1,245 research outputs found
Fractional Hereditariness of Lipid Membranes: Instabilities and Linearized Evolution
In this work lipid ordering phase changes arising in planar membrane bilayers
is investigated both accounting for elas- ticity alone and for effective
viscoelastic response of such assemblies. The mechanical response of such
membranes is studied by minimizing the Gibbs free energy which penalizes
perturbations of the changes of areal stretch and their gradients only [1]. As
material instabilities arise whenever areal stretches characterizing
homogeneous configurations lie inside the spinoidal zone of the free energy
density, bifurcations from such configurations are shown to occur as
oscillatory perturbations of the in-plane displacement. Experimental
observations [2] show a power-law in-plane viscous behavior of lipid structures
allowing for an effective viscoelastic behavior of lipid membranes [3], which
falls in the framework of Fractional Hereditariness. A suitable generalization
of the variational principle invoked for the elasticity is applied in this
case, and the corresponding Euler-Lagrange equation is found together with a
set of bound- ary and initial conditions. Separation of variables allows for
showing how Fractional Hereditariness owes bifurcated modes with a larger
number of spatial oscillations than the corresponding elastic analog. Indeed,
the available range of areal stresses for material instabilities is found to
increase with respect to the purely elastic case. Nevertheless, the time
evolution of the perturbations solving the Euler-Lagrange equation above
exhibits time-decay and the large number of spatial oscillation slowly relaxes,
thereby keeping the features of a long-tail type time-response.Comment: 21 pages, 11 figures, special issu
Galaxies into the Dark Ages
We consider the capabilities of current and future large facilities operating
at 2\,mm to 3\,mm wavelength to detect and image the [CII] 158\,m line
from galaxies into the cosmic "dark ages" ( to 20). The [CII] line
may prove to be a powerful tool in determining spectroscopic redshifts, and
galaxy dynamics, for the first galaxies. We emphasize that the nature, and even
existence, of such extreme redshift galaxies, remains at the frontier of open
questions in galaxy formation. In 40\,hr, ALMA has the sensitivity to detect
the integrated [CII] line emission from a moderate metallicity, active
star-forming galaxy [; star formation rate (SFR) =
5\,\,yr], at at a significance of 6. The
next-generation Very Large Array (ngVLA) will detect the integrated [CII] line
emission from a Milky-Way like star formation rate galaxy (, SFR = 1\,\,yr), at at a significance
of 6. Imaging simulations show that the ngVLA can determine rotation
dynamics for active star-forming galaxies at , if they exist. Based
on our very limited knowledge of the extreme redshift Universe, we calculate
the count rate in blind, volumetric surveys for [CII] emission at
to 20. The detection rates in blind surveys will be slow (of order unity per
40\,hr pointing). However, the observations are well suited to commensal
searches. We compare [CII] with the [OIII] 88m line, and other ancillary
information in high galaxies that would aid these studies.Comment: 11pages, 8 figures, Accepted for the Astrophysical Journa
Lyman Alpha Emitter Evolution in the Reionization Epoch
Combining cosmological SPH simulations with a previously developed Lyman
Alpha production/transmission model and the Early Reionization Model (ERM,
reionization ends at redshift z~7), we obtain Lyman Alpha and UV Luminosity
Functions (LFs) for Lyman Alpha Emitters (LAEs) for redshifts between 5.7 and
7.6. Matching model results to observations at z~5.7 requires escape fractions
of Lyman Alpha, f_alpha=0.3, and UV (non-ionizing) continuum photons, f_c=0.22,
corresponding to a color excess, E(B-V)=0.15. We find that (i) f_c increases
towards higher redshifts, due the decreasing mean dust content of galaxies,
(ii) the evolution of f_alpha/f_c hints at the dust content of the ISM becoming
progressively inhomogeneous/clumped with decreasing redshift. The clustering
photoionization boost is important during the initial reionization phases but
has little effect on the Lyman Alpha LF for a highly ionized IGM. Halo
(stellar) masses are in the range 10.0 < \log M_h < 11.8 (8.1 < \log M_* <
10.4) with M_h \propto M_*^{0.64}. The star formation rates are between 3-120
solar masses per year, mass-weighted mean ages are greater than 20 Myr at all
redshifts, while the mean stellar metallicity increases from Z=0.12 to 0.22
solar metallicity from z~7.6 to z~5.7; both age and metallicity positively
correlate with stellar mass. The brightest LAEs are all characterized by large
star formation rates and intermediate ages (~200 Myr), while objects in the
faint end of the Lyman Alpha LF show large age and star formation rate spreads.
With no more free parameters, the Spectral Energy Distributions of three LAE at
z~5.7 observed by Lai et al. (2007) are well reproduced by an intermediate age
(182-220 Myr) stellar population and the above E(B-V) value.Comment: 13 pages, 9 figures, accepted to MNRA
Terminology and tensions within evidence-informed decision-making in South Africa over a 15-year period
In this article, we examine a key premise underlying evidence-informed decisionmaking (EIDM) – that research is for all, including service users and potential users, service providers and a wide range of decision-makers, from those running local services to national government officials and international agencies. Qualitative data collected on terminology used when writing and talking about EIDM over a period of 15 years during the implementation of a number of capacity development programmes in South Africa were combined with critical reflections in practice. Findings reveal that tensions exist in the titles and terminology used to describe the relationships between academia and government or between research and policy, and that these tensions have shifted over time, but not necessarily diminished. An analysis and critique of this terminology is provided to identify and unpack these tensions, which challenge the central premise of 'research for all'. The perpetuation of divisive labels that profile people, of job titles and specific terminology that describe agency, as well as the use of technical language, continues to exclude people from the approach. These have the effect of setting up users against producers of evidence. In conclusion, we challenge the advocates of the EIDM approach to review language and terminology to be more inclusive, to enable relationship-building and ease the process of engagement to ensure evidence-informed decision-making is true to its premise that research is for all
Spatial Distributions of Multiple Dust Components in the PPN/PN Dust Shells
We investigate spatial distributions of specific dust components in the
circumstellar shells of a proto-planetary nebula candidate, HD 179821, and a
planetary nebula, BD3639, by means of spectral imaging. With
high-resolution ground-based images and ISO spectra in the mid-infrared, we can
derive ``dust feature only'' maps by subtracting synthesized continuum maps
from the observed images at the feature wavelength. Such spatially detailed
information will help to develop models for the evolution of dust grains around
evolved stars.Comment: 4 pages + 7 figures, to appear in the proceedings of the conference,
"Post-AGB Objects (proto-planetary nebulae) as a Phase of Stellar Evolution",
Torun, Poland, July 5-7, 2000, eds. R. Szczerba, R. Tylenda, and S.K. Gorny.
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