2,911 research outputs found
Studies in the vegetational history of Greece
The vegetational history of Greece was studied by means of pollen analysis. Six pollen diagrams from various parts of the Greek mainland are presented, which between them cover a time from around 30,000 years ago to the present day. The development of the vegetation from a steppe during the time of the last glaciation through a transitional stage in the late-glacial period to a post-glacial forest, and the degradation of the vegetation to the present day scrub is represented by three main pollen assemblage zones. Zone I covers part or all of the time of the last glacial period when the dominant vegetation was an Artemisia steppe with little woodland, in response to a cool dry climate with a temperature comparable with that of northern Europe today but probably less rainfall than is usual in Greece now. Zone II covers the vegetational development during the time of the late-glacial and early post-glacial where evidence from southern Greece shows a pioneer scrub vegetation being succeeded by mixed-oak forest in response to a temperature and rainfall increase, with a temporary reversion to cool and dry conditions deduced from a vegetational reversion. This sequence probably corresponds to the lower Dryas-Allerd-Upper Dryas sequence known in northern Europe. Zone III covers the degradation of the forest vegetation which appears to have taken place earlier in southern Greece than in the north, where there appears to have been substantial forest up to at least the year A.D. and possibly until Medieval times before clearance by man and animal. Two periods of olive cultivation are recognisable, one in Middle Bronze Age – Mycenean times and the other in Late Dark Ages – Early Classical times. There is no sign of climatic change in the Post-glacial sequence. The pollen rain from some modern Greek vegetation shows that tara which are abundant there may be seriously under-represented in the pollen rain
Evidence for polarised boron in Co-B and Fe-B alloys
By exploiting the tunability of synchrotron radiation in measurements of spin-resolved photoemission it has proved possible to obtain information on the polarisation of the valence electrons of Co-B and Fe-B amorphous magnetic alloys, Both the spin-integrated and spin-resolved energy distribution curves show a marked dependence on photon energy indicating that the p states of boron hybridise with the d states of the transition metals giving rise to mixed states in the binding energy range 1 to 5 eV, The observed polarisation and spin-resolved densities of states imply that in the above restricted energy range there is a net negative polarisation of the boron states
Ultrathin epitaxial Fe films in vicinal GaAs(001): A study by spin-resolved photoelectron spectroscopy
Thin epitaxial Fe films have been grown on vicinal GaAs(001) substrates and their remanent magnetic properties and the degree of substrate atom diffusion investigated using synchrotron-based photoelectron spectroscopy. The vicinal Fe films, though exhibiting greater As diffusion than their singular homologues, displayed better film quality both from the structural and the magnetic points of view. The spin-resolved valence spectra of the vicinal films resemble those for crystalline bulk Fe at lower film thicknesses than for singular films
A tale of two sites – I. Inferring the properties of minihalo-hosted galaxies from current observations
The very first galaxies that started the cosmic dawn likely resided in so-called ‘minihaloes’, with masses of ∼105–108 M, accreting their gas from the intergalactic medium through H2 cooling. Such molecularly cooled galaxies (MCGs) mostly formed in pristine environments, hosted massive, metal-free stars, and were eventually sterilized by the build-up of a disassociating (Lyman–Werner; LW) background. Therefore, their properties might be very different from the galaxies we see in the later Universe. Although MCGs are probably too faint to be observed directly, we could nevertheless infer their properties from the imprint they leave in the cosmic 21-cm signal. Here we quantify this imprint by extending the public simulation code 21CMFAST to allow for a distinct population of MCGs. We allow MCGs to have different properties from other galaxies, including unique scaling relations for their stellar-to-halo mass ratios, ionizing escape fractions, and spectral energy distributions. We track inhomogeneous recombinations, disassociative LW feedback, and photoheating from reionization. After demonstrating how MCGs can shape the 21-cm signal, we explore to what extent current observations can already place constraints on their properties. The cosmic microwave background optical depth from Planck sets an upper limit on the product of the ionizing escape fraction and the stellar mass in MCGs. When including also the timing of the putative EDGES absorption signal, we find an additional strong degeneracy between the stellar mass and the X-ray luminosity of MCGs. If proven to be of cosmic origin, the timing of the EDGES signal would have been set by MCGs
Results from EDGES High-Band: II. Constraints on Parameters of Early Galaxies
We use the sky-average spectrum measured by EDGES High-Band ( MHz) to
constrain parameters of early galaxies independent of the absorption feature at
~MHz reported by Bowman et al. (2018). These parameters represent
traditional models of cosmic dawn and the epoch of reionization produced with
the 21cmFAST simulation code (Mesinger & Furlanetto 2007, Mesinger et al.
2011). The parameters considered are: (1) the UV ionizing efficiency (),
(2) minimum halo virial temperature hosting efficient star-forming galaxies
(), (3) integrated soft-band X-ray luminosity (), and (4) minimum X-ray energy escaping the first
galaxies (), corresponding to a typical H column
density for attenuation through the interstellar medium. The High-Band spectrum
disfavors high values of and , which correspond
to signals with late absorption troughs and sharp reionization transitions. It
also disfavors intermediate values of , which
produce relatively deep and narrow troughs within the band. Specifically, we
rule out
( C.L.). We then combine the EDGES High-Band data with constraints on the
electron scattering optical depth from Planck and the hydrogen neutral fraction
from high- quasars. This produces a lower degeneracy between and
than that reported in Greig & Mesinger (2017a) using
the Planck and quasar constraints alone. Our main result in this combined
analysis is the estimate ~~ ( C.L.). We leave for future work the evaluation of
~cm models using simultaneously data from EDGES Low- and High-Band.Comment: Accepted in Ap
The restoration of Loch Leven, Scotland, UK
This paper reviews the progress made towards the restoration of Loch Leven, the largest lake in lowland Scotland, over the last 20 years. In particular, the importance of direct regulation and of setting water quality objectives and targets is examined. Various means of engaging with stakeholders and the general public are also considered. Success criteria and catchment management initiatives are described and briefly reviewed
An investigation of the laser-induced zircon 'matrix effect'
This study aims to improve our understanding of the current limitations to high-precision U-Pb analysis of zircon by LA-ICP-MS by investigating the underlying causes of variation in ablation behaviour between different zircon matrices. Multiple factors such as: the degree of accumulated radiation damage, trace element composition; crystal colour; and crystallographic orientation are all systematically investigated. Due to the marked decrease in elastic moduli of natural zircon crystals with increasing radiation damage, the accumulation of this damage is the dominant factor controlling the rate of ablation for partially damaged to highly metamict zircon samples. There are slight differences, however, in ablation behaviour between highly crystalline matrices that cannot be attributed solely to differences in the degree of accumulated radiation damage. These differences are associated with structural weakening (i.e., decrease in elastic moduli and overall lower mechanical resistance) caused by an increasing degree of cation substitution in some of the zircon samples. Effects of crystallographic orientation and of crystal opacity (i.e., colour) on ablation behaviour are negligible compared to the combined influences of accumulated radiation damage and trace element substitution into the zircon structure. Experiments performed on natural and annealed zircon grains reveal that the reduction in ablation rates observed for the treated samples compared to the untreated grains is proportional to the degree of structural reconstitution achieved after annealing. Thermal annealing of natural zircon at temperatures >1000 °C results in much more uniform ablation characteristics. This 'homogenisation' of ablation behaviour between zircon matrices produces a decrease in the laser-induced matrix effects and subsequent improvement in the accuracy of 206Pb/238U ratio determinations by LA-ICP-MS
Large Magellanic Cloud Planetary Nebula Morphology: Probing Stellar Populations and Evolution
Planetary Nebulae (PNe) in the Large Magellanic Cloud (LMC) offer the unique
opportunity to study both the Population and evolution of low- and
intermediate-mass stars, by means of the morphological type of the nebula.
Using observations from our LMC PN morphological survey, and including images
available in the HST Data Archive, and published chemical abundances, we find
that asymmetry in PNe is strongly correlated with a younger stellar Population,
as indicated by the abundance of elements that are unaltered by stellar
evolution (Ne, Ar, S). While similar results have been obtained for Galactic
PNe, this is the first demonstration of the relationship for extra-galactic
PNe. We also examine the relation between morphology and abundance of the
products of stellar evolution. We found that asymmetric PNe have higher
nitrogen and lower carbon abundances than symmetric PNe. Our two main results
are broadly consistent with the predictions of stellar evolution if the
progenitors of asymmetric PNe have on average larger masses than the
progenitors of symmetric PNe. The results bear on the question of formation
mechanisms for asymmetric PNe, specifically, that the genesis of PNe structure
should relate strongly to the Population type, and by inference the mass, of
the progenitor star, and less strongly on whether the central star is a member
of a close binary system.Comment: The Astrophysical Journal Letters, in press 4 figure
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