9,270 research outputs found
Art Historical Explanation Of Paintings And The Need For An Aesthetics Of Agency
Why should a person, and in the context of this conference particularly an art historian, take seriously the notion of the aesthetic, its discovery and/or rediscovery? Aesthetics might after all be considered at best something of a distraction from bread and butter historical and sociological analysis, and at worst entirely incompatible with it. Pursuing the line further it might be urged that, since on the one hand aesthetics is about 'how things appear'—i.e. is subject to individual predilection, taste and feeling—and on the other, historical analysis is about the careful and scholarly reconstruction of a past social reality, the two must be at loggerheads. What the art historian writes about on a weekday whilst wearing her hard hat at the office must not be confused with what she personally feels, wandering around a gallery in her woolly hat at the weekend
Survival of habitable planets in unstable planetary systems
Many observed giant planets lie on eccentric orbits. Such orbits could be the
result of strong scatterings with other giant planets. The same dynamical
instability that produces these scatterings may also cause habitable planets in
interior orbits to become ejected, destroyed, or be transported out of the
habitable zone. We say that a habitable planet has resilient habitability if it
is able to avoid ejections and collisions and its orbit remains inside the
habitable zone. Here we model the orbital evolution of rocky planets in
planetary systems where giant planets become dynamically unstable. We measure
the resilience of habitable planets as a function of the observed, present-day
masses and orbits of the giant planets. We find that the survival rate of
habitable planets depends strongly on the giant planet architecture. Equal-mass
planetary systems are far more destructive than systems with giant planets of
unequal masses. We also establish a link with observation; we find that giant
planets with present-day eccentricities higher than 0.4 almost never have a
habitable interior planet. For a giant planet with an present-day eccentricity
of 0.2 and semimajor axis of 5 AU orbiting a Sun-like star, 50% of the orbits
in the habitable zone are resilient to the instability. As semimajor axis
increases and eccentricity decreases, a higher fraction of habitable planets
survive and remain habitable. However, if the habitable planet has rocky
siblings, there is a significant risk of rocky planet collisions that would
sterilize the planet.Comment: Accepted to MNRA
How to form planetesimals from mm-sized chondrules and chondrule aggregates
The size distribution of asteroids and Kuiper belt objects in the solar
system is difficult to reconcile with a bottom-up formation scenario due to the
observed scarcity of objects smaller than 100 km in size. Instead,
planetesimals appear to form top-down, with large km bodies forming
from the rapid gravitational collapse of dense clumps of small solid particles.
In this paper we investigate the conditions under which solid particles can
form dense clumps in a protoplanetary disk. We use a hydrodynamic code to model
the interaction between solid particles and the gas inside a shearing box
inside the disk, considering particle sizes from sub-millimeter-sized
chondrules to meter-sized rocks. We find that particles down to millimeter
sizes can form dense particle clouds through the run-away convergence of radial
drift known as the streaming instability. We make a map of the range of
conditions (strength of turbulence, particle mass-loading, disk mass, and
distance to the star) which are prone to producing dense particle clumps.
Finally, we estimate the distribution of collision speeds between mm-sized
particles. We calculate the rate of sticking collisions and obtain a robust
upper limit on the particle growth timescale of years. This means
that mm-sized chondrule aggregates can grow on a timescale much smaller than
the disk accretion timescale ( years). Our results suggest a
pathway from the mm-sized grains found in primitive meteorites to fully formed
asteroids. We speculate that asteroids may form from a positive feedback loop
in which coagualation leads to particle clumping driven by the streaming
instability. This clumping, in turn reduces collision speeds and enhances
coagulation.} Future simulations should model coagulation and the streaming
instability together to explore this feedback loop further.Comment: 20 pages. Accepted for publication in A&
Toward an initial mass function for giant planets
The distribution of exoplanet masses is not primordial. After the initial
stage of planet formation is complete, the gravitational interactions between
planets can lead to the physical collision of two planets, or the ejection of
one or more planets from the system. When this occurs, the remaining planets
are typically left in more eccentric orbits. Here we use present-day
eccentricities of the observed exoplanet population to reconstruct the initial
mass function of exoplanets before the onset of dynamical instability. We
developed a Bayesian framework that combines data from N-body simulations with
present-day observations to compute a probability distribution for the planets
that were ejected or collided in the past. Integrating across the exoplanet
population, we obtained an estimate of the initial mass function of exoplanets.
We find that the ejected planets are primarily sub-Saturn type planets. While
the present-day distribution appears to be bimodal, with peaks around and , this bimodality does not seem to be
primordial. Instead, planets around appear to be
preferentially removed by dynamical instabilities. Attempts to reproduce
exoplanet populations using population synthesis codes should be mindful of the
fact that the present population has been depleted of intermediate-mass
planets. Future work should explore how the system architecture and
multiplicity might alter our results.Comment: 10 pages, 9 figures; submitted to MNRA
Expanding alliance: ANZUS cooperation and Asia–Pacific security
Is an alliance conceived as a bulwark against a resurgence of Japanese militarism and which cut its military and intelligence teeth in the Cold War is still relevant to today’s strategic concerns?
Overview
The alliance between Australia and the US, underpinned by the formal ANZUS Treaty of 1951, continues to be a central part of Australian defence and security thinking and an instrument of American policy in the Asia–Pacific.
How is it that an alliance conceived as a bulwark against a resurgence of Japanese militarism and which cut its military and intelligence teeth in the Cold War is still relevant to today’s strategic concerns? The answer is partly—and importantly—that the core values of the ANZUS members are strongly aligned, and successive Australian governments and American presidential administrations have seen great value in working with like-minded partners to ensure Asia–Pacific security. Far from becoming a historical curiosity, today it’s not just relevant, but of greater importance than has been the case in the past few decades.
To explore new ideas on how to strengthen the US–Australia alliance, ASPI conducted a high-level strategic dialogue in Honolulu in July this year. Discussions canvassed the future strategic environment; the forthcoming Australian Defence White Paper; budget, sovereignty and expectation risks; and cooperation in the maritime, land, air, cyber, space and intelligence domains.
A key purpose of the Honolulu dialogue was to help ASPI develop policy recommendations on the alliance relationship for government. This report is the product of those discussions
Planetesimal formation by the streaming instability in a photoevaporating disk
Recent years have seen growing interest in the streaming instability as a
candidate mechanism to produce planetesimals. However, these investigations
have been limited to small-scale simulations. We now present the results of a
global protoplanetary disk evolution model that incorporates planetesimal
formation by the streaming instability, along with viscous accretion,
photoevaporation by EUV, FUV, and X-ray photons, dust evolution, the water ice
line, and stratified turbulence. Our simulations produce massive (60-130
) planetesimal belts beyond 100 au and up to of
planetesimals in the middle regions (3-100 au). Our most comprehensive model
forms 8 of planetesimals inside 3 au, where they can give rise to
terrestrial planets. The planetesimal mass formed in the inner disk depends
critically on the timing of the formation of an inner cavity in the disk by
high-energy photons. Our results show that the combination of photoevaporation
and the streaming instability are efficient at converting the solid component
of protoplanetary disks into planetesimals. Our model, however, does not form
enough early planetesimals in the inner and middle regions of the disk to give
rise to giant planets and super-Earths with gaseous envelopes. Additional
processes such as particle pileups and mass loss driven by MHD winds may be
needed to drive the formation of early planetesimal generations in the planet
forming regions of protoplanetary disks.Comment: 20 pages, 12 figures; accepted to Ap
Ten Years of Universal Primary Technology Education in England and Wales - What have we learnt?
In this paper I will attempt to outline the long and complex history of National Curriculum technology at primary level, celebrating the successes as well as analysing the mistakes. It is, in the words of the chairman of the National Curriculum Council (NCC), a 'lesson for us all' (Graham 1993) and makes salutary reading for any country or state about to embark upon the implementation of its own primary technology curriculum. The story in England and Wales is one of 'free-market curriculum making' in which: "; ... policy for technology education has emerged in an unplanned way by a process of action and reaction, involving initiatives and proposals from a variety of stakeholders ... "; (Layton 1995, p. 114
Representation of multiple engineering viewpoints in Computer Aided Design through computer-interpretable descriptive markup
The aim of this work was to find a way of representing multiple interpretations of a product design with the same CAD model and in a way that allowed reduction of the manual work of producing the viewpoint specific models of the product through automation The approach presented is the recording of multiple viewpoint-interpretations of a product design with a CAD product model using descriptive, by-reference (stand-off) computer interpretable markup of the model.EThOS - Electronic Theses Online ServiceGBUnited Kingdo
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