501 research outputs found
Museums, conversations, and learning
In this study, 178 groups of visitors were interviewed and recorded during their visits to museums. Three clusters of elements were shown to influence learning: the identity of the visitors, their response to the learning environment, and their explanatory engagement during the visit. A structural equation model using these variables fit well. Further examination revealed that not all conversational behavior was supportive of learning; some actions, such as making frequent personal connections, were detrimental to learning; additionally, silent contemplation was modestly associated with learning. This paper discusses these findings through the experiences of four couples whose outcome measures placed them at the extreme high or low end of the learning distribution
Aerosols are not Spherical Cows: Using Discrete Dipole Approximation to Model the Properties of Fractal Particles
The optical properties of particulate-matter aerosols, within the context of
exoplanet and brown dwarf atmospheres, are compared using three different
models: Mie theory, Modified Mean Field (MMF) Theory, and Discrete Dipole
Approximation (DDA). Previous results have demonstrated that fractal haze
particles (MMF and DDA) absorb much less long-wavelength radiation than their
spherical counterparts (Mie), however it is shown here that the opposite can
also be true if a more varying refractive index profile is used. Additionally,
it is demonstrated that absorption and scattering cross-sections, as well as
the asymmetry parameter, are underestimated if Mie theory is used. Although DDA
can be used to obtain more accurate results, it is known to be much more
computationally intensive; to avoid this, the use of low-resolution aerosol
models is explored, which could dramatically speed up the process of obtaining
accurate computations of optical cross-sections within a certain parameter
space. The validity of DDA is probed for wavelengths of interest for
observations of aerosols within exoplanet and brown dwarf atmospheres (0.2 to
15 micrometres). Finally, novel code is presented to compare the results of
Mie, MMF and DDA theories (CORAL: Comparison Of Radiative AnaLyses), as well as
to increase and decrease the resolution of DDA shape files accordingly
(SPHERIFY). Both codes can be applied to a range of other interesting
astrophysical environments in addition to exoplanet atmospheres, for example
dust grains within protoplanetary disks.Comment: 24 pages, 23 figures, accepted for publication in "Monthly Notices of
the Royal Astronomical Society
The material role of digital media in connecting with, within, and beyond museums
The connective potentials of digital media have been positioned as a key part of a contemporary museum visitor experience. Using a sociology of translation, we construct a network of visitor experiences using data from a digital media engagement project at a large and multi-sited museum in the United Kingdom. These experiences relate to (dis)connections with the museum, museum objects, and other visitors. Through this analysis we disclose the often contradictory roles of the non-human, including and going beyond the digital, as contributors to the success and failure of attempts to change museum visitor experiences through engagement activities rooted in narratives of participation and connectivity
Collisional Evolution of Irregular Satellite Swarms: Detectable Dust around Solar System and Extrasolar Planets
Since the 1980's it has been becoming increasingly clear that the Solar
System's irregular satellites are collisionally evolved. We derive a general
model for the collisional evolution of an irregular satellite swarm and apply
it to the Solar System and extrasolar planets. Our model reproduces the Solar
System's complement of observed irregulars well, and suggests that the
competition between grain-grain collisions and Poynting-Robertson (PR) drag
helps set the fate of the dust. Because swarm collision rates decrease over
time the main dust sink can change with time, and may help unravel the
accretion history of synchronously rotating regular satellites that show
brightness asymmetries. Some level of dust must be present on AU scales around
the Solar System's giant planets, which we predict may be at detectable levels.
We also predict whether dust produced by extrasolar circumplanetary swarms can
be detected. The coronagraphic instruments on JWST will have the ability to
detect the dust generated by these swarms, which are most detectable around
planets that orbit at tens of AU from the youngest stars. Because the
collisional decay of swarms is relatively insensitive to planet mass, swarms
can be much brighter than their host planets and allow discovery of
Neptune-mass planets that would otherwise remain invisible. This dust may have
already been detected. The observations of the planet Fomalhaut b can be
explained as scattered light from dust produced by the collisional decay of an
irregular satellite swarm around a 10 Earth-mass planet. Such a swarm comprises
about 5 Lunar masses worth of irregular satellites. Finally, we consider what
happens if Fomalhaut b passes through Fomalhaut's main debris ring, which
allows the circumplanetary swarm to be replenished through collisions with ring
planetesimals. (abridged)Comment: accepted to MNRA
Are debris disks self-stirred?
This paper considers the evidence that debris disks are self-stirred by the
formation of Plutos. A model for the dust produced during self-stirring is
applied to statistics for A stars. As there is no significant difference
between excesses of A-stars <50Myr old, we focus on reproducing the broad
trends, the "rise and fall" of the fraction of stars with excesses. Using a
population model, we find that the statistics and trends can be reproduced with
a self-stirring model of planetesimal belts with radii distributed between
15-120AU. Disks must have this 15AU minimum radius to show a peak in disk
fraction, rather than a monotonic decline. Populations of extended disks with
fixed inner and/or outer radii fail to fit the statistics, due mainly to the
slow 70um evolution as stirring moves further out in the disk. This conclusion,
that debris disks are narrow belts, is independent of the significance of 24um
trends for young A-stars. We show that the statistics can also be reproduced
with a model in which disks are stirred by secular perturbations from a nearby
eccentric planet. Detailed imaging is therefore the best way to characterise
the stirring mechanism. From a more detailed look at beta Pictoris Moving Group
and TW Hydrae Association A-stars we find that the disk around beta Pictoris is
likely the result of secular stirring by the proposed planet at ~10AU; the
structure of the HR 4796A disk also points to sculpting by a planet. The two
other stars with disks, HR 7012 and eta Tel, possess transient hot dust, though
the outer eta Tel disk is consistent with a self-stirred origin. Planet
formation provides a natural explanation for the belt-like nature of debris
disks, with inner regions cleared by planets that may also stir the disk, and
the outer edges set by where planetesimals can form. [abridged]Comment: Accepted to MNRA
Extreme debris disk variability : exploring the diverse outcomes of large asteroid impacts during the era of terrestrial planet formation
The most dramatic phases of terrestrial planet formation are thought to be oligarchic and chaotic growth, on timescales of up to 100─200 Myr, when violent impacts occur between large planetesimals of sizes up to protoplanets. Such events are marked by the production of large amounts of debris, as has been observed in some exceptionally bright and young debris disks (termed extreme debris disks). Here we report five years of Spitzer measurements of such systems around two young solar-type stars: ID8 and P1121. The short-term (weekly to monthly) and long-term (yearly) disk variability is consistent with the aftermaths of large impacts involving large asteroid-sized bodies. We demonstrate that an impact-produced clump of optically thick dust, under the influence of the dynamical and viewing geometry effects, can produce short-term modulation in the disk light curves. The long-term disk flux variation is related to the collisional evolution within the impact-produced fragments once released into a circumstellar orbit. The time-variable behavior observed in the P1121 system is consistent with a hypervelocity impact prior to 2012 that produced vapor condensates as the dominant impact product. Two distinct short-term modulations in the ID8 system suggest two violent impacts at different times and locations. Its long-term variation is consistent with the collisional evolution of two different populations of impact-produced debris dominated by either vapor condensates or escaping boulders. The bright, variable emission from the dust produced in large impacts from extreme debris disks provides a unique opportunity to study violent events during the era of terrestrial planet formation
Dusty Planetary Systems
Extensive photometric stellar surveys show that many main sequence stars show
emission at infrared and longer wavelengths that is in excess of the stellar
photosphere; this emission is thought to arise from circumstellar dust. The
presence of dust disks is confirmed by spatially resolved imaging at infrared
to millimeter wavelengths (tracing the dust thermal emission), and at optical
to near infrared wavelengths (tracing the dust scattered light). Because the
expected lifetime of these dust particles is much shorter than the age of the
stars (>10 Myr), it is inferred that this solid material not primordial, i.e.
the remaining from the placental cloud of gas and dust where the star was born,
but instead is replenished by dust-producing planetesimals. These planetesimals
are analogous to the asteroids, comets and Kuiper Belt objects (KBOs) in our
Solar system that produce the interplanetary dust that gives rise to the
zodiacal light (tracing the inner component of the Solar system debris disk).
The presence of these "debris disks" around stars with a wide range of masses,
luminosities, and metallicities, with and without binary companions, is
evidence that planetesimal formation is a robust process that can take place
under a wide range of conditions. This chapter is divided in two parts. Part I
discusses how the study of the Solar system debris disk and the study of debris
disks around other stars can help us learn about the formation, evolution and
diversity of planetary systems by shedding light on the frequency and timing of
planetesimal formation, the location and physical properties of the
planetesimals, the presence of long-period planets, and the dynamical and
collisional evolution of the system. Part II reviews the physical processes
that affect dust particles in the gas-free environment of a debris disk and
their effect on the dust particle size and spatial distribution.Comment: 68 pages, 25 figures. To be published in "Solar and Planetary
Systems" (P. Kalas and L. French, Eds.), Volume 3 of the series "Planets,
Stars and Stellar Systems" (T.D. Oswalt, Editor-in-chief), Springer 201
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