18,959 research outputs found
The automated multi-stage substructuring system for NASTRAN
The substructuring capability developed for eventual installation in Level 16 is now operational in a test version of NASTRAN. Its features are summarized. These include the user-oriented, Case Control type control language, the automated multi-stage matrix processing, the independent direct access data storage facilities, and the static and normal modes solution capabilities. A complete problem analysis sequence is presented with card-by-card description of the user input
The nature of turbulence in OMC1 at the star forming scale: observations and simulations
Aim: To study turbulence in the Orion Molecular Cloud (OMC1) by comparing
observed and simulated characteristics of the gas motions.
Method: Using a dataset of vibrationally excited H2 emission in OMC1
containing radial velocity and brightness which covers scales from 70AU to
30000AU, we present the transversal structure functions and the scaling of the
structure functions with their order. These are compared with the predictions
of two-dimensional projections of simulations of supersonic hydrodynamic
turbulence.
Results: The structure functions of OMC1 are not well represented by power
laws, but show clear deviations below 2000AU. However, using the technique of
extended self-similarity, power laws are recovered at scales down to 160AU. The
scaling of the higher order structure functions with order deviates from the
standard scaling for supersonic turbulence. This is explained as a selection
effect of preferentially observing the shocked part of the gas and the scaling
can be reproduced using line-of-sight integrated velocity data from subsets of
supersonic turbulence simulations. These subsets select regions of strong flow
convergence and high density associated with shock structure. Deviations of the
structure functions in OMC1 from power laws cannot however be reproduced in
simulations and remains an outstanding issue.Comment: 12 pages, 8 figures, accepted A&A. Revised in response to referee.
For higher resolution, see http://www.astro.phys.au.dk/~maikeng/sim_paper
Observations of spatial and velocity structure in the Orion Molecular Cloud
Observations are reported of H2 IR emission in the S(1) v=1-0 line at 2.121
microns in the Orion Molecular Cloud, OMC1, using the GriF instrument on the
Canada-France-Hawaii Telescope. GriF is a combination of adaptive optics and
Fabry-Perot interferometry, yielding a spatial resolution of 0.15" to 0.18" and
a velocity discrimination as high as 1 km/s. Thanks to the high spatial and
velocity resolution of the GriF data, 193 bright H2 emission regions can be
identified in OMC1. The general characteristics of these features are described
in terms of radial velocities, brightness and spatial displacement of maxima of
velocity and brightness, the latter to yield the orientation of flows in the
plane of the sky. Strong spatial correlation between velocity and bright H2
emission is found and serves to identify many features as shocks. Important
results are: (i) velocities of the excited gas illustrate the presence of a
zone to the south of BN-IRc2 and Peak 1, and the west of Peak 2, where there is
a powerful blue-shifted outflow with an average velocity of -18 km/s. This is
shown to be the NIR counterpart of an outflow identified in the radio from
source I, a very young O-star. (ii) There is a band of weak velocity features
(<5 km/s) in Peak 1 which may share a common origin through an explosive event,
in the BN-IRc2 region, with the fast-moving fingers (or bullets) to the NW of
OMC1. (iii) A proportion of the flows are likely to represent sites of low mass
star formation and several regions show multiple outflows, probably indicative
of multiple star formation within OMC1. The high spatial and velocity
resolution of the GriF data show these and other features in more detail than
has previously been possible.Comment: 27 pages, 19 figures, submitted to A&A Version 2: Several additions,
including a section on protostellar candidates in OMC1, have been made based
on the referee's suggestions v3: corrected typograph
Individual Control of Risk: Seat Belt Use, Subjective Norms and the Theory of Reasoned Action
When faced with a risk for which an inexpensive solution is available, individuals often choose the risk rather than the solution. Protection from certain kinds of risks, e.g., using seat belts or condoms or insulating against radon, is largely under personal control, but individuals often choose not to comply with behaviors which would reduce the risk. The Theory of Reasoned Action (TRA) has been used to predict when individuals will comply. The authors attempted to validate aspects of the TRA by the use of scenarios. Factor analysis of their data supports the theory that intention is a major determinate of behavior but fails to establish the influence of scenarios on subjects\u27 intention to wear seat belts
Enhancement of prompt photons in ultrarelativistic proton-proton collisions from nonlinear gluon evolution at small-
In this paper we estimate the influence of nonlinear gluon evolution in the
production of prompt photons at the LHC pp collider. We assume the validity of
collinear factorization and consider the EHKQS parton distributions, which are
solutions of the GLR-MQ evolution equations and describe quite well the DESY
HERA data, as input in our calculations. We find that both single and
double photon production are enhanced for low- photons and central
rapidities, while this effect is absent for the high- photons. The
implications of this effect for the Quark-Gluon Plasma searches and for the QCD
background to Higgs are also discussed.Comment: 4 pages, 4 figures. Version to be published in Physical Review
Modified Fragmentation Function from Quark Recombination
Within the framework of the constituent quark model, it is shown that the
single hadron fragmentation function of a parton can be expressed as a
convolution of shower diquark or triquark distribution function and quark
recombination probability, if the interference between amplitudes of quark
recombination with different momenta is neglected. The recombination
probability is determined by the hadron's wavefunction in the constituent quark
model. The shower diquark or triquark distribution functions of a fragmenting
jet are defined in terms of overlapping matrices of constituent quarks and
parton field operators. They are similar in form to dihadron or trihadron
fragmentation functions in terms of parton operator and hadron states.
Extending the formalism to the field theory at finite temperature, we
automatically derive contributions to the effective single hadron fragmentation
function from the recombination of shower and thermal constituent quarks. Such
contributions involve single or diquark distribution functions which in turn
can be related to diquark or triquark distribution functions via sum rules. We
also derive QCD evolution equations for quark distribution functions that in
turn determine the evolution of the effective jet fragmentation functions in a
thermal medium.Comment: 23 pages in RevTex with 8 postscript figure
Near-Infrared Spectroscopy of Molecular Hydrogen Emission in Four Reflection Nebulae: NGC 1333, NGC 2023, NGC 2068, and NGC 7023
We present near-infrared spectroscopy of fluorescent molecular hydrogen (H_2)
emission from NGC 1333, NGC 2023, NGC 2068, and NGC 7023 and derive the
physical properties of the molecular material in these reflection nebulae. Our
observations of NGC 2023 and NGC 7023 and the physical parameters we derive for
these nebulae are in good agreement with previous studies. Both NGC 1333 and
NGC 2068 have no previously-published analysis of near-infrared spectra. Our
study reveals that the rotational-vibrational states of molecular hydrogen in
NGC 1333 are populated quite differently from NGC 2023 and NGC 7023. We
determine that the relatively weak UV field illuminating NGC 1333 is the
primary cause of the difference. Further, we find that the density of the
emitting material in NGC 1333 is of much lower density, with n ~ 10^2 - 10^4
cm^-3. NGC 2068 has molecular hydrogen line ratios more similar to those of NGC
7023 and NGC 2023. Our model fits to this nebula show that the bright,
H_2-emitting material may have a density as high as n ~ 10^5 cm^-3, similar to
what we find for NGC 2023 and NGC 7023. Our spectra of NGC 2023 and NGC 7023
show significant changes in both the near-infrared continuum and H_2 intensity
along the slit and offsets between the peaks of the H_2 and continuum emission.
We find that these brightness changes may correspond to real changes in the
density and temperatures of the emitting region, although uncertainties in the
total column of emitting material along a given line of sight complicates the
interpretation. The spatial difference in the peak of the H_2 and near-infrared
continuum peaks in NGC 2023 and NGC 7023 shows that the near-infrared continuum
is due to a material which can survive closer to the star than H_2 can.Comment: Submitted for publication in ApJ. 34 pages including 12 embedded
postscript figures. Also available at
http://www.astronomy.ohio-state.edu/~martini/pub
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