12,361 research outputs found
ADP correspondence system: Unsolicited proposal evaluation tracking application
A complete description of a correspondence control system, designed to be used by non-ADP clerical personnel is provided. In addition to operating instructions, sufficient design and conceptual information is provided to allow use or adaption of the system in related applications. The complete COBOL program and documentation are available
How to identify the youngest protostars
We study the transition from a prestellar core to a Class 0 protostar, using
SPH to simulate the dynamical evolution, and a Monte Carlo radiative transfer
code to generate the SED and isophotal maps. For a prestellar core illuminated
by the standard interstellar radiation field, the luminosity is low and the SED
peaks at ~190 micron. Once a protostar has formed, the luminosity rises (due to
a growing contribution from accretion onto the protostar) and the peak of the
SED shifts to shorter wavelengths (~80-100 micron). However, by the end of the
Class 0 phase, the accretion rate is falling, the luminosity has decreased, and
the peak of the SED shifts back towards longer wavelengths (90-150 micron). In
our simulations, the density of material around the protostar remains
sufficiently high well into the Class 0 phase that the protostar only becomes
visible in the NIR if it is displaced from the centre dynamically. Raw submm/mm
maps of Class 0 protostars tend to be much more centrally condensed than those
of prestellar cores. However, when convolved with a typical telescope beam, the
difference in central concentration is less marked, although the Class 0
protostars appear more circular. Our results suggest that, if a core is deemed
to be prestellar on the basis of having no associated IRAS source, no cm radio
emission, and no outflow, but it has a circular appearance and an SED which
peaks at wavelengths below ~170 micron, it may well contain a very young Class
0 protostar.Comment: Accepted by A&A (avaliable with high-res images at
http://carina.astro.cf.ac.uk/pub/Dimitrios.Stamatellos/publications
Simulating star formation in molecular cloud cores I. The influence of low levels of turbulence on fragmentation and multiplicity
We present the results of an ensemble of simulations of the collapse and
fragmentation of dense star-forming cores. We show that even with very low
levels of turbulence the outcome is usually a binary, or higher-order multiple,
system. We take as the initial conditions for these simulations a typical
low-mass core, based on the average properties of a large sample of observed
cores. All the simulated cores start with a mass of , a
flattened central density profile, a ratio of thermal to gravitational energy
and a ratio of turbulent to gravitational energy
. Even this low level of turbulence is sufficient to
produce multiple star formation in 80% of the cores; the mean number of stars
and brown dwarfs formed from a single core is 4.55, and the maximum is 10. At
the outset, the cores have no large-scale rotation. The only difference between
each individual simulation is the detailed structure of the turbulent velocity
field. The multiple systems formed in the simulations have properties
consistent with observed multiple systems. Dynamical evolution tends
preferentially to eject lower mass stars and brown dwarves whilst hardening the
remaining binaries so that the median semi-major axis of binaries formed is
au. Ejected objects are usually single low-mass stars and brown
dwarfs, yielding a strong correlation between mass and multiplicity. Our
simulations suggest a natural mechanism for forming binary stars that does not
require large-scale rotation, capture, or large amounts of turbulence.Comment: 20 pages, 12 figures submitted to A&
Dynamics from diffraction
A model-independent approach for the extraction of detailed
lattice dynamical information from neutron powder diffraction data is described. The technique is based on a statistical analysis of atomistic configurations generated using reverse Monte Carlo structural refinement.
Phonon dispersion curves extracted in this way are shown to
reproduce many of the important features found in those determined independently using neutron triple-axis spectroscopy. The extent to which diffraction data are sensitive to lattice dynamics is explored in a
range of materials. The prospect that such detailed dynamical information might be accessible using comparatively facile experiments such as neutron
powder diffraction is incredibly valuable when studying systems for which established spectroscopic methods are prohibitive or
inappropriate
Adsorption and binding dynamics of graphene-supported phospholipid membranes using the QCM-D technique
We report on the adsorption dynamics of phospholipid membranes on
graphene-coated substrates using the quartz crystal microbalance with
dissipation monitoring (QCM-D) technique. We compare the lipid vescle
interaction and membranne formation on gold and silicon dioxide QCM crystal
surfaces with their graphene oxide (GO) and reduced (r)GO coated counterparts,
and report on the different lipid structures obtained. We establish graphene
derivative coatings as support surfaces with tuneable hydrophobicity for the
formation of controllable lipid structures. One structure of interest formed
are lipid monolayer membrannes which were formed on rGO, which are otherwise
challenging to produce. We also demonstrate and monitor biotin-avidin binding
on such a membranne, which will then serve as a platform for a wide range of
biosensing applications. The QCM-D technique could be extended to both
fundamental studies and applications of other covalent and non-covalent
interactions in 2-dimensional materials
Simulating star formation in molecular cloud cores IV. The role of turbulence and thermodynamics
We perform SPH simulations of the collapse and fragmentation of low-mass
cores having different initial levels of turbulence
(alpha_turb=0.05,0.10,0.25). We use a new treatment of the energy equation
which captures the transport of cooling radiation against opacity due to both
dust and gas (including the effects of dust sublimation, molecules, and H^-
ions). We also perform comparison simulations using a standard barotropic
equation of state. We find that -- when compared with the barotropic equation
of state -- our more realistic treatment of the energy equation results in more
protostellar objects being formed, and a higher proportion of brown dwarfs; the
multiplicity frequency is essentially unchanged, but the multiple systems tend
to have shorter periods (by a factor ~3), higher eccentricities, and higher
mass ratios. The reason for this is that small fragments are able to cool more
effectively with the new treatment, as compared with the barotropic equation of
state. We find that the process of fragmentation is often bimodal. The first
protostar to form is usually, at the end, the most massive, i.e. the primary.
However, frequently a disc-like structure subsequently forms round this
primary, and then, once it has accumulated sufficient mass, quickly fragments
to produce several secondaries. We believe that this delayed fragmentation of a
disc-like structure is likely to be an important source of very low-mass
hydrogen-burning stars and brown dwarfs.Comment: 14 pages, 8 figures. Accepted for publication by A&
Emergence of long-range order in BaTiO3 from local symmetry-breaking distortions
By using a symmetry motivated basis to evaluate local distortions against
pair distribution function data (PDF), we show without prior bias, that the
off-centre Ti displacements in the archetypal ferroelectric BaTiO3 are zone
centred and rhombohedral-like in nature across its known ferroelectric and
paraelectric phases. With our newly-gained insight we construct a simple Monte
Carlo (MC) model which captures our main experimental findings and demonstrate
how the rich crystallographic phase diagram of BaTiO3 emerges from correlations
of local symmetry-breaking distortions alone. Our results strongly support the
order-disorder picture for these phase transitions, but can also be reconciled
with the soft-mode theory of BaTiO3 that is supported by some spectroscopic
techniques.Comment: 5 pages, 3 figure
Structure determination of disordered materials from diffraction data
We show that the information gained in spectroscopic experiments regarding
the number and distribution of atomic environments can be used as a valuable
constraint in the refinement of the atomic-scale structures of nanostructured
or amorphous materials from pair distribution function (PDF) data. We
illustrate the effectiveness of this approach for three paradigmatic disordered
systems: molecular C60, a-Si, and a-SiO2 . Much improved atomistic models are
attained in each case without any a-priori assumptions regarding coordination
number or local geometry. We propose that this approach may form the basis for
a generalised methodology for structure "solution" from PDF data applicable to
network, nanostructured and molecular systems alike.Comment: 4 pages, 3 figures, set out as for PR
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