3,205 research outputs found
Radiation-Driven Warping: The Origin of Warps and Precession in Accretion Disks
A geometrically thin, optically thick, warped accretion disk with a central
source of luminosity is subject to non-axisymmetric forces due to radiation
pressure; the resulting torque acts to modify the warp. In a recent paper,
\cite{pri96} used a local analysis to show that initially planar accretion
disks are unstable to warping driven by radiation torque. Here we extend this
work with a global analysis of the stable and unstable modes. We confirm
Pringle's conclusion that thin centrally-illuminated accretion disks are
generically unstable to warping via this mechanism; we discuss the
time-evolution and likely steady-state of such systems and show specifically
that this mechanism can explain the warping of the disk of water masers in NGC
4258 and the 164-day precession period of the accretion disk in SS 433.
Radiation-driven warping and precession provides a robust mechanism for
producing warped, precessing accretion disks in active galactic nuclei and
X-ray binary systems.Comment: 16 pages, latex, 3 figure
Charter School Funding: Inequity Expands
The revenue study is based on Fiscal Year 2010â11 (FY11) data for each of 30 selected states plus the District of Columbia (D.C.). Traditional school districts and public charter schools were analyzed and aggregated âstatewide.â For each state, one to three âfocus areasâ were selected based on larger concentrations of charter students â most focus areas are large cities, some are metropolitan counties. Traditional school districts and charter schools were analyzed separately in each focus area. The analytic team collected and analyzed all revenues, public and private, flowing to traditional district and public charter schools. FY11 funding includes Federal, State, Local, Other, PublicIndeterminate, and Indeterminate sources
The Productivity of Public Charter Schools
This is the first national study of the productivity of public charter schools relative to district schools. This report is a follow up to the charter school revenue study, Charter School Funding: Inequity Expands, released in April 2014 by the School Choice Demonstration Project at the University of Arkansas. That study was authored by the same research team that crafted this report. In the revenue study, per pupil revenues for public charter schools and traditional public schools (TPS) were compared. The research team found that during the 2010-11 school year (FY11), charter-school students across 30 states and the District of Columbia on average received $3,814 less in funding than TPS students, a funding gap of 28.4 percent
The Bolocam 1.1 mm Lockman Hole Galaxy Survey: SHARC II 350 micron Photometry and Implications for Spectral Models, Dust Temperatures, and Redshift Estimation
We present 350 micron photometry of all 17 galaxy candidates in the Lockman
Hole detected in a 1.1 mm Bolocam survey. Several of the galaxies were
previously detected at 850 microns, at 1.2 mm, in the infrared by Spitzer, and
in the radio. Nine of the Bolocam galaxy candidates were detected at 350
microns and two new candidates were serendipitously detected at 350 microns
(bringing the total in the literature detected in this way to three). Five of
the galaxies have published spectroscopic redshifts, enabling investigation of
the implied temperature ranges and a comparison of photometric redshift
techniques.
Lambda = 350 microns lies near the spectral energy distribution peak for z =
2.5 thermally emitting galaxies. Thus, luminosities can be measured without
extrapolating to the peak from detection wavelengths of lambda > 850 microns.
Characteristically, the galaxy luminosities lie in the range 1.0 - 1.2 x 10^13
L_solar, with dust temperatures in the range of 40 K to 70 K, depending on the
choice of spectral index and wavelength of unit optical depth. The implied dust
masses are 3 - 5 x 10^8 M_solar. We find that the far-infrared to radio
relation for star-forming ULIRGs systematically overpredicts the radio
luminosities and overestimates redshifts on the order of Delta z ~ 1, whereas
redshifts based on either on submillimeter data alone or the 1.6 micron stellar
bump and PAH features are more accurate.Comment: In Press (to appear in Astrophysical Journal, ApJ 20 May 2006 v643 1)
47 pages, 10 figures, 4 table
Mapping Children's Discussions of Evidence in Science to Assess Collaboration and Argumentation
The research reported in this paper concerns the development of children's skills of interpreting and evaluating evidence in science. Previous studies have shown that school teaching often places limited emphasis on the development of these skills, which are necessary for children to engage in scientific debate and decision-making. The research, undertaken in the UK, involved four collaborative decision-making activities to stimulate group discussion, each was carried out with five groups of four children (10-11 years old). The research shows how the children evaluated evidence for possible choices and judged whether their evidence was sufficient to support a particular conclusion or the rejection of alternative conclusions. A mapping technique was developed to analyse the discussions and identify different "levels" of argumentation. The authors conclude that suitable collaborative activities that focus on the discussion of evidence can be developed to exercise children's ability to argue effectively in making decisions
Ionizing Photons and EUV Excesses in Clusters of Galaxies
Observations with the Extreme Ultraviolet Explorer satellite are purported to
show extreme ultraviolet (EUV) and soft X-ray excesses in several clusters of
galaxies (Bonamente, Lieu & Mittaz 2001). If interpreted as thermal emission,
this would imply the presence of warm (T\sim 10^6 K) gas in these clusters with
a mass comparable to that of gas at coronal temperatures. If true, this would
have profound implications for our understanding of galaxy clusters and the
distribution of baryons in the universe. Here we show that because of the large
ionizing photon emissivities of gas at such low temperatures, the ionizing
photon fluxes seen by disk galaxies in the observed clusters can be very large,
resulting in minimum emission measures from neutral gas in such disks as high
as 100 cm^(-6) pc. This result is essentially independent of the mechanism
actually responsible for producing the alleged EUV excesses. The predicted
emission measures in Abell 1795 (z=0.063) are about an order of magnitude
larger than seen in the Reynolds layer of the Galaxy, providing a
straightforward observational test of the reality of the EUV excess. New
tunable filter H alpha images and WFPC images from the Hubble Space Telescope
archive do not support the existence of the claimed EUV excess.Comment: To appear in ApJ Letters; 11 pages, 3 figure
Observations of the Extended Distribution of Ionized Hydrogen in the Plane of M31
We have used the Wisconsin H-Alpha Mapper (WHAM) to observe the spatially
extended distribution of ionized hydrogen in M31 beyond the stellar disk. We
obtained five sets of observations, centered near the photometric major axis of
M31, that extend from the center of the galaxy to just off the edge of the
southwestern HI disk. Beyond the bright stellar disk, but within the HI disk,
weak H-alpha is detected with an intensity I(H-alpha) = 0.05 (+0.01 / -0.02)
Rayleighs. Since M31 is inclined 77 degrees with respect to the line of sight,
this implies that the ambient intergalactic ionizing flux onto each side of M31
is Phi_0 <= 1.6 x 10^4 photons cm^-2 s^-1. Just beyond the outer boundary of
the HI disk we find no significant detection of H-alpha and place an upper
limit I(H-alpha) <= 0.019 Rayleighs.Comment: To appear in ApJ Letters; 12 pages, 4 figure
Power, Propulsion, and Communications for Microspacecraft Missions
The development of small sized, low weight spacecraft should lead to reduced scientific mission costs by lowering fabrication and launch costs. An order of magnitude reduction in spacecraft size can be obtained by miniaturizing components. Additional reductions in spacecraft weight, size, and cost can be obtained by utilizing the synergy that exists between different spacecraft systems. The state-of-the-art of three major systems, spacecraft power, propulsion, and communications is discussed. Potential strategies to exploit the synergy between these systems and/or the payload are identified. Benefits of several of these synergies are discussed
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