1,738 research outputs found
Gravitational multipole moments from Noether charges
We define the mass and current multipole moments for an arbitrary theory of
gravity in terms of canonical Noether charges associated with specific residual
transformations in canonical harmonic gauge, which we call multipole
symmetries. We show that our definition exactly matches Thorne's mass and
current multipole moments in Einstein gravity, which are defined in terms of
metric components. For radiative configurations, the total multipole charges --
including the contributions from the source and the radiation -- are given by
surface charges at spatial infinity, while the source multipole moments are
naturally identified by surface integrals in the near-zone or, alternatively,
from a regularization of the Noether charges at null infinity. The conservation
of total multipole charges is used to derive the variation of source multipole
moments in the near-zone in terms of the flux of multipole charges at null
infinity.Comment: v1: 22 pages + 13 pages of appendices, 1 figure; v2: published
version in JHE
The First Stars: formation under X-ray feedback
We investigate the impact of a cosmic X-ray background (CXB) on Population
III stars forming in a minihalo at . Using the smoothed particle
hydrodynamics code GADGET-2, we attain sufficient numerical resolution to
follow gas collapsing into the centre of the minihalo from cosmological initial
conditions up to densities of , at which point we form
sink particles. This allows us to study how the presence of a CXB affects the
formation of H and HD in the gas prior to becoming fully molecular. Using a
suite of simulations for a range of possible CXB models, we follow each
simulation for 5000\yr after the first sink particle forms. The CXB provides
two competing effects, with X-rays both heating the gas and increasing the free
electron fraction, allowing more H to form. X-ray heating dominates below
, while the additional H cooling becomes more
important above . The gas becomes optically thick to
X-rays as it exits the quasi-hydrostatic `loitering phase,' such that the
primary impact of the CXB is to cool the gas at intermediate densities,
resulting in an earlier onset of baryonic collapse into the dark matter halo.
At the highest densities, self-shielding results in similar thermodynamic
behaviour across a wide range of CXB strengths. Consequently, we find that star
formation is relatively insensitive to the presence of a CXB; both the number
and the characteristic mass of the stars formed remains quite similar even as
the strength of the CXB varies by several orders of magnitude.Comment: Accepted for publication in MNRAS. Includes improved treatment of
X-ray optical depth. 13 pages, 12 figure
Alx1, a member of the Cart1/Alx3/Alx4 subfamily of Paired-class homeodomain proteins, is an essential component of the gene network controlling skeletogenic fate specification in the sea urchin embryo
In the sea urchin embryo, the large micromeres and their progeny function as a critical signaling center and execute a complex morphogenetic program. We have identified a new and essential component of the gene network that controls large micromere specification, the homeodomain protein Alx1. Alx1 is expressed exclusively by cells of the large micromere lineage beginning in the first interphase after the large micromeres are born. Morpholino studies demonstrate that Alx1 is essential at an early stage of specification and controls downstream genes required for epithelial-mesenchymal transition and biomineralization. Expression of Alx1 is cell autonomous and regulated maternally through Ć-catenin and its downstream effector, Pmar1. Alx1 expression can be activated in other cell lineages at much later stages of development, however, through a regulative pathway of skeletogenesis that is responsive to cell signaling. The Alx1 protein is highly conserved among euechinoid sea urchins and is closely related to the Cart1/Alx3/Alx4 family of vertebrate homeodomain proteins. In vertebrates, these proteins regulate the formation of skeletal elements of the limbs, face and neck. Our findings suggest that the ancestral deuterostome had a population of biomineral-forming mesenchyme cells that expressed an Alx1-like protein
Expression of skeletogenic genes during arm regeneration in the brittle star Amphiura filiformis.
The brittle star Amphiura filiformis, which regenerates its arms post autotomy, is emerging as a useful model for studying the molecular underpinnings of regeneration, aided by the recent availability of some molecular resources. During regeneration a blastema initially is formed distally to the amputation site, and then a rapid rebuild is obtained by adding metameric units, which will eventually differentiate and become fully functional. In this work we first characterize the developmental process of the regenerating arms using two differentiation markers for muscle and skeletal structures - Afi-trop-1 and Afi-Ī±coll. Both genes are not expressed in the blastema and newly added undifferentiated metameric units. Their expression at different regenerating stages shows an early segregation of muscle and skeletal cells during the regenerating process, long before the metameric units become functional. We then studied the expression of a set of genes orthologous of the sea urchin transcription factors involved in the development of skeletal and non-skeletal mesoderm: Afi-ets1/2, Afi-alx1, Afi-tbr, Afi-foxB and Afi-gataC. We found that Afi-ets1/2, Afi-alx1, Afi-foxB and Afi-gataC are all expressed at the blastemal stage. As regeneration progresses those genes are expressed in a similar small undifferentiated domain beneath the distal growth cap, while in more advanced metameric units they become restricted to different skeletal domains. Afi-foxB becomes expressed in non-skeletal structures. This suggests that they might play a combinatorial role only in the early cell specification process and that subsequently they function independently in the differentiation of different structures. Afi-tbr is not present in the adult arm tissue at any stage of regeneration. In situ hybridization results have been confirmed with a new strategy for quantitative PCR (QPCR), using a subdivision of the three stages of regeneration into proximal (differentiated) and distal (undifferentiated) arm segments
Changes in behavioural response of Mediterranean Seabass (Dicenthratus labrax L.) under different feeding distributions
Captive-induced behavioural deviations may
involve many aspects of fish behaviour such as
swimming activity and enhancement of individual
aggressiveness. We studied seabass
(Dicentrarchus labrax) behaviour as a function
of manual and automatic feeding distribution
modes. Under manual mode, the food is distributed
over an extended area for a longer period,
and its precise location is not always predictable,
while with pneumatic automatic feeders,
fish receive the same amount of resource,
which is concentrated in the same surface area
over a shorter period. We compared seabass
behaviour under automatic and manual conditions
collecting video image recordings before,
during, and after feeding distribution, in the
morning and in afternoon, on two different
days, and analysing data within independent
sessions of measurements. Feeding modes significantly
affected swimming behaviour: automatically-
fed fish were characterised by vertical
movements through the water column (towards
the surface and bottom) and by horizontal
swimming. Manually-fed fish were instead
characterised by sharp direction changes during
their swimming, mostly towards the surface.
Feeding distribution induced changes in
collision frequency and elicited aggressive
behaviour. In particular, agonistic behaviour
(i.e. a fish attacks another fish) was almost
exclusively recorded during the feeding under
automatic distribution, whereas it was constantly
expressed during all the distribution
phases under manual mod
Mathematical Patterns and Cognitive Architectures
Mathematical patterns are an important subclass of the class of patterns. The main task of this paper is examining a particular proposal concerning the nature of mathematical patterns and some elements of the cognitive architecture an agent should have to recognize them
On Representing Concepts in High-dimensional Linear Spaces
Producing a mathematical model of concepts is a very important issue
in artificial intelligence, because if such a model were found this, besides being
a very interesting result in its own right, would also contribute to the emergence
of what we could call the \u2018mathematics of thought.\u2019 One of the most interesting
attempts made in this direction is P. Gardenfors\u2019 theory of conceptual spaces, a \ua8
theory which is mostly presented by its author in an informal way. The main aim
of the present article is contributing to Gardenfors\u2019 theory of conceptual spaces \ua8
by discussing some of the advantages which derive from the possibility of representing
concepts in high-dimensional linear spaces
Application of Biotests for the Determination of Soil Ecotoxicity after Exposure to Biodegradable Plastics
Biodegradable plastics are mostly applied in packaging materials (e.g., shopping bags), waste collection bags, catering products, and agricultural applications. In this last case, degradation takes place directly in soil where biodegradable plastic products are intentionally left after use (e.g., mulch films for weeds control). Due to the growing volumes of biodegradable polymers and plastics, interest in their environmental safety is increasing and more research is carried out. Some attempt has been made to apply biotests, used in other sectors of environmental sciences, in the assessment of biodegradable plastics safety. In this work, the quality of soils after biodegradation of the bioplastics Mater-Bi has been assessed with a large array of biotests based on model organisms representative of the different trophic levels in the food chains of the edaphic and aquatic ecosystems. Mater-Bi was degraded under controlled conditions for 6 months at a 1% concentration. The selected organisms included bacteria and protozoa (Vibrio fischeri and Dictyostelium discoideum, respectively), the green alga Pseudokirchneriella subcapitata, plants (the monocotyledon Sorghum saccharatum and the dicotyledon Lepidium sativum), and invertebrates animals (Daphnia magna, a freshwater crustacean, and the Oligochaeta earthworm Eisenia andrei), using both acute and chronic endpoints. The results of the applied ecotoxicological tests showed that the Mater-Bi materials tested at very high doses did not affect the soil quality. Soil exposed to Mater-Bi has no noxious effects on edaphic organisms; in particular, mono and dicotyledon plants results, indicate that Mater-Bi plastic products are innocuous for agricultural uses. The use of more sensitive chronic endpoints allows to exclude possible effects at population level. This is the first time that such a comprehensive approach is applied to the assessment of possible ecotoxicity effects induced by biodegradable plastics in soil and represents a possible starting point for improved standardized testing schemes
Creativity in Conceptual Spaces
The main aim of this paper is contributing to what in the last few years has been known as computational creativity. This will be done by showing the relevance of a particular mathematical representation of G"ardenfors's conceptual spaces to the problem of modelling a phenomenon which plays a central role in producing novel and fruitful representations of perceptual patterns: analogy
Ultrastructural and molecular analysis of the origin and differentiation of cells mediating brittle star skeletal regeneration
Background: Regeneration is the ability to re-grow body parts or tissues after trauma, and it is widespread across
metazoans. Cells involved in regeneration can arise from a pool of undifferentiated proliferative cells or be recruited
from pre-existing differentiated tissues. Both mechanisms have been described in different phyla; however, the
cellular and molecular mechanisms employed by different animals to restore lost tissues as well as the source of
cells involved in regeneration remain largely unknown. Echinoderms are a clade of deuterostome invertebrates that
show striking larval and adult regenerative abilities in all extant classes. Here, we use the brittle star Amphiura
filiformis to investigate the origin and differentiation of cells involved in skeletal regeneration using a combination
of microscopy techniques and molecular markers.
Results: Our ultrastructural analyses at different regenerative stages identify a population of morphologically
undifferentiated cells which appear in close contact with the proliferating epithelium of the regenerating aboral
coelomic cavity. These cells express skeletogenic marker genes, such as the transcription factor alx1 and the
differentiation genes c-lectin and msp130L, and display a gradient of morphological differentiation from the aboral
coelomic cavity towards the epidermis. Cells closer to the epidermis, which are in contact with developing spicules,
have the morphology of mature skeletal cells (sclerocytes), and express several skeletogenic transcription factors
and differentiation genes. Moreover, as regeneration progresses, sclerocytes show a different combinatorial
expression of genes in various skeletal elements.
Conclusions: We hypothesize that sclerocyte precursors originate from the epithelium of the proliferating aboral
coelomic cavity. As these cells migrate towards the epidermis, they differentiate and start secreting spicules.
Moreover, our study shows that molecular and cellular processes involved in skeletal regeneration resemble those
used during skeletal development, hinting at a possible conservation of developmental programmes during adult
regeneration. Finally, we highlight that many genes involved in echinoderm skeletogenesis also play a role in
vertebrate skeleton formation, suggesting a possible common origin of the deuterostome endoskeleton pathway
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