6,101 research outputs found
On the chemistry of hydrides of N atoms and O ions
Previous work by various authors has suggested that the detection by
Herschel/HIFI of nitrogen hydrides along the low density lines of sight towards
G10.6-0.4 (W31C) cannot be accounted for by gas-phase chemical models. In this
paper we investigate the role of surface reactions on dust grains in diffuse
regions, and we find that formation of the hydrides by surface reactions on
dust grains with efficiency comparable to that for H formation reconciles
models with observations of nitrogen hydrides. However, similar surface
reactions do not contribute significantly to the hydrides of O ions
detected by Herschel/HIFI present along many sight lines in the Galaxy. The
O hydrides can be accounted for by conventional gas-phase chemistry either
in diffuse clouds of very low density with normal cosmic ray fluxes or in
somewhat denser diffuse clouds with high cosmic ray fluxes. Hydride chemistry
in dense dark clouds appears to be dominated by gas-phase ion-molecule
reactions.Comment: 19 pages, 4 figures, 4 tables Accepted for publication in Ap
Between the historical languages and the reconstructed language : an alternative approach to the Gerundive + “Dative of Agent” construction in Indo-European
It is argued by Hettrich (1990) that the “dative of agent” construction in the Indo-European languages most likely continues a construction inherited from Proto-Indo-European. In two recent proposals (Danesi 2013, Luraghi 2016), it is argued that the “dative of agent” contains no agent at all, although the two proposals differ with regard to the reconstructability of the “dative of agent” construction. Luraghi argues that it is an independent secondary development from an original beneficiary function (cf. Hettrich 1990), while Danesi maintains that the construction is reconstructable for an earlier proto-stage. Elaborating on Danesi’s approach, we analyze gerundives with the “dative of agent” in six different Indo-European languages that bridge the east–west divide, namely, Sanskrit, Avestan, Ancient Greek, Latin, Tocharian, and Lithuanian. Scrutiny of the data reveals similarities at a morphosyntactic level, a semantic level (i.e. modal meaning and low degree of transitivity), and also, to some extent, at an etymological level. An analysis involving a modal reading of the predicate, with a dative subject and a nominative object, is better equipped to account for the particulars of the “gerundive + nominative + dative” construction than the traditional agentive/passive analysis. The proposal is couched within the theoretical framework of Construction Grammar, in which the basic unit of language is the Construction, i.e. a form–function correspondence, and no principled distinction between lexical items and complex syntactic structures is assumed. As these structures are by definition units of comparanda, required by the Comparative Method, they can be successfully utilized in the reconstruction of a proto-construction for Proto-Indo-European
Six-dimensional nilpotent Lie algebras
We give a full classification of 6-dimensional nilpotent Lie algebras over an
arbitrary field, including fields that are not algebraically closed and fields
of characteristic~2. To achieve the classification we use the action of the
automorphism group on the second cohomology space, as isomorphism types of
nilpotent Lie algebras correspond to orbits of subspaces under this action. In
some cases, these orbits are determined using geometric invariants, such as the
Gram determinant or the Arf invariant. As a byproduct, we completely determine,
for a 4-dimensional vector space , the orbits of \GL(V) on the set of
2-dimensional subspaces of .Comment: Corrected a small error in Theorem 4.
Bayesian analysis of the multivariate dependence of three transition water ecosystem classifications
The Water Framework Directive (WFD) recognizes benthic macroinvertebrates as a good biological quality element for transitional waters as they are the most exposed to natural variability patterns characteristic of these ecosystems, due to their life cycles and space-use behavior. Here, we address the ecological status classification issue for three lagoons in Apulia, using benthic macroinvertebrates and three proposed multimetric indices (namely M-AMBI, BITS and ISS), likely to respond differently to different sources of stress and natural variability. Lagoon classification is based on discretization by standard classification boundaries with only partial consideration of the natural variability of ecosystem properties and possible inaccuracies of the classification procedures. In order to investigate the possible contrasting behavior of the three classifications, we propose Bayesian hierarchical models in which the multimetric indices and their discrete counterparts are jointly modeled as function of abiotic covariates, external anthropogenic pressures indicators and spatio-temporal effects
Shark Declines in the Mediterranean Sea
Summarizes a study of population and biomass trends of large sharks in the Mediterranean, and highlights the risk of some species becoming extinct as a result of unintended capture in fishing gear, targeted shark fishing, and human population pressure
Statistical analysis of Ni nanowires breaking processes: a numerical simulation study
A statistical analysis of the breaking behavior of Ni nanowires is presented.
Using molecular dynamic simulations, we have determined the time evolution of
both the nanowire atomic structure and its minimum cross section (Sm(t)).
Accumulating thousands of independent breaking events, Sm histograms are built
and used to study the influence of the temperature, the crystalline stretching
direction and the initial nanowire size. The proportion of monomers, dimers and
more complex structures at the latest stages of the breaking process are
calculated, finding important differences among results obtained for different
nanowire orientations and sizes. Three main cases have been observed. (A) [111]
stretching direction and large nanowire sizes: the wire evolves from more
complex structures to monomers and dimers prior its rupture; well ordered
structures is presented during the breaking process. (B) Large nanowires
stretched along the [100] and [110] directions: the system mainly breaks from
complex structures (low probability of finding monomers and dimers), having
disordered regions during their breakage; at room temperature, a huge histogram
peak around Sm=5 appears, showing the presence of long staggered pentagonal Ni
wires with ...-5-1-5-... structure. (C) Initial wire size is small: strong size
effects independently on the temperature and stretching direction. Finally, the
local structure around monomers and dimmers do not depend on the stretching
direction. These configurations differ from those usually chosen in static
studies of conductance.Comment: 18 pages, 13 figure
Real space investigation of structural changes at the metal-insulator transition in VO2
Synchrotron X-ray total scattering studies of structural changes in rutile
VO2 at the metal-insulator transition temperature of 340 K reveal that
monoclinic and tetragonal phases of VO2 coexist in equilibrium, as expected for
a first-order phase transition. No evidence for any distinct intermediate phase
is seen. Unbiased local structure studies of the changes in V--V distances
through the phase transition, using reverse Monte Carlo methods, support the
idea of phase coexistence and point to the high degree of correlation in the
dimerized low-temperature structure. No evidence for short range V--V
correlations that would be suggestive of local dimers is found in the metallic
phase.Comment: 4 pages, 5 figure
After the Fall: Legacy Effects of Biogenic Structure on Wind-Generated Ecosystem Processes Following Mussel Bed Collapse
Blue mussels (Mytilus edulis) are ecosystem engineers with strong effects on species diversity and abundances. Mussel beds appear to be declining in the Gulf of Maine, apparently due to climate change and predation by the invasive green crab, Carcinus maenas. As mussels die, they create a legacy of large expanses of shell biogenic structure. In Maine, USA, we used bottom traps to examine effects of four bottom cover types (i.e., live mussels, whole shells, fragmented shells, bare sediment) and wind condition (i.e., days with high, intermediate, and low values) on flow-related ecosystem processes. Significant differences in transport of sediment, meiofauna, and macrofauna were found among cover types and days, with no significant interaction between the two factors. Wind condition had positive effects on transport. Shell hash, especially fragmented shells, had negative effects, possibly because it acted as bed armor to reduce wind-generated erosion and resuspension. Copepods had the greatest mobility and shortest turnover times (0.15 d), followed by nematodes (1.96 d) and the macrofauna dominant, Tubificoides benedeni (2.35 d). Shell legacy effects may play an important role in soft-bottom system responses to wind-generated ecosystem processes, particularly in collapsed mussel beds, with implications for recolonization, connectivity, and the creation and maintenance of spatial pattern
A study of methanol and silicon monoxide production through episodic explosions of grain mantles in the Central Molecular Zone
Methanol (CHOH) is found to be abundant and widespread towards the
Central Molecular Zone, the inner few hundred parsecs of our Galaxy. Its origin
is, however, not fully understood. It was proposed that the high cosmic ray
ionisation rate in this region could lead to a more efficient non-thermal
desorption of this species formed on grain surfaces, but it would also mean
that this species is destroyed in a relatively short timescale. In a first
step, we run chemical models with a high cosmic ray ionisation rate and find
that this scenario can only reproduce the lowest abundances of methanol derived
in this region (10-10). In a second step, we investigate
another scenario based on episodic explosions of grain mantles. We find a good
agreement between the predicted abundances of methanol and the observations. We
find that the dominant route for the formation of methanol is through
hydrogenation of CO on the grains followed by the desorption due to the grain
mantle explosion. The cyclic aspect of this model can explain the widespread
presence of methanol without requiring any additional mechanism. We also model
silicon monoxide (SiO), another species detected in several molecular clouds of
the Galactic Centre. An agreement is found with observations for a high
depletion of Si (Si/H 10) with respect to the solar abundance.Comment: Accepted in MNRA
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