18,928 research outputs found
Identifying biotic determinants of historic American eel (Anguilla rostrata) distributions
Traditionally, ecologists studying large scale patterns in species distributions emphasize abiotic variables over biotic interactions. Noting that both abiotic & biotic variables likely determine distributions of all organisms, many ecologists now aim for a more comprehensive view of species distributions, inclusive of both abiotic and biotic components (Soberón 2007)
Formulating a State Approach to Professional Development
When viewed from the perspective of an entire state\u27s needs, the challenges of designing professional development to meet the requirements of the federal No Child Left Behind legislation of 2001 are daunting. In Oklahoma, the concerns about delivering to rural and urban populations which contain a variety of underserved populations are further complicated by the differences in the way science and mathematics are structured as disciplines. We describe two model programs, one in science and one in mathematics, which take much different approaches. However, the programs have three common elements that make them highly successful. Each program engages teachers strongly, seeks to change learning by altering both teachers\u27 behavior and content knowledge, and is continuously reflective
Predictions for The Very Early Afterglow and The Optical Flash
According to the internal-external shocks model for -ray bursts
(GRBs), the GRB is produced by internal shocks within a relativistic flow while
the afterglow is produced by external shocks with the ISM. We explore the early
afterglow emission. For short GRBs the peak of the afterglow will be delayed,
typically, by few dozens of seconds after the burst. For long GRBs the early
afterglow emission will overlap the GRB signal. We calculate the expected
spectrum and the light curves of the early afterglow in the optical, X-ray and
-ray bands. These characteristics provide a way to discriminate
between late internal shocks emission (part of the GRB) and the early afterglow
signal. If such a delayed emission, with the characteristics of the early
afterglow, will be detected it can be used both to prove the internal shock
scenario as producing the GRB, as well as to measure the initial Lorentz factor
of the relativistic flow. The reverse shock, at its peak, contains energy which
is comparable to that of the GRB itself, but has a much lower temperature than
that of the forward shock so it radiates at considerably lower frequencies. The
reverse shock dominates the early optical emission, and an optical flash
brighter than 15th magnitude, is expected together with the forward shock peak
at x-rays or -rays. If this optical flash is not observed, strong
limitations can be put on the baryonic contents of the relativistic shell
deriving the GRBs, leading to a magnetically dominated energy density.Comment: 23 pages including 4 figure
Launch vehicle effluent measurements during the August 20, 1977, Titan 3 launch at Air Force Eastern Test Range
Airborne effluent measurements within the launch cloud and visible and infrared measurements of cloud physical behavior are discussed. Airborne effluent measurements include concentrations of HCl, Cl2, NO, NOX, and particulates as a function of time during each sampling pass through the exhaust cloud. The particle size distribution was measured for each pass through the cloud. Mass concentration as a function of particle diameter was measured over the size range of 0.05- to 25 micron diameter, and particle number density was measured as a function of diameter over a size range of 0.5 to 7.5 micron. Effluent concentrations in the cloud ranged from about 30 ppm several minutes after launch to about 1 to 2 ppm at 100 minutes. Maximum Cl2 concentrations were about 40 to 55 ppb and by 20 minutes were less than 1.0 ppb. A tabulated listing of the airborne data is given in the appendix. Usable cloud imaging data were limited to the first 16 minutes after launch
Preliminary results of fisheries investigation associated with Skylab-3
The author has identified the following significant results. This investigation is to establish the feasibility of utilizing remotely sensed data acquired from aircraft and satellite platforms to provide information concerning the distribution and abundance of oceanic gamefish. Data from the test area in the northeastern Gulf of Mexico has made possible the identification of fisheries significant environmental parameters for white marlin. Predictive models based on catch data and surface truth information have been developed and have demonstrated potential for reducing search significantly by identifying areas which have a high probability of being productive. Three of the parameters utilized by the model, chlorophyll-a, sea surface temperature, and turbidity have been inferred from aircraft sensor data. Cloud cover and delayed receipt have inhibited the use of Skylab data. The first step toward establishing the feasibility of utilizing remotely sensed data to assess amd monitor the distribution of ocean gamefish has been taken with the successful identification of fisheries significant oceanographic parameters and the demonstration of the capability of measuring most of these parameters remotely
Tidally Triggered Star Formation in Close Pairs of Galaxies: Major and Minor Interactions
We study star formation in a sample of 345 galaxies in 167 pairs and compact
groups drawn from the original CfA2 Redshift Survey and from a follow-up search
for companions. We construct our sample with attention to including pairs with
luminosity contrast |\Delta m_R| >= 2. These 57 galaxies with |\Delta m_R| >= 2
provide a set of nearby representative cases of minor interactions, a central
feature of the hierarchical galaxy formation model. Here we report the
redshifts and positions of the 345 galaxies in our sample, and of 136 galaxies
in apparent pairs that are superpositions. In the pairs sample as a whole,
there are strong correlations between the equivalent width of the H\alpha
emission line and the projected spatial and the line-of-sight velocity
separation of the pair. For pairs of small luminosity contrast, |\Delta m_R| <
2, the member galaxies show a correlation between the equivalent width of
H\alpha and the projected spatial separation of the pair. However, for pairs
with large luminosity contrast, |\Delta m_R| >= 2, we detect no correlation
between the equivalent width of H\alpha and the projected spatial separation.
The relative luminosity of the companion galaxy is more important in a
gravitational tidal interaction than the intrinsic luminosity of the galaxy.
Central star formation across the entire pairs sample depends strongly on the
luminosity ratio, |\Delta m_R|, a reasonable proxy for the mass ratio of the
pair; pairs composed of similarly luminous galaxies produce the strongest
bursts of star formation. Pairs with |\Delta m_R| >= 2 rarely have EW(H\alpha)
>~ 70 Ang.Comment: Minor revisions following journal proof
On the Maximum Mass of Accreting Primordial Supermassive Stars
Supermassive primordial stars are suspected to be the progenitors of the most
massive quasars at z~6. Previous studies of such stars were either unable to
resolve hydrodynamical timescales or considered stars in isolation, not in the
extreme accretion flows in which they actually form. Therefore, they could not
self-consistently predict their final masses at collapse, or those of the
resulting supermassive black hole seeds, but rather invoked comparison to
simple polytropic models. Here, we systematically examine the birth, evolution
and collapse of accreting non-rotating supermassive stars under accretion rates
of 0.01-10 solar masses per year, using the stellar evolution code KEPLER. Our
approach includes post-Newtonian corrections to the stellar structure and an
adaptive nuclear network, and can transition to following the hydrodynamic
evolution of supermassive stars after they encounter the general relativistic
instability. We find that this instability triggers the collapse of the star at
masses of 150,000-330,000 solar masses for accretion rates of 0.1-10 solar
masses per year, and that the final mass of the star scales roughly
logarithmically with the rate. The structure of the star, and thus its
stability against collapse, is sensitive to the treatment of convection, and
the heat content of the outer accreted envelope. Comparison with other codes
suggests differences here may lead to small deviations in the evolutionary
state of the star as a function of time, that worsen with accretion rate. Since
the general relativistic instability leads to the immediate death of these
stars, our models place an upper limit on the masses of the first quasars at
birth.Comment: 5 pages, 4 figures. Accepted ApJ letter
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