We present abundances for seven stars in the (extremely) low-metallicity tail of the Sculptor dwarf spheroidal galaxy, from spectra taken with X-shooter on the ESO VLT. Targets were selected from the ...Ca II triplet (CaT) survey of the dwarf abundances and radial velocities team (DART) using the latest calibration. Of the seven extremely metal-poor candidates, five stars are confirmed to be extremely metal-poor (i.e., Fe/H < −3 dex), with Fe/H = –3.47 ± 0.07 for our most metal-poor star. All have Fe/H ≤ −2.5 dex from the measurement of individual Fe lines. These values are in agreement with the CaT predictions to within error bars. None of the seven stars is found to be carbon-rich. We estimate a 2–13% possibility of this being a pure chance effect, which could indicate a lower fraction of carbon-rich extremely metal-poor stars in Sculptor compared to the Milky Way halo. The α/Fe ratios show a range from +0.5 to –0.5, a larger variation than seen in Galactic samples although typically consistent within 1–2σ. One star seems mildly iron-enhanced. Our program stars show no deviations from the Galactic abundance trends in chromium and the heavy elements barium and strontium. Sodium abundances are, however, below the Galactic values for several stars. Overall, we conclude that the CaT lines are a successful metallicity indicator down to the extremely metal-poor regime and that the extremely metal-poor stars in the Sculptor dwarf galaxy are chemically more similar to their Milky Way halo equivalents than the more metal-rich population of stars.
We have investigated the response of superhydrogenated gas-phase coronene cations upon soft x-ray absorption. Carbon (1s)⟶π^{⋆} transitions were resonantly excited at hν=285 eV. The resulting core ...hole is then filled in an Auger decay process, with the excess energy being released in the form of an Auger electron. Predominantly highly excited dications are thus formed, which cool down by hydrogen emission. In superhydrogenated systems, the additional H atoms act as a buffer, quenching loss of native H atoms and molecular fragmentation. Dissociation and transition state energies for several H loss channels were computed by means of density functional theory. Using these energies as input into an Arrhenius-type cascade model, very good agreement with the experimental data is found. The results have important implications for the survival of polyaromatic hydrocarbons in the interstellar medium and reflect key aspects of graphene hydrogenation.
Near-Edge X‑ray Absorption Mass Spectrometry of a Gas-Phase Peptide González-Magaña, O; Reitsma, G; Tiemens, M ...
The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory,
11/2012, Volume:
116, Issue:
44
Journal Article
Peer reviewed
We have studied the dissociation of the gas-phase protonated peptide leucine enkephalin YGGFL+H+ upon X-ray absorption in the region of the C K-edge. The yield of photodissociation products was ...recorded as a function of photon energy. The total photoabsorption yield is qualitatively similar to near-edge X-ray absorption fine structure (NEXAFS) spectra recorded from condensed phase peptides and proteins. Fragment specificity reveals distinct quantitative differences between spectra obtained for different masses. Fragmentation channels can be assigned to specific electronic transitions some of which are site specific. For instance, C 1s → π★ excitations in the leucine enkephalin aromatic side chains lead to relatively little fragmentation, whereas such excitations along the peptide backbone induce strong fragmentation.
Plate kinematic reconstructions play an essential role in our understanding of global geodynamics, but become increasingly difficult to constrain back in geological time due to the subduction of ...oceanic lithosphere. Here, we attempt to kinematically reconstruct the Cretaceous and older plate tectonic history of the Caribbean Plate within the Mesozoic Panthalassa (paleo‐Pacific) Ocean. To this end, we present new paleomagnetic data from Jurassic and Cretaceous oceanic sedimentary and volcanic Large Igneous Province‐related rocks of the Nicoya Peninsula and Murciélago Islands of northwestern Costa Rica. We use these data, in combination with constraints from marine magnetic anomalies to infer the age of the lithospheric basement, seismic tomography to locate deep‐mantle plume generation zones, and general kinematic feasibility, to test different reconstruction scenarios connecting the Caribbean Plate to the Farallon Plate as restored from Pacific spreading records. Our resulting reconstruction implies that the western Caribbean subduction zone initiated around 100 Ma, in an intraoceanic setting, breaking up oceanic lithosphere of at least 70 Myr old.
Key Points
We reconstructed the Jurassic‐Cretaceous plate tectonic history of the Caribbean Plate within the Mesozoic Panthalassa Ocean
New paleomagnetic data indicate that northwestern Costa Rica was located within 10 degrees from the equator throughout the Cretaceous
The western Caribbean subduction zone initiated in Late Cretaceous time, in an intraoceanic setting, breaking up lithosphere of at least 70 Myr old
The Andes are the world's most biodiverse mountain chain, encompassing a complex array of ecosystems from tropical rainforests to alpine habitats. We provide a synthesis of Andean vascular plant ...diversity by estimating a list of all species with publicly available records, which we integrate with a phylogenetic dataset of 14 501 Neotropical plant species in 194 clades. We find that (i) the Andean flora comprises at least 28 691 georeferenced species documented to date, (ii) Northern Andean mid-elevation cloud forests are the most species-rich Andean ecosystems, (iii) the Andes are a key source and sink of Neotropical plant diversity, and (iv) the Andes, Amazonia, and other Neotropical biomes have had a considerable amount of biotic interchange through time.
We present an evolutionary and floristic synthesis of Andean plant diversity and evolution across time and space.Uplift of the Andes varied across time and space. Particularly, the fast uplift rates between 8 and 5 Ma in the Northern Andes may have favoured plant diversification.Using online specimen databases, we suggest that the Andean flora comprises at least 28 691 species. We identify North Andean montane forests as the potential species richest area.Using a biogeographic analysis on a dataset of 14 501 Neotropical species in 194 clades, we reveal that the Andes are both a key source and sink of Neotropical vascular plant biodiversity. We unveil strong biogeographical links between the Andes, Amazonia, and Central America.We highlight a number of critical research gaps, notably major Andean plant groups are still understudied, and fewer studies exist for the Central and Southern Andes. Filling these gaps will allow a more holistic understanding of Andean floras and provide essential tools for their conservation.
Aims. Molecular hydrogen is the most abundant molecule in the Universe. It is thought that a large portion of H2 forms by association of hydrogen atoms to polycyclic aromatic hydrocarbons (PAHs). We ...model the influence of PAHs on total H2 formation rates in photodissociation regions (PDRs) and assess the effect of these formation rates on the total cloud structure. Methods. We set up a chemical kinetic model at steady state in a PDR environment and included radiative transfer to calculate the chemistry at different depths in the PDR. This model includes known dust grain chemistry for the formation of H2 and a H2 formation mechanism on PAHs. Since H2 formation on PAHs is impeded by thermal barriers, this pathway is only efficient at higher temperatures (T> 200 K). At these temperatures the conventional route of H2 formation via H atoms physisorbed on dust grains is no longer feasible, so the PAH mechanism enlarges the region where H2 formation is possible. Results. We find that PAHs have a significant influence on the structure of PDRs. The extinction at which the transition from atomic to molecular hydrogen occurs strongly depends on the presence of PAHs, especially for PDRs with a strong external radiation field. A sharp spatial transition between fully dehydrogenated PAHs on the outside of the cloud and normally hydrogenated PAHs on the inside is found. As a proof of concept, we use coronene to show that H2 forms very efficiently on PAHs, and that this process can reproduce the high H2 formation rates derived in several PDRs.
Aims. Molecular hydrogen is the most abundant molecule in the Universe. It is thought that a large portion of H sub(2) forms by association of hydrogen atoms to polycyclic aromatic hydrocarbons ...(PAHs). We model the influence of PAHs on total H sub(2) formation rates in photodissociation regions (PDRs) and assess the effect of these formation rates on the total cloud structure. Methods. We set up a chemical kinetic model at steady state in a PDR environment and included radiative transfer to calculate the chemistry at different depths in the PDR. This model includes known dust grain chemistry for the formation of H sub(2) and a H sub(2) formation mechanism on PAHs. Since H sub(2) formation on PAHs is impeded by thermal barriers, this pathway is only efficient at higher temperatures (T> 200 K). At these temperatures the conventional route of H sub(2) formation via H atoms physisorbed on dust grains is no longer feasible, so the PAH mechanism enlarges the region where H sub(2) formation is possible. Results. We find that PAHs have a significant influence on the structure of PDRs. The extinction at which the transition from atomic to molecular hydrogen occurs strongly depends on the presence of PAHs, especially for PDRs with a strong external radiation field. A sharp spatial transition between fully dehydrogenated PAHs on the outside of the cloud and normally hydrogenated PAHs on the inside is found. As a proof of concept, we use coronene to show that H sub(2) forms very efficiently on PAHs, and that this process can reproduce the high H sub(2) formation rates derived in several PDRs.
PDF
