824 research outputs found
Time maintenance of user clocks via the tracking and data relay satellite system
A system is described which uses the Tracking and Data Relay Satellite System (TDRSS) itself to compare the user satellite clock with a clock at the White Sands station that is referenced to Universal Time Coordinated (UTC). No command of the spacecraft by the system is required, and actual on-board clock corrections are made by the spacecraft control center at its discretion. Computer models were constructed using basic orbital parameters for user and TDRS satellites. With only first-order corrections and simple averaging techniques for constant clock rates, error measurement precision of better than one microsecond was obtained. More sophisticated computations should allow considerable improvement over this
Preparing Students to be Effective Information Technology Project Managers: Can More Focus on Dialogue and Communication Processes Help?
The overall importance of dialogue and communication processes to scientific and technical problem solving is already well established in communication and project management literature (e.g., Bohm, 1999; Isaacs, 1999; Schwalbe, 1999; Kerzner, 1998). However, very little of this literature has been specifically applied to the fast-growing field of Information Technology/Information Systems (IT/IS) project management and to the pedagogical issues surrounding the teaching of students how to be more effective in working together as a team to solve IT/IS systems development problems. Many of the articles and books that address the subject have focused on some of the more obvious needs for better listening skills, more effective written and oral communication skills, techniques for helping to get your point across, etc. These are important but there is also a need for a more in depth look at the problem by applying some of the more ideological concepts related to “dialogue” introduced by Bohm and Isaacs
Massive star formation via high accretion rates and early disk-driven outflows
We present an investigation of massive star formation that results from the
gravitational collapse of massive, magnetized molecular cloud cores. We
investigate this by means of highly resolved, numerical simulations of initial
magnetized Bonnor-Ebert-Spheres that undergo collapse and cooling. By comparing
three different cases - an isothermal collapse, a collapse with radiative
cooling, and a magnetized collapse - we show that massive stars assemble
quickly with mass accretion rates exceeding 10^-3 Msol/yr. We confirm that the
mass accretion during the collapsing phase is much more efficient than
predicted by selfsimilar collapse solutions, i.e. dM/dt ~ c^3/G. We find that
during protostellar assembly the mass accretion reaches 20 - 100 c^3/G.
Furthermore, we determined the self-consistent structure of bipolar outflows
that are produced in our three dimensional magnetized collapse simulations.
These outflows produce cavities out of which radiation pressure can be
released, thereby reducing the limitations on the final mass of massive stars
formed by gravitational collapse. Moreover, we argue that the extraction of
angular momentum by disk-threaded magnetic fields and/or by the appearance of
bars with spiral arms significantly enhance the mass accretion rate, thereby
helping the massive protostar to assemble more quickly.Comment: 22 pages, 12 figures, aastex style, accepted for publication in ApJ,
see http://www.ita.uni-heidelberg.de/~banerjee/publications/MassiveStars.pdf
for high resolution figure
Long-term monitoring of Scripps’s Murrelet and Guadalupe Murrelet at San Clemente Island, California: evaluation of baseline data in 2012–2016
San Clemente Island (SCI) supports one of the smallest Scripps’s Murrelet (Synthliboramphus scrippsi; SCMU) colonies in the world, and perhaps the only colony of Guadalupe Murrelets (S. hypoleucus; GUMU) in California. In 2012–2016, the U.S. Navy sponsored development of a long-term murrelet monitoring program at SCI that utilized nocturnal spotlight surveys, night-lighting at-sea captures, and nest monitoring. Standardized spotlight survey transects were established in nearshore waters off breeding areas at Seal Cove and southeast SCI (SESCI). Baseline mean spotlight counts were 29 ± 15 murrelets (n = 31) at Seal Cove in 2013–2016 and 21 ± 10 murrelets (n = 15) at SESCI in 2014–2016. We banded 201 SCMU captured in congregations at Seal Cove (n = 158) and SESCI (n = 43); 12% of the SCMU from Seal Cove and 7% from SESCI were recaptured ≥1 year after banding. We also banded 21 GUMU at Seal Cove, but none were recaptured. Murrelet nests or eggs were found in 6 shoreline breeding “refuges” at Seal Cove and SESCI that were seldom if ever visited by island foxes (Urocyon littoralis clementae) and feral cats (Felis catus). Incubating SCMU were observed in 4 nest sites, but in 8 other sites only eggs or eggshells were found. Overall hatching success was very low (12%; n = 17 clutches) in 2012–2016, apparently due to intraspecific competition for limited nest crevices at Seal Cove and predation (or possibly abandonment and subsequent egg scavenging) by foxes or black rats (Rattus rattus) at SESCI. Using spotlight survey data, we estimated 115 murrelet pairs (range 79–208) at SCI, including 110 pairs (range 76–199) of SCMU and 5 pairs (range 3–9) of GUMU, although a GUMU nest has not yet been found. Power analyses of Seal Cove spotlight data indicated that surveys conducted over 9 nights per year for 20 years could reliably (power ≥ 0.90) detect minimum population changes of ± 1.7% per annum. Additional efforts are needed to (1) confirm the breeding status of GUMU; (2) investigate alternative methods of rat control to increase hatching success in murrelet breeding refuges; and (3) enhance breeding habitats to reduce intraspecific competition for nest sites and increase the number of monitored nests
Genetic structure of Tribolium castaneum populations in mills
We investigated the genetic diversity and differentiation among nine populations of Tribolium castaneum using eight polymorphic loci, including microsatellites and other insertion-deletion polymorphisms (=”indels”). Samples were collected in food processing/storage facilities located in Kansas, Nebraska, California, Louisiana, Florida and Puerto Rico. Standard population genetic analysis was applied, and an assignment test was used to assign individuals to their genetic population. All loci were polymorphic across populations, with the number of alleles per locus-population combination varying from three to fourteen. Among 72 locus-by-population combinations, 31 deviated significantly from Hardy-Weinberg equilibrium, which was associated with a deficiency in heterozygosity. Tribolium castaneum populations show some level of genetic structuring. Genetic differentiation between populations, using FST estimates, was significant, with FST varying from 0.018 to 0.149. AMOVA indicated that 8.32% of the variation in allele frequency resulted from comparisons among populations. Genetic distance was not significantly correlated with geographic distance. Correct assignment to the genetic population was possible in only 56% of all individuals. Together, these results revealed that geographically distinct populations of T. castaneum had low to moderate levels of genetic differentiation that was not correlated with geographic distance, and the genotypic profile of the individuals did not provide enough information for fingerprinting them with their source population. Keywords: Tribolium castaneum, Population genetics, Genetic structure, FST, Genetic fingerprintin
Global Nonradial Instabilities of Dynamically Collapsing Gas Spheres
Self-similar solutions provide good descriptions for the gravitational
collapse of spherical clouds or stars when the gas obeys a polytropic equation
of state, (with ). We study the behaviors of
nonradial perturbations in the similarity solutions of Larson, Penston and
Yahil, which describe the evolution of the collapsing cloud prior to core
formation. Our global stability analysis reveals the existence of unstable
bar-modes () when . In particular, for the collapse of
isothermal spheres, which applies to the early stages of star formation, the
density perturbation relative to the background, , increases as ,
where denotes the epoch of core formation, and is the cloud
central density. Thus, the isothermal cloud tends to evolve into an ellipsoidal
shape (prolate bar or oblate disk, depending on initial conditions) as the
collapse proceeds. In the context of Type II supernovae, core collapse is
described by the equation of state, and our analysis
indicates that there is no growing mode (with density perturbation) in the
collapsing core before the proto-neutron star forms, although nonradial
perturbations can grow during the subsequent accretion of the outer core and
envelope onto the neutron star. We also carry out a global stability analysis
for the self-similar expansion-wave solution found by Shu, which describes the
post-collapse accretion (``inside-out'' collapse) of isothermal gas onto a
protostar. We show that this solution is unstable to perturbations of all
's, although the growth rates are unknown.Comment: 28 pages including 7 ps figures; Minor changes in the discussion; To
be published in ApJ (V.540, Sept.10, 2000 issue
Naturally occurring Ehrlichia chaffeensis infection in coyotes from Oklahoma.
A nested polymerase chain reaction assay was used to determine the presence of Ehrlichia chaffeensis, E. canis, and E. ewingii DNA in blood samples of free-ranging coyotes from central and northcentral Oklahoma. Of the 21 coyotes examined, 15 (71%) were positive for E. chaffeensis DNA; none was positive for E. canis or E. ewingii. Results suggest that E. chaffeensis infections are common in free-ranging coyotes in Oklahoma and that these wild canids could play a role in the epidemiology of human monocytotropic ehrlichiosis
The structure of protostellar envelopes derived from submillimeter continuum images
High dynamic range imaging of submillimeter dust emission from the envelopes
of eight young protostars in the Taurus and Perseus star-forming regions has
been carried out using the SCUBA submillimeter camera on the James Clerk
Maxwell Telescope. Good correspondence between the spectral classifications of
the protostars and the spatial distributions of their dust emission is
observed, in the sense that those with cooler spectral energy distributions
also have a larger fraction of the submillimeter flux originating in an
extended envelope compared with a disk. This results from the cool sources
having more massive envelopes rather than warm sources having larger disks.
Azimuthally-averaged radial profiles of the dust emission are used to derive
the power-law index of the envelope density distributions, p (defined by rho
proportional to r^-p), and most of the sources are found to have values of p
consistent with those predicted by models of cloud collapse. However, the
youngest protostars in our sample, L1527 and HH211-mm, deviate significantly
from the theoretical predictions, exhibiting values of p somewhat lower than
can be accounted for by existing models. For L1527 heating of the envelope by
shocks where the outflow impinges on the surrounding medium may explain our
result. For HH211-mm another explanation is needed, and one possibility is that
a shallow density profile is being maintained in the outer envelope by magnetic
fields and/or turbulence. If this is the case star formation must be determined
by the rate at which the support is lost from the cloud, rather than the
hydrodynamical properties of the envelope, such as the sound speed.Comment: Accepted for publication in the Astrophysical Journa
High-Mass Proto-Stellar Candidates - II : Density structure from dust continuum and CS emission
We present a detailed 1.2 mm continuum and CS spectral line study of a large
sample of 69 massive star forming regions in very early stages of evolution,
most of them prior to building up an ultracompact HII region. The continuum
data show a zoo of different morphologies and give detailed information on the
spatial distributions, the masses, column densities and average densities of
the whole sample. Fitting the radial intensity profiles shows that three
parameters are needed to describe the spatial distribution of the sources:
constant emission from the center out to a few arcsec radius followed by a
first power law intensity distribution which steepens further outside into a
second power law distribution. The mean inner power law intensity index mi
(I~r^(-mi)) is 1.2 corresponding to density indices p (n~r^(-p)) of 1.6. In
total the density distribution of our massive star formations sites seem to be
not too different from their low-mass counterparts, but we show that setting
tight constrains on the density indices is very difficult and subject to many
possible errors. The local densities we derive from CS calculations are higher
(up to one order of magnitude) than the mean densities we find via the
mm-continuum. Such inhomogeneous density distribution reflects most likely the
ubiquitous phenomenon of clumping and fragmentation in molecular clouds.
Linewidth-mass relations show a departure from virial equilibrium in the stages
of strongly collapsing cores.Comment: 15 pages, 13 jpeg-figures. Astrophysical Journal, in pres
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