calculations.
The system has been implemented for the language C, and is designed for easy modification for other languages such as Fortran
Disentangling effective temperatures of individual eclipsing binary components by means of color-index constraining
Eclipsing binary stars are gratifying objects because of their unique
geometrical properties upon which all important physical parameters such as
masses, radii, temperatures, luminosities and distance may be obtained in
absolute scale. This poses strict demand on the model to be free of systematic
effects that would influence the results later used for calibrations, catalogs
and evolution theory. We present an objective scheme of obtaining individual
temperatures of both binary system components by means of color-index
constraining, with the only requirement that the observational data-set is
acquired in a standard photometric system. We show that for a modest case of
two similar main-sequence components the erroneous approach of assuming the
temperature of the primary star from the color index yields temperatures which
are systematically wrong by ~100K.Comment: 6 pages, 3 figures, 1 table; to appear in proceedings of the Close
Binaries in the 21st Century conference in Syros, Greec
Parametrization of C-shocks. Evolution of the Sputtering of Grains
Context: The detection of narrow SiO lines toward the young shocks of the
L1448-mm outflow has been interpreted as a signature of the magnetic precursor
of C-shocks. In contrast with the low SiO abundances (<10E-12) in the ambient
gas, the narrow SiO emission at almost ambient velocities reveals enhanced SiO
abundances of 10E-11. This enhancement has been proposed to be produced by the
sputtering of the grain mantles at the first stages of C-shocks. However,
modelling of the sputtering of grains has usually averaged the SiO abundances
over the dissipation region of C-shocks, which cannot explain the recent
observations. Aims: To model the evolution of the gas phase abundances of SiO,
CH3OH and H2O, produced by the sputtering of grains as the shock propagates
through the ambient gas. Methods: We propose a parametric model to describe the
physical structure of C-shocks as a function of time. Using the known
sputtering yields for water mantles (with minor constituents like silicon and
CH3OH) and olivine cores by collisions with H2, He, C, O, Si, Fe and CO, we
follow the evolution of the abundances of silicon, CH3OH and H2O ejected from
grains. Results: The evolution of these abundances shows that CO seems to be
the most efficient sputtering agent in low velocity shocks. The velocity
threshold for the sputtering of silicon from the grain mantles is reduced by
5-10 km s-1 by CO compared to other models. The sputtering by CO can generate
SiO abundances of 10E-11 at the early stages of low velocity shocks, consistent
with those observed in the magnetic precursor of L1448-mm. Our model also
satisfactorily reproduce the progressive enhancement of SiO, CH3OH and H2O
observed in this outflow by the coexistence of two shocks with vs=30 and
60kms-1 within the same region.Comment: 12 pages, 7 figures, accepted for publication in A&
Star formation in disk galaxies driven by primordial H_2
We show that gaseous \HI disks of primordial composition irradiated by an
external radiation field can develop a multiphase medium with temperatures
between 10^2 and 10^4 K due to the formation of molecular hydrogen. For a given
\HI column density there is a critical value of the radiation field below which
only the cold \HI phase can exist. Due to a time decreasing quasar background,
the gas starts cooling slowly after recombination until the lowest stable
temperature in the warm phase is reached at a critical redshift .
Below this redshift the formation of molecular hydrogen promotes a rapid
transition towards the cold \HI phase. We find that disks of protogalaxies with
10^{20}\simlt N_{HI}\simlt 10^{21} cm^{-2} are gravitationally stable at
K and can start their star formation history only at z \simlt
z_{cr}\sim 2, after the gas in the central portion of the disk has cooled to
temperatures T\simlt 300 K. Such a delayed starbust phase in galaxies of low
gas surface density and low dynamical mass can disrupt the disks and cause them
to fade away. These objects could contribute significantly to the faint blue
galaxy population.Comment: 16 pages (LaTeX), 2 Figures to be published in Astrophysical Journal
Letter
The Spatial Distribution of Atomic Carbon Emission in the Giant Molecular Cloud NGC 604-2
We have mapped a giant molecular cloud in the giant HII region NGC 604 in M33
in the 492 GHz ^3P_1 -- ^3P_0 transition of neutral atomic carbon using the
James Clerk Maxwell Telescope. We find the distribution of the [CI] emission to
be asymmetric with respect to the CO J=1--0 emission, with the peak of the [CI]
emission offset towards the direction of the center of the HII region. In
addition, the line ratio I_{[CI]}/I_{CO} is highest (~ 0.2) facing the HII
region and lowest (< 0.1) away from it. These asymmetries indicate an edge-on
morphology where the [CI] emission is strongest on the side of the cloud facing
the center of the HII region, and not detected at all on the opposite side This
suggests that the sources of the incident flux creating C from the dissociation
of CO are the massive stars of the HII region. The lowest line ratios are
similar to what is observed in Galactic molecular clouds, while the highest are
similar to starburst galaxies and other regions of intense star formation. The
column density ratio, N(C)/N(H_2) is a few times 10^{-6}, in general agreement
with models of photodissociation regions.Comment: Accepted for publication in ApJ. 8 pages, 5 figures, 3 table
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