864 research outputs found
A "Kane's Dynamics" Model for the Active Rack Isolation System
In the late 1980s, microgravity researchers began to voice their concern that umbilical-transmitted energy could significantly degrade the acceleration environment of microgravity space science experiments onboard manned spacecraft. Since umbilicals are necessary for many experiments, control designers began to seek ways to compensate for these "indirect" disturbances. Hampton, et al., used the Kane s method to develop a model of the active rack isolation system (ARIS) that includes (1) actuator control forces, (2) direct disturbance forces, and (3) indirect, actuator-transmitted disturbances. Their model does not, however, include the indirect, umbilical-transmitted disturbances. Since the umbilical stiffnesses are not negligible, these indirect disturbances must be included in the model. Until the umbilicals have been appropriately included, the model will be incomplete. This Technical Memorandum presents a nonlinear model of ARIS with umbilicals included. Model verification was achieved by utilizing two commercial-off-the-shelf software tools. Various forces and moments were applied to the model to yield simulated responses of the system. Plots of the simulation results show how various critical points on an ARIS-outfitted international standard payload rack behave under the application of direct disturbances, indirect disturbances, and control forces. Simulations also show system response to a variety of initial conditions
SNPS/INDELS identification in genomic regions of abc transporters in susceptible and ivermectin-resistant Haemonchus contortus
A Test of the Standard Hypothesis for the Origin of the HI Holes in Holmberg II
The nearby irregular galaxy Holmberg II has been extensively mapped in HI
using the Very Large Array (VLA), revealing intricate structure in its
interstellar gas component (Puche et al. 1992). An analysis of these structures
shows the neutral gas to contain a number of expanding HI holes. The formation
of the HI holes has been attributed to multiple supernova events occurring
within wind-blown shells around young, massive star clusters, with as many as
10-200 supernovae required to produce many of the holes. From the sizes and
expansion velocities of the holes, Puche et al. assigned ages of ~10^7 to 10^8
years. If the supernova scenario for the formation of the HI holes is correct,
it implies the existence of star clusters with a substantial population of
late-B, A and F main sequence stars at the centers of the holes. Many of these
clusters should be detectable in deep ground-based CCD images of the galaxy. In
order to test the supernova hypothesis for the formation of the HI holes, we
have obtained and analyzed deep broad-band BVR and narrow-band H-alpha images
of Ho II. We compare the optical and HI data and search for evidence of the
expected star clusters in and around the HI holes. We also use the HI data to
constrain models of the expected remnant stellar population. We show that in
several of the holes the observed upper limits for the remnant cluster
brightness are strongly inconsistent with the SNe hypothesis described in Puche
et al. Moreover, many of the HI holes are located in regions of very low
optical surface brightness which show no indication of recent star formation.
Here we present our findings and explore possible alternative explanations for
the existence of the HI holes in Ho II, including the suggestion that some of
the holes were produced by Gamma-ray burst events.Comment: 30 pages, including 6 tables and 3 images. To appear in Astron.
Journal (June 1999
A Fractal Analysis of the HI Emission from the Large Magellanic Cloud
A composite map of HI in the LMC using the ATCA interferometer and the Parkes
multibeam telescope was analyzed in several ways in an attempt to characterize
the structure of the neutral gas and to find an origin for it. Fourier
transform power spectra in 1D, 2D, and in the azimuthal direction were found to
be approximate power laws over 2 decades in length. Delta-variance methods also
showed the same power-law structure. Detailed models of these data were made
using line-of-sight integrals over fractals that are analogous to those
generated by simulations of turbulence with and without phase transitions. The
results suggested a way to measure directly for the first time the
line-of-sight thickness of the cool component of the HI disk of a nearly
face-on galaxy. The signature of this thickness was found to be present in all
of the measured power spectra.
The character of the HI structure in the LMC was also viewed by comparing
positive and negative images of the integrated emission. The geometric
structure of the high-emission regions was found to be filamentary, whereas the
geometric structure of the low-emission (intercloud) regions was found to be
patchy and round. This result suggests that compressive events formed the
high-emission regions, and expansion events, whether from explosions or
turbulence, formed the low-emission regions. The character of the structure was
also investigated as a function of scale using unsharp masks.
All of these results suggest that most of the ISM in the LMC is fractal,
presumably the result of pervasive turbulence, self-gravity, and self-similar
stirring.Comment: 30 pages, 21 figures, scheduled for ApJ Vol 548n1, Feb 10, 200
Reduzindo intervenções de rotina durante o trabalho de parto e parto: primeiro, não causar dano [Reducing routine interventions during labor and delivery: First, do no harm.]
Multifractal Scaling, Geometrical Diversity, and Hierarchical Structure in the Cool Interstellar Medium
Multifractal scaling (MFS) refers to structures that can be described as a
collection of interwoven fractal subsets which exhibit power-law spatial
scaling behavior with a range of scaling exponents (concentration, or
singularity, strengths) and dimensions. The existence of MFS implies an
underlying multiplicative (or hierarchical, or cascade) process. Panoramic
column density images of several nearby star- forming cloud complexes,
constructed from IRAS data and justified in an appendix, are shown to exhibit
such multifractal scaling, which we interpret as indirect but quantitative
evidence for nested hierarchical structure. The relation between the dimensions
of the subsets and their concentration strengths (the "multifractal spectrum'')
appears to satisfactorily order the observed regions in terms of the mixture of
geometries present: strong point-like concentrations, line- like filaments or
fronts, and space-filling diffuse structures. This multifractal spectrum is a
global property of the regions studied, and does not rely on any operational
definition of "clouds.'' The range of forms of the multifractal spectrum among
the regions studied implies that the column density structures do not form a
universality class, in contrast to indications for velocity and passive scalar
fields in incompressible turbulence, providing another indication that the
physics of highly compressible interstellar gas dynamics differs fundamentally
from incompressible turbulence. (Abstract truncated)Comment: 27 pages, (LaTeX), 13 figures, 1 table, submitted to Astrophysical
Journa
The initial stellar mass function from random sampling in hierarchical clouds II: statistical fluctuations and a mass dependence for starbirth positions and times
Observed variations in the slope of the initial stellar mass function are
shown to be consistent with a model in which the protostellar gas is randomly
sampled from hierarchical clouds at a rate proportional to the square root of
the local density. RMS variations in the IMF slope around the Salpeter value
are +/- 0.4 when only 100 stars are observed, and +/- 0.1 when 1000 stars are
observed. The hierarchical-sampling model also reproduces the tendency for
massive stars to form closer to the center of a cloud, at a time somewhat later
than the formation time of the lower mass stars. The assumed density dependence
for the star formation rate is shown to be appropriate for turbulence
compression, magnetic diffusion, gravitational collapse, and clump or
wavepacket coalescence. The low mass flattening in the IMF comes from the
inability of gas to form stars below the thermal Jeans mass at typical
temperatures and pressures. Consideration of heating and cooling processes
indicate why the thermal Jeans mass should be nearly constant in normal
environments, and why it might increase in some starburst regions. The steep
IMF in the extreme field is not explained by the model, but other origins are
suggested.Comment: 21 pages, 8 figures, scheduled for ApJ vol. 515, April 10, 199
Structure Function Scaling in the Taurus and Perseus Molecular Cloud Complexes
We compute the structure function scaling of the integrated intensity images
of two J=1-0 13CO maps of Taurus and Perseus. The scaling exponents of the
structure functions follow the velocity scaling of supersonic turbulence,
suggesting that turbulence plays an important role in the fragmentation of cold
interstellar clouds. The data also allows to verify the validity of the two
basic assumptions of the hierarchical symmetry model, originally proposed for
the derivation of the velocity structure function scaling. This shows that the
same hierarchical symmetry holds for the projected density field of cold
interstellar clouds.Comment: submitted to Ap
The spatial distribution of star and cluster formation in M51
Aims. We study the connection between spatially resolved star formation and
young star clusters across the disc of M51. Methods. We combine star cluster
data based on B, V, and I-band Hubble Space Telescope ACS imaging, together
with new WFPC2 U-band photometry to derive ages, masses, and extinctions of
1580 resolved star clusters using SSP models. This data is combined with data
on the spatially resolved star formation rates and gas surface densities, as
well as Halpha and 20cm radio-continuum (RC) emission, which allows us to study
the spatial correlations between star formation and star clusters. Two-point
autocorrelation functions are used to study the clustering of star clusters as
a function of spatial scale and age. Results. We find that the clustering of
star clusters among themselves decreases both with spatial scale and age,
consistent with hierarchical star formation. The slope of the autocorrelation
functions are consistent with projected fractal dimensions in the range of
1.2-1.6, which is similar to other galaxies, therefore suggesting that the
fractal dimension of hierarchical star formation is universal. Both star and
cluster formation peak at a galactocentric radius of 2.5 and 5 kpc, which we
tentatively attribute to the presence of the 4:1 resonance and the co-rotation
radius. The positions of the youngest (<10 Myr) star clusters show the
strongest correlation with the spiral arms, Halpha, and the RC emission, and
these correlations decrease with age. The azimuthal distribution of clusters in
terms of kinematic age away from the spiral arms indicates that the majority of
the clusters formed 5-20 Myr before their parental gas cloud reached the centre
of the spiral arm.Comment: 14 pages, 21 figures, accepted for publication in A&
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