1,530 research outputs found
The Neon Abundance of Galactic Wolf-Rayet Stars
The fast, dense winds which characterize Wolf-Rayet (WR) stars obscure
their underlying cores, and complicate the verification of evolving
core and nucleosynthesis models. Core evolution can be probed by
measuring abundances of wind-borne nuclear processed elements,
partially overcoming this limitation. Using ground-based mid-infrared
spectroscopy and the 12.81um [NeII] emission line measured in
four Galactic WR stars, we estimate neon abundances and compare to
long-standing predictions from evolved-core models. For the WC star
WR121, this abundance is found to be >~11x the cosmic
value, in good agreement with predictions. For the three less-evolved
WN stars, little neon enhancement above cosmic values is measured, as
expected. We discuss the impact of clumping in WR winds on this
measurement, and the promise of using metal abundance ratios to
eliminate sensitivity to wind density and ionization structure.Comment: Accepted for publication in ApJ; 9 pages, 2 color figures, 4 table
Teachers as leaders in a knowledge society: encouraging signs of a new professionalism
[Abstract]: Challenges confronting schools worldwide are greater than ever,and, likewise, many teachers possess capabilities, talents, and formal credentials more sophisticated than ever. However, the responsibility and authority accorded
to teachers have not grown significantly, nor has the image of teaching as a profession advanced significantly. The question becomes, what are the implications for the image and status of the teaching profession as the concept of knowledge society takes a firm hold in the industrialized world? This article addresses the philosophical underpinnings of teacher leadership manifested in case studies where schools sought to achieve the generation of new knowledge as part of a process of whole-school revitalization. Specifically, this article reports on Australian research that has illuminated the work of teacher leaders engaged in the IDEAS project, a joint school revitalization initiative of the University
of Southern Queensland and the Queensland Department of Education and the Arts
Autonomous three-dimensional formation flight for a swarm of unmanned aerial vehicles
This paper investigates the development of a new guidance algorithm for a formation of unmanned aerial vehicles. Using the new approach of bifurcating potential fields, it is shown that a formation of unmanned aerial vehicles can be successfully controlled such that verifiable autonomous patterns are achieved, with a simple parameter switch allowing for transitions between patterns. The key contribution that this paper presents is in the development of a new bounded bifurcating potential field that avoids saturating the vehicle actuators, which is essential for real or safety-critical applications. To demonstrate this, a guidance and control method is developed, based on a six-degreeof-freedom linearized aircraft model, showing that, in simulation, three-dimensional formation flight for a swarm of unmanned aerial vehicles can be achieved
Field-warmed soil carbon changes imply high 21st-century modeling uncertainty
The feedback between planetary warming and soil carbon loss has been the
focus of considerable scientific attention in recent decades, due to its
potential to accelerate anthropogenic climate change. The soil carbon
temperature sensitivity is traditionally estimated from short-term
respiration measurements â either from laboratory incubations that are
artificially manipulated or from field measurements that cannot distinguish
between plant and microbial respiration. To address these limitations of
previous approaches, we developed a new method to estimate soil temperature
sensitivity (Q10) of soil carbon directly from warming-induced changes
in soil carbon stocks measured in 36 field experiments across the world.
Variations in warming magnitude and control organic carbon percentage
explained much of field-warmed organic carbon percentage
(R2âŻâ=ââŻ0.96), revealing Q10 across sites of 2.2 [1.6, 2.7]
95âŻ% confidence interval (CI). When these field-derived Q10 values
were extrapolated over the 21st century using a post hoc correction of 20
Coupled Model Intercomparison Project Phase 5 (CMIP5) Earth system model
outputs, the multi-model mean soil carbon stock changes shifted from the
previous value of 88âŻÂ±âŻ153âŻPg carbon (weighted meanâŻÂ±âŻ1âŻSD) to
19âŻÂ±âŻ155âŻPg carbon with a Q10-driven 95âŻ%âŻCI of
248âŻÂ±âŻ191 to â95âŻÂ±âŻ209âŻPg carbon. On average, incorporating
the field-derived Q10 values into Earth system model simulations led to
reductions in the projected amount of carbon sequestered in the soil over the
21st century. However, the considerable parameter uncertainty led to
extremely high variability in soil carbon stock projections within each
model; intra-model uncertainty driven by the field-derived Q10 was as
great as that between model variation. This study demonstrates that data
integration should capture the variation of the system, as well as mean
trends
Massive stars and the energy balance of the interstellar medium. II. The 35 solar mass star and a solution to the "missing wind problem"
We continue our numerical analysis of the morphological and energetic
influence of massive stars on their ambient interstellar medium for a 35 solar
mass star that evolves from the main sequence through red supergiant and
Wolf-Rayet phases, until it ultimately explodes as a supernova. We find that
structure formation in the circumstellar gas during the early main-sequence
evolution occurs as in the 60 solar mass case but is much less pronounced
because of the lower mechanical wind luminosity of the star. Since on the other
hand the shell-like structure of the HII region is largely preserved, effects
that rely on this symmetry become more important. At the end of the stellar
lifetime 1% of the energy released as Lyman continuum radiation and stellar
wind has been transferred to the circumstellar gas. From this fraction 10% is
kinetic energy of bulk motion, 36% is thermal energy, and the remaining 54% is
ionization energy of hydrogen. The sweeping up of the slow red supergiant wind
by the fast Wolf-Rayet wind produces remarkable morphological structures and
emission signatures, which are compared with existing observations of the
Wolf-Rayet bubble S308. Our model reproduces the correct order of magnitude of
observed X-ray luminosity, the temperature of the emitting plasma as well as
the limb brightening of the intensity profile. This is remarkable, because
current analytical and numerical models of Wolf-Rayet bubbles fail to
consistently explain these features. A key result is that almost the entire
X-ray emission in this stage comes from the shell of red supergiant wind swept
up by the shocked Wolf-Rayet wind rather than from the shocked Wolf-Rayet wind
itself as hitherto assumed and modeled. This offers a possible solution to what
is called the ``missing wind problem'' of Wolf-Rayet bubbles.Comment: 52 pages, 20 figures, 2 tables, accepted for publication in the
Astrophysical Journa
Physical Properties of Wolf-Rayet Stars
The striking broad emission line spectroscopic appearance of Wolf-Rayet (WR)
stars has long defied analysis, due to the extreme physical conditions within
their line and continuum forming regions. Recently, model atmosphere studies
have advanced sufficiently to enable the determination of stellar temperatures,
luminosities, abundances, ionizing fluxes and wind properties. The observed
distributions of nitrogen (WN) and carbon (WC) sequence WR stars in the Milky
Way and in nearby star forming galaxies are discussed; these imply lower limits
to progenitor masses of ~25, 40, 75 Msun for hydrogen-depleted (He-burning) WN,
WC, and H-rich (H-burning) WN stars, respectively. WR stars in massive star
binaries permit studies of wind-wind interactions and dust formation in WC
systems. They also show that WR stars have typical masses of 10-25 Msun,
extending up to 80 Msun for H-rich WN stars. Theoretical and observational
evidence that WR winds depend on metallicity is presented, with implications
for evolutionary models, ionizing fluxes, and the role of WR stars within the
context of core-collapse supernovae and long-duration gamma ray bursts.Comment: 76 pages, 8 figures. Minor revision to "Annual Review of Astronomy &
Astrophysics" review article Volume 45 (2007) following editors comments.
Version with full resolution figures is available from
ftp://astro1.shef.ac.uk/pub/pac/AnnRev_revised.pd
Active flow control systems architectures for civil transport aircraft
Copyright @ 2010 American Institute of Aeronautics and AstronauticsThis paper considers the effect of choice of actuator technology and associated power systems architecture on the mass cost and power consumption of implementing active flow control systems on civil transport aircraft. The research method is based on the use of a mass model that includes a mass due to systems hardware and a mass due to the system energy usage. An Airbus A320 aircraft wing is used as a case-study application. The mass model parameters are based on first-principle physical analysis of electric and pneumatic power systems combined with empirical data on system hardware from existing equipment suppliers. Flow control methods include direct fluidic, electromechanical-fluidic, and electrofluidic actuator technologies. The mass cost of electrical power distribution is shown to be considerably less than that for pneumatic systems; however, this advantage is reduced by the requirement for relatively heavy electrical power management and conversion systems. A tradeoff exists between system power efficiency and the system hardware mass required to achieve this efficiency. For short-duration operation the flow control solution is driven toward lighter but less power-efficient systems, whereas for long-duration operation there is benefit in considering heavier but more efficient systems. It is estimated that a practical electromechanical-fluidic system for flow separation control may have a mass up to 40% of the slat mass for a leading-edge application and 5% of flap mass for a trailing-edge application.This work is funded by the Sixth European Union Framework Programme as part of the AVERT project (Contract No. AST5-CT-2006-030914
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