2,597 research outputs found
Open questions in the study of population III star formation
The first stars were key drivers of early cosmic evolution. We review the
main physical elements of the current consensus view, positing that the first
stars were predominantly very massive. We continue with a discussion of
important open questions that confront the standard model. Among them are
uncertainties in the atomic and molecular physics of the hydrogen and helium
gas, the multiplicity of stars that form in minihalos, and the possible
existence of two separate modes of metal-free star formation.Comment: 15 pages, 2 figures. To appear in the conference proceedings for IAU
Symposium 255: Low-Metallicity Star Formation: From the First Stars to Dwarf
Galaxie
Designing electronic collaborative learning environments
Electronic collaborative learning environments for learning and working are in vogue. Designers design them according to their own constructivist interpretations of what collaborative learning is and what it should achieve. Educators employ them with different educational approaches and in diverse situations to achieve different ends. Students use them, sometimes very enthusiastically, but often in a perfunctory way. Finally, researchers study them and—as is usually the case when apples and oranges are compared—find no conclusive evidence as to whether or not they work, where they do or do not work, when they do or do not work and, most importantly, why, they do or do not work. This contribution presents an affordance framework for such collaborative learning environments; an interaction design procedure for designing, developing, and implementing them; and an educational affordance approach to the use of tasks in those environments. It also presents the results of three projects dealing with these three issues
The Birth of a Galaxy: Primordial Metal Enrichment and Stellar Populations
By definition, Population III stars are metal-free, and their protostellar
collapse is driven by molecular hydrogen cooling in the gas-phase, leading to
large characteristic masses. Population II stars with lower characteristic
masses form when the star-forming gas reaches a critical metallicity of 10^{-6}
- 10^{-3.5} Z_\odot. We present an adaptive mesh refinement radiation
hydrodynamics simulation that follows the transition from Population III to II
star formation. The maximum spatial resolution of 1 comoving parsec allows for
individual molecular clouds to be well-resolved and their stellar associations
to be studied in detail. We model stellar radiative feedback with adaptive ray
tracing. A top-heavy initial mass function for the Population III stars is
considered, resulting in a plausible distribution of pair-instability
supernovae and associated metal enrichment. We find that the gas fraction
recovers from 5 percent to nearly the cosmic fraction in halos with merger
histories rich in halos above 10^7 solar masses. A single pair-instability
supernova is sufficient to enrich the host halo to a metallicity floor of
10^{-3} Z_\odot and to transition to Population II star formation. This
provides a natural explanation for the observed floor on damped Lyman alpha
(DLA) systems metallicities reported in the literature, which is of this order.
We find that stellar metallicities do not necessarily trace stellar ages, as
mergers of halos with established stellar populations can create superpositions
of t-Z evolutionary tracks. A bimodal metallicity distribution is created after
a starburst occurs when the halo can cool efficiently through atomic line
cooling.Comment: 11 pages, 7 figures; replaced with accepted version to ApJ;
additional movies and images can be found at
http://www.astro.princeton.edu/~jwise/research/GalaxyBirth.htm
17 ways to say yes:Toward nuanced tone of voice in AAC and speech technology
People with complex communication needs who use speech-generating devices have very little expressive control over their tone of voice. Despite its importance in human interaction, the issue of tone of voice remains all but absent from AAC research and development however. In this paper, we describe three interdisciplinary projects, past, present and future: The critical design collection Six Speaking Chairs has provoked deeper discussion and inspired a social model of tone of voice; the speculative concept Speech Hedge illustrates challenges and opportunities in designing more expressive user interfaces; the pilot project Tonetable could enable participatory research and seed a research network around tone of voice. We speculate that more radical interactions might expand frontiers of AAC and disrupt speech technology as a whole
Quasiparticle spin susceptibility in heavy-fermion superconductors : An NMR study compared with specific heat results
Quasi-particle spin susceptibility () for various heavy-fermion
(HF) superconductors are discussed on the basis of the experimental results of
electronic specific heat (), NMR Knight shift () and NMR
relaxation rate () within the framework of the Fermi liquid model for a
Kramers doublet crystal electric field (CEF) ground state.
is calculated from the enhanced Sommerfeld coefficient and
from the quasi-particle Korringa relation
via the relation of
where is the hyperfine
coupling constant, the Abogadoro's number and the Bohr magneton.
For the even-parity (spin-singlet) superconductors CeCuSi, CeCoIn
and UPdAl, the fractional decrease in the Knight shift, , below the superconducting transition temperature () is due to
the decrease of the spin susceptibility of heavy quasi-particle estimated
consistently from and . This result
allows us to conclude that the heavy quasi-particles form the spin-singlet
Cooper pairs in CeCuSi, CeCoIn and UPdAl. On the other
hand, no reduction in the Knight shift is observed in UPt and
UNiAl, nevertheless the estimated values of and
are large enough to be probed experimentally. The odd-parity
superconductivity is therefore concluded in these compounds. The NMR result
provides a convincing way to classify the HF superconductors into either even-
or odd- parity paring together with the identification for the gap structure,
as long as the system has Kramers degeneracy.Comment: 11 pages, 3 tables, 5 figures, RevTex4(LaTex2e
The First Stars: Mass Growth Under Protostellar Feedback
We perform three-dimensional cosmological simulations to examine the growth
of metal-free, Population III (Pop III) stars under radiative feedback. We
begin our simulation at z=100 and trace the evolution of gas and dark matter
until the formation of the first minihalo. We then follow the collapse of the
gas within the minihalo up to densities of n = 10^12 cm^-3, at which point we
replace the high-density particles with a sink particle to represent the
growing protostar. We model the effect of Lyman-Werner (LW) radiation emitted
by the protostar, and employ a ray-tracing scheme to follow the growth of the
surrounding H II region over the next 5000 yr. We find that a disk assembles
around the first protostar, and that radiative feedback will not prevent
further fragmentation of the disk to form multiple Pop III stars. Ionization of
neutral hydrogen and photodissociation of H_2 by LW radiation leads to heating
of the dense gas to several thousand Kelvin, and this warm region expands
outward at the gas sound speed. Once the extent of this warm region becomes
equivalent to the size of the disk, the disk mass declines while the accretion
rate onto the protostars is reduced by an order of magnitude. This occurs when
the largest sink has grown to ~ 20 M_sol while the second sink has grown to 7
M_sol, and we estimate the main sink will approach an asymptotic value of ~ 30
M_sol by the time it reaches the main sequence. Our simulation thus indicates
that the most likely outcome is a massive Pop III binary. However, we simulate
only one minihalo, and the statistical variation between minihaloes may be
substantial. If Pop III stars were typically unable to grow to more than a few
tens of solar masses, this would have important consequences for the occurence
of pair-instability supernovae in the early Universe as well as the Pop III
chemical signature in the oldest stars observable today.Comment: 21 pages, 11 figures, to appear in MNRA
Localization and chiral symmetry in 2+1 flavor domain wall QCD
We present results for the dependence of the residual mass of domain wall
fermions (DWF) on the size of the fifth dimension and its relation to the
density and localization properties of low-lying eigenvectors of the
corresponding hermitian Wilson Dirac operator relevant to simulations of 2+1
flavor domain wall QCD. Using the DBW2 and Iwasaki gauge actions, we generate
ensembles of configurations with a space-time volume and an
extent of 8 in the fifth dimension for the sea quarks. We demonstrate the
existence of a regime where the degree of locality, the size of chiral symmetry
breaking and the rate of topology change can be acceptable for inverse lattice
spacings GeV.Comment: 59 Pages, 23 figures, 1 MPG linke
Lessons from using iPads to understand young children's creativity
This paper explores how iPads can be used as part of a child-centred data collection approach to understanding young children’s creativity. Evidence is presented from a pilot study about 3- to 5-year-old children’s creative play. Researchers’ reflective accounts of children’s engagement with iPad video diaries and free to use apps were logged across two early educational settings over a three-month period. Findings suggest that iPads offer a mechanism to allow children to express their creative play and to encourage involvement in the research process. However, bespoke research software to use with early years children is required to improve this process
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