12,660 research outputs found
Dynamics of the Young Binary LMC Cluster NGC 1850
In this paper we have examined the age and internal dynamics of the young
binary LMC cluster NGC 1850 using BV CCD images and echelle spectra of 52
supergiants. Isochrone fits to a BV color-magnitude diagram revealed that the
primary cluster has an age of Myr while the secondary member
has Myr. BV surface brightness profiles were constructed out
to R 40 pc, and single-component King-Michie (KM) models were applied. The
total cluster luminosity varied from L = 2.60 - 2.65
L\sol\ and L = 1.25 - 1.35 as the anisotropy radius
varied from infinity to three times the scale radius with the isotropic models
providing the best agreement with the data. Of the 52 stars with echelle
spectra, a subset of 36 were used to study the cluster dynamics. The KM radial
velocity distributions were fitted to these velocities yielding total cluster
masses of 5.4 - 5.9 M\sol\ corresponding to M/L =
0.02 M\sol/L\sol\ or M/L = 0.05 M\sol/L\sol.
A rotational signal in the radial velocities has been detected at the 93\%
confidence level implying a rotation axis at a position angle of 100\deg. A
variety of rotating models were fit to the velocity data assuming cluster
ellipticities of . These models provided slightly better
agreement with the radial velocity data than the KM models and had masses that
were systematically lower by a few percent. The preferred value for the slope
of a power-law IMF is a relatively shallow, x = 0.29 \pmm{+0.3}{-0.8}
assuming the B-band M/L or x = 0.71 \pmm{+0.2}{-0.4} for the V-band.Comment: 41 pages (figures available via anonymous FTP as described below
SSME structural dynamic model development
A mathematical model of the Space Shuttle Main Engine (SSME) as a complete assembly, with detailed emphasis on LOX and High Fuel Turbopumps is developed. The advantages of both complete engine dynamics, and high fidelity modeling are incorporated. Development of this model, some results, and projected applications are discussed
Trains, tails and loops of partially adsorbed semi-flexible filaments
Polymer adsorption is a fundamental problem in statistical mechanics that has
direct relevance to diverse disciplines ranging from biological lubrication to
stability of colloidal suspensions. We combine experiments with computer
simulations to investigate depletion induced adsorption of semi-flexible
polymers onto a hard-wall. Three dimensional filament configurations of
partially adsorbed F-actin polymers are visualized with total internal
reflection fluorescence microscopy. This information is used to determine the
location of the adsorption/desorption transition and extract the statistics of
trains, tails and loops of partially adsorbed filament configurations. In
contrast to long flexible filaments which primarily desorb by the formation of
loops, the desorption of stiff, finite-sized filaments is largely driven by
fluctuating filament tails. Simulations quantitatively reproduce our
experimental data and allow us to extract universal laws that explain scaling
of the adsorption-desorption transition with relevant microscopic parameters.
Our results demonstrate how the adhesion strength, filament stiffness, length,
as well as the configurational space accessible to the desorbed filament can be
used to design the characteristics of filament adsorption and thus engineer
properties of composite biopolymeric materials
Alternative model for the administration and analysis of research-based assessments
Research-based assessments represent a valuable tool for both instructors and
researchers interested in improving undergraduate physics education. However,
the historical model for disseminating and propagating conceptual and
attitudinal assessments developed by the physics education research (PER)
community has not resulted in widespread adoption of these assessments within
the broader community of physics instructors. Within this historical model,
assessment developers create high quality, validated assessments, make them
available for a wide range of instructors to use, and provide minimal (if any)
support to assist with administration or analysis of the results. Here, we
present and discuss an alternative model for assessment dissemination, which is
characterized by centralized data collection and analysis. This model provides
a greater degree of support for both researchers and instructors in order to
more explicitly support adoption of research-based assessments. Specifically,
we describe our experiences developing a centralized, automated system for an
attitudinal assessment we previously created to examine students'
epistemologies and expectations about experimental physics. This system
provides a proof-of-concept that we use to discuss the advantages associated
with centralized administration and data collection for research-based
assessments in PER. We also discuss the challenges that we encountered while
developing, maintaining, and automating this system. Ultimately, we argue that
centralized administration and data collection for standardized assessments is
a viable and potentially advantageous alternative to the default model
characterized by decentralized administration and analysis. Moreover, with the
help of online administration and automation, this model can support the
long-term sustainability of centralized assessment systems.Comment: 7 pages, 1 figure, accepted in Phys. Rev. PE
Beat Cepheid Period Ratios from OPAL Opacities
The discovery of a large number of beat Cepheids in the Large Magellanic
Cloud in the MACHO survey, provides an opportunity to compare the
characteristics of such Cepheids over a range of metallicities. We produced a
large grid of linear nonadiabatic pulsation models using the OPAL opacity
tables and with compositions corresponding to those of the Milky Way, and the
Large and Small Magellanic Clouds. Using the relationship between the period
ratio and the main pulsation period, we are able to define a range of models
which correspond to the observed beat Cepheids, and thereby constrain the
physical characteristics of the LMC beat Cepheids. We are also able to make
some predictions about the nature of the yet-to-be-discovered SMC beat
Cepheids.Comment: 20 pages, 6 embedded Postscript figures, uses aasms4.sty. Accepted
for publication in the 1997 September A
The crystal structure of parkinsonite, nominally Pb7MoO9Cl2: a naturally occurring Aurivillius phase
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Large-effect flowering time mutations reveal conditionally adaptive paths through fitness landscapes in Arabidopsis thaliana.
Contrary to previous assumptions that most mutations are deleterious, there is increasing evidence for persistence of large-effect mutations in natural populations. A possible explanation for these observations is that mutant phenotypes and fitness may depend upon the specific environmental conditions to which a mutant is exposed. Here, we tested this hypothesis by growing large-effect flowering time mutants of Arabidopsis thaliana in multiple field sites and seasons to quantify their fitness effects in realistic natural conditions. By constructing environment-specific fitness landscapes based on flowering time and branching architecture, we observed that a subset of mutations increased fitness, but only in specific environments. These mutations increased fitness via different paths: through shifting flowering time, branching, or both. Branching was under stronger selection, but flowering time was more genetically variable, pointing to the importance of indirect selection on mutations through their pleiotropic effects on multiple phenotypes. Finally, mutations in hub genes with greater connectedness in their regulatory networks had greater effects on both phenotypes and fitness. Together, these findings indicate that large-effect mutations may persist in populations because they influence traits that are adaptive only under specific environmental conditions. Understanding their evolutionary dynamics therefore requires measuring their effects in multiple natural environments
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