6,749 research outputs found
Mixing Time Scales in a Supernova-Driven Interstellar Medium
We study the mixing of chemical species in the interstellar medium (ISM).
Recent observations suggest that the distribution of species such as deuterium
in the ISM may be far from homogeneous. This raises the question of how long it
takes for inhomogeneities to be erased in the ISM, and how this depends on the
length scale of the inhomogeneities. We added a tracer field to the
three-dimensional, supernova-driven ISM model of Avillez (2000) to study mixing
and dispersal in kiloparsec-scale simulations of the ISM with different
supernova (SN) rates and different inhomogeneity length scales. We find several
surprising results. Classical mixing length theory fails to predict the very
weak dependence of mixing time on length scale that we find on scales of
25--500 pc. Derived diffusion coefficients increase exponentially with time,
rather than remaining constant. The variance of composition declines
exponentially, with a time constant of tens of Myr, so that large differences
fade faster than small ones. The time constant depends on the inverse square
root of the supernova rate. One major reason for these results is that even
with numerical diffusion exceeding physical values, gas does not mix quickly
between hot and cold regions.Comment: 23 pages, 14 figures that include 7 simulation images and 19 plots,
accepted for publication at Ap
Citrate and isocitrate in plant metabolism
The relevance of citrate and isocitrate metabolism in plants is discussed in connection with the different pathways for
their conversions. The routes for citrate and isocitrate conversions are incorporated into the system of cross-linked metabolic
processes and may provide carbon skeletons for nitrogen assimilation and reducing equivalents for biosynthetic reactions,
support the functioning of the glyoxylate cycle and play an important role in the TCA and energy metabolism as a whole.
The possibility of the coupling of citrate and isocitrate metabolism with various electron transport systems is discussed from
the point of view of the efficiency of the balancing cellular NAD P H Ž . Ž . rNAD P and ATP q rADP ratios. The role of citrate
and isocitrate and their derivations as potent effectors of some enzymes is considered. Special attention is paid to the
enzymes associated with citrate and isocitrate metabolism and to the mechanisms which regulate their activity. The
possibilities of the coordination of the main processes of energy and biosynthetic metabolism at the level of citrate and
isocitrate distribution are discussed.The authors are grateful to Dr., Professor, Academician of the Russian Academy of Sciences, V.P.
ŽSkulachev Moscow State University, Institute of.
Molecular Biology for carefully and critically reading the manuscript and stimulating discussions. We
also thank Dr. Teresa M. Santos for her help during
the preparation of the manuscript. The work of Prof.
Tatyana N. Popova in Madeira University was supported by the grant no. 9r96r1r0618 of the Portuguese Foundation for Support of the Scientific
Community JNICT and the Madeiran Scientific and Ž .
Technological Centre CITMA . Ž .info:eu-repo/semantics/publishedVersio
Heat exchange between two interacting nanoparticles beyond the fluctuation-dissipation regime
We show that the observed non-monotonic behavior of the thermal conductance
between two nanoparticles when they are brought into contact is originated by
an intricate phase space dynamics. Here it is assumed that this dynamics
results from the thermally activated jumping through a rough energy landscape.
A hierarchy of relaxation times plays the key role in the description of this
complex phase space behaviour. Our theory enables us to analyze the heat
transfer just before and at the moment of contact.Comment: 4 pages, 1 figure, approved for publication in Physical Review
Letter
Recovery of substrates and macro-benthos after fishing trials with a new Portuguese clam dredge
The effect of dredging on bottom structure was assessed, to estimate the damage inflicted on the benthic macrofauna left on the dredge path and to evaluate aggregations of scavengers within the track. Sediment
suspended during dredging rapidly resettled both on sand and sandy-mud bottoms. Dredge tracks were deeper on sandy-mud sediments and persisted longer than in sandy sediments. The disappearance of tracks in both sediment types depended both on current strength and wave action. Damage and mortality induced by dredging on the macrobenthic animals left on the dredge path was relatively low. Post-fishing diver observations showed that damaged animals left on the dredge path rapidly attracted scavengers, mainly Ophiura albida. Immediately after the tow ophiuras reached densities 8 times greater in the track
region than in the background. However, they dispersed again rapidly because only few damaged fauna were available on the dredge tracks. Undamaged or slightly damaged shellfish started to rebury immediately
after escaping from the dredge
Stellar rotational periods in the planet hosting open cluster Praesepe
By using the dense coverage of the extrasolar planet survey project HATNet,
we Fourier analyze 381 high-probability members of the nearby open cluster
Praesepe (Beehive/M44/NGC 2632). In addition to the detection of 10 variables
(of \delta Scuti and other types), we identify 180 rotational variables
(including the two known planet hosts). This sample increases the number of
known rotational variables in this cluster for spectral classes earlier than M
by more than a factor of three. These stars closely follow a color/magnitude --
period relation from early F to late K stars. We approximate this relation by
polynomials for an easier reference to the rotational characteristics in
different colors. The total (peak-to-peak) amplitudes of the large majority
(94%) of these variables span the range of 0.005 to 0.04 mag. The periods cover
a range from 2.5 to 15 days. These data strongly confirm that Praesepe and the
Hyades have the same gyrochronological ages. Regarding the two planet hosts,
Pr0211 (the one with the shorter orbital period) has a rotational period that
is ~2 days shorter than the one expected from the main rotational pattern in
this cluster. This, together with other examples discussed in the paper, may
hint that star-planet interaction via tidal dissipation can be significant in
some cases in the rotational evolution of stars hosting Hot Jupiters.Comment: 17 pages, 13 figures, 5 tables; accepted for publication in MNRA
Stability of the Duality Gap in Linear Optimization
In this paper we consider the duality gap function g that measures the difference between the optimal values of the primal problem and of the dual problem in linear programming and in linear semi-infinite programming. We analyze its behavior when the data defining these problems may be perturbed, considering seven different scenarios. In particular we find some stability results by proving that, under mild conditions, either the duality gap of the perturbed problems is zero or + ∞ around the given data, or g has an infinite jump at it. We also give conditions guaranteeing that those data providing a finite duality gap are limits of sequences of data providing zero duality gap for sufficiently small perturbations, which is a generic result.This research was partially supported by MINECO of Spain and FEDER of EU, Grant MTM2014-59179-C2-01 and SECTyP-UNCuyo Res. 4540/13-R
Review. The evolution of research regarding the economics of irrigation water
This work analyses the main research trends (subjects, methodology used, countries of the authors and data) in the
economics of irrigation water during the last 10 years (2000-2009). For this purpose, a quantitative methodology has
been used which is new to this sphere, based on the review of a representative sample of 332 papers published in the
15 most important journals focused on this field of science indexed in the databases of the Institute for Scientific
Information (ISI), the Science Citation Index (SCI) and the Social Science Citation Index (SCCI). The results obtained
confirm: a) the notable growth in the number of papers published, especially in the last three years, b) the high degree
of collaboration between authors, including those of different origin, for their performance c) the prominence of the
USA, Australia, India and Spain as the countries of the first authors and origin of the data, d) the greater attention
given to subjects related with «investment project analysis», «production planning» and, especially, «production
function and productivity of water», and e) the predominance of empirical studies that use basic analysis approaches
(cost analysis, investment evaluation, etc.).El presente trabajo analiza las principales tendencias de investigación (temáticas, metodologías utilizadas, países de
los autores y de los datos) en economía del agua de riego en los últimos 10 años (2000-2009). Para ello, se ha utilizado
una metodología cuantitativa, novedosa en este ámbito, basada en la revisión de una muestra representativa de 332
artículos publicados en las 15 revistas más importantes en este ámbito científico, indexadas en las bases de datos del
Institute for Scientific Information (ISI), el Science Citation Index (SCI) y el Social Science Citation Index (SCCI). Los
resultados obtenidos confirman: a) el notable crecimiento del número de artículos publicados, sobre todo, en los tres
últimos años, b) la elevada colaboración entre los autores para su realización, incluso de diferente procedencia, c) el
protagonismo de EEUU, Australia, India y España como países de los primeros autores y de procedencia de los datos,
d) la mayor atención prestada a los temas relacionados con el «análisis de proyectos de inversión», la «planificación de
la producción» y, especialmente, a las «funciones de producción y la productividad del agua», y e) el predominio de
los trabajos empíricos que utilizan técnicas de análisis básicas (análisis de costes, evaluación de inversiones, etc.)
Selected Applications of Linear Semi-Infinite Systems Theory
In this paper we, firstly, review the main known results on systems of an arbitrary (possibly infinite) number of weak linear inequalities posed in the Euclidean space Rn (i.e., with n unknowns), and, secondly, show the potential power of this theoretical tool by developing in detail two significant applications, one to computational geometry: the Voronoi cells, and the other to mathematical analysis: approximate subdifferentials, recovering known results in both fields and proving new ones. In particular, this paper completes the existing theory of farthest Voronoi cells of infinite sets of sites by appealing to well-known results on linear semi-infinite systems.This research was partially supported by PGC2018-097960-B-C22 of the Ministerio de Ciencia, Innovación y Universidades (MCIU), the Agencia Estatal de Investigación (AEI), and the European Regional Development Fund (ERDF); by CONICET, Argentina, Res D No 4198/17; and by Universidad Nacional de Cuyo, Secretaría de Investigación, Internacionales y Posgrado (SIIP), Res. 3922/19-R, Cod.06/D227, Argentina
The Voronoi inverse mapping
Given an arbitrary set T in the Euclidean space whose elements are called sites, and a particular site s, the Voronoi cell of s , denoted by VT(s)VT(s), consists of all points closer to s than to any other site. The Voronoi mapping of s , denoted by ψsψs, associates to each set T∋sT∋s the Voronoi cell VT(s)VT(s) of s w.r.t. T . These Voronoi cells are solution sets of linear inequality systems, so they are closed convex sets. In this paper we study the Voronoi inverse problem consisting in computing, for a given closed convex set F∋sF∋s, the family of sets T∋sT∋s such that ψs(T)=Fψs(T)=F. More in detail, the paper analyzes relationships between the elements of this family, ψs−1(F), and the linear representations of F , provides explicit formulas for maximal and minimal elements of ψs−1(F), and studies the closure operator that assigns, to each closed set T containing s , the largest element of ψs−1(F), where F=VT(s)F=VT(s).This work has been supported by MINECO of Spain and ERDF from EC, Grants MTM2011-29064-C03-01, MTM2011-29064-C03-02, Australian Research Council’s Discovery Projects funding scheme (project number DP140103213), Severo Ochoa Programme for Centres of Excellence in R&D (SEV-2015-0563), and SECTyP-UNCuyo, Argentina, Grant Res. 4540/13-R. The second author is affiliated to MOVE (Markets, Organizations and Votes in Economics)
Biogeographic history and cryptic diversity of saxicolous Tropiduridae lizards endemic to the semiarid Caatinga
Background: Phylogeographic research has advanced in South America, with increasing efforts on taxa from the dry diagonal biomes. However, the diversification of endemic fauna from the semiarid Caatinga biome in northeastern Brazil is still poorly known. Here we targeted saxicolous lizards of the Tropidurus semitaeniatus species group to better understand the evolutionary history of these endemic taxa and the Caatinga. We estimated a time-calibrated phylogeny for the species group based on two mitochondrial and two nuclear genes and jointly estimated the species limits and species tree within the group. We also devoted a denser phylogeographic sampling of the T. semitaeniatus complex to explore migration patterns, and the spatiotemporal diffusion history to verify a possible role of the São Francisco River as a promoter of differentiation in this saxicolous group of lizards. Results: Phylogenetic analysis detected high cryptic genetic diversity, occurrence of unique microendemic lineages associated with older highlands, and a speciation history that took place during the Pliocene-Pleistocene transition. Species delimitation detected five evolutionary entities within the T. semitaeniatus species group, albeit with low support. Thus, additional data are needed for a more accurate definition of species limits and interspecific relationships within this group. Spatiotemporal analyses reconstructed the geographic origin of the T. semitaeniatus species complex to be located north of the present-day course of the São Francisco River, followed by dispersal that expanded its distribution towards the northwest and south. Gene flow estimates showed higher migration rates into the lineages located north of the São Francisco River. Conclusions: The phylogenetic and population structures are intrinsically associated with stable rock surfaces and landscape rearrangements, such as the establishment of drainage basins located to the northern and southern distribution ranges. The T. semitaeniatus complex preserved high genetic diversity during range expansion, possibly as a result of frequent long-distance dispersal events. Our results indicate that both the current course of the São Francisco River and its paleo-courses had an important role in promoting diversification of the Caatinga endemic T. semitaeniatus species group
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