39,789 research outputs found
Fullerenes and proto-fullerenes in interstellar carbon dust
Laboratory spectra of hydrogenated amorphous carbon (HAC) particles prepared
under a variety of conditions show spectral features at 7.05, 8.5, 17.4 and
18.9 {\mu}m (1418, 1176, 575 & 529 cm-1) that have been associated with
emission from C60 molecules. These lines occur in the spectra even though C60
molecules as such are not present in our samples. It appears that these four
spectral lines in HAC can instead be associated with precursor molecules or
"proto-fullerenes" that subsequently react to yield C60. We develop a model
tracing the evolution and de-hydrogenation of HAC dust and show that the
observation of an emission feature at 16.4 {\mu}m (610 cm-1) in astronomical
spectra signals the presence of the pentagonal carbon rings required for the
formation of fullerenes. We suggest that the set of four IR emission lines
previously identified with C60 in many objects that also show the 16.4 {\mu}m
feature and other polycyclic aromatic hydrocarbon bands arise from
proto-fullerenes rather than C60. Tc1 is an example of a source in which
de-hydrogenation has proceeded to the point where only fullerenes are present.Comment: 17 pages, 2 figures, accepted ApJ Letter
Comparison between Eulerian diagnostics and finite-size Lyapunov exponents computed from altimetry in the Algerian basin
Transport and mixing properties of surface currents can be detected from
altimetric data by both Eulerian and Lagrangian diagnostics. In contrast with
Eulerian diagnostics, Lagrangian tools like the local Lyapunov exponents have
the advantage of exploiting both spatial and temporal variability of the
velocity field and are in principle able to unveil subgrid filaments generated
by chaotic stirring. However, one may wonder whether this theoretical advantage
is of practical interest in real-data, mesoscale and submesoscale analysis,
because of the uncertainties and resolution of altimetric products, and the
non-passive nature of biogeochemical tracers. Here we compare the ability of
standard Eulerian diagnostics and the finite-size Lyapunov exponent in
detecting instantaneaous and climatological transport and mixing properties. By
comparing with sea-surface temperature patterns, we find that the two
diagnostics provide similar results for slowly evolving eddies like the first
Alboran gyre. However, the Lyapunov exponent is also able to predict the
(sub-)mesoscale filamentary process occuring along the Algerian current and
above the Balearic Abyssal Plain. Such filaments are also observed, with some
mismatch, in sea-surface temperature patterns. Climatologies of Lyapunov
exponents do not show any compact relation with other Eulerian diagnostics,
unveiling a different structure even at the basin scale. We conclude that
filamentation dynamics can be detected by reprocessing available altimetric
data with Lagrangian tools, giving insight into (sub-)mesoscale stirring
processes relevant to tracer observations and complementing traditional
Eulerian diagnostics
Lagrangian transport in a microtidal coastal area: the Bay of Palma, island of Mallorca, Spain
Coastal transport in the Bay of Palma, a small region in the island of
Mallorca, Spain, is characterized in terms of Lagrangian descriptors. The data
sets used for this study are the output for two months (one in autumn and one
in summer) of a high resolution numerical model, ROMS, forced atmospherically
and with a spatial resolution of 300 m. The two months were selected because
its different wind regime, which is the main driver of the sea dynamics in this
area. Finite-size Lyapunov Exponents (FSLEs) were used to locate
semi-persistent Lagrangian coherent structures (LCS) and to understand the
different flow regimes in the Bay. The different wind directions and regularity
in the two months have a clear impact on the surface Bay dynamics, whereas only
topographic features appear clearly in the bottom structures. The fluid
interchange between the Bay and the open ocean was tudied by computing particle
trajectories and Residence Times (RT) maps. The escape rate of particles out of
the Bay is qualitatively different, with a 32 more of escape rate of
particles to the ocean in October than in July, owing to the different
geometric characteristics of the flow. We show that LCSs separate regions with
different transport properties by displaying spatial distributions of residence
times on synoptic Lagrangian maps together with the location of the LCSs.
Correlations between the time-dependent behavior of FSLE and RT are also
investigated, showing a negative dependence when the stirring characterized by
FSLE values moves particles in the direction of escape
The reduction of plankton biomass induced by mesoscale stirring: a modeling study in the Benguela upwelling
Recent studies, both based on remote sensed data and coupled models, showed a
reduction of biological productivity due to vigorous horizontal stirring in
upwelling areas. In order to better understand this phenomenon, we consider a
system of oceanic flow from the Benguela area coupled with a simple
biogeochemical model of Nutrient-Phyto-Zooplankton (NPZ) type. For the flow
three different surface velocity fields are considered: one derived from
satellite altimetry data, and the other two from a regional numerical model at
two different spatial resolutions. We compute horizontal particle dispersion in
terms of Lyapunov Exponents, and analyzed their correlations with phytoplankton
concentrations. Our modelling approach confirms that in the south Benguela
there is a reduction of biological activity when stirring is increased.
Two-dimensional offshore advection and latitudinal difference in Primary
Production, also mediated by the flow, seem to be the dominant processes
involved. We estimate that mesoscale processes are responsible for 30 to 50% of
the offshore fluxes of biological tracers. In the northern area, other factors
not taken into account in our simulation are influencing the ecosystem. We
suggest explanations for these results in the context of studies performed in
other eastern boundary upwelling areas
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