726 research outputs found
Experimental evidence of delocalized states in random dimer superlattices
We study the electronic properties of GaAs-AlGaAs superlattices with
intentional correlated disorder by means of photoluminescence and vertical dc
resistance. The results are compared to those obtained in ordered and
uncorrelated disordered superlattices. We report the first experimental
evidence that spatial correlations inhibit localization of states in disordered
low-dimensional systems, as our previous theoretical calculations suggested, in
contrast to the earlier belief that all eigenstates are localized.Comment: 4 pages, 5 figures. Physical Review Letters (in press
Conditioned place avoidance in the planaria Schmidtea mediterranea: A pre-clinical invertebrate model of anxiety-related disorders
The objective of the present study was to develop a model of avoidance learning and its extinction in planarians (Schmidtea mediterranea). Based on previous experiments showing conditioned place preference, we developed a procedure to investigate conditioned place avoidance (CPA) using shock as an unconditioned stimulus (US) and an automated tracking system to record the animals’ behaviour. In Experiment 1, we assessed the unconditioned properties of different shock intensities by measuring post shock activity. In two subsequent experiments we investigated CPA using different designs, surfaces as conditioned stimuli (CSs; rough and smooth), and different US intensities (5V and 10V). In general, we observed the successful development of CPA. However, CPA was stronger with higher shock intensities, and we found that, in our preparation, a rough surface is best at entering into an association with the shock than a smooth surface. Finally, we also observed extinction of CPA. The evidence of CPA and its extinction in flatworms validates the planaria as a pre-clinical model for the study of avoidance learning, a hallmark of anxiety disorders
Evolving temporal association rules with genetic algorithms
A novel framework for mining temporal association rules by discovering itemsets with a genetic algorithm is introduced. Metaheuristics have been applied to association rule mining, we show the efficacy of extending this to another variant - temporal association rule mining. Our framework is an enhancement to existing temporal association rule mining methods as it employs a genetic algorithm to simultaneously search the rule space and temporal space. A methodology for validating the ability of the proposed framework isolates target temporal itemsets in synthetic datasets. The Iterative Rule Learning method successfully discovers these targets in datasets with varying levels of difficulty
Isolation and analysis of phospholipids in dairy foods
The lipid fraction of milk is one of the most complex matrixes in foodstuffs due to the presence of a high number of moieties with
different physical and chemical properties. Glycerolipids include glycerol and two fatty acids esterified in positions sn-1 and sn-2
with higher concentration of unsaturated fatty acids than in the triglyceride fraction of milk. Sphingolipids consist of a sphingoid
base linked to a fatty acid across an amide bond.Their amphiphilic nature makes them suitable to be added into a variety of foods
and recent investigations showthat phospholipids,mainly phosphatidylserine and sphingomyelin, can exert antimicrobial, antiviral,
and anticancer activities as well as positive effects in Alzheimer’s disease, stress, and memory decline. Polar lipids can be found as
natural constituents in the membranes of all living organisms with soybean and eggs as the principal industrial sources, yet they
have low contents in phosphatidylserine and sphingomyelin. Animal products are rich sources of these compounds but since there
are legal restrictions to avoid transmission of prions,milk and dairy products are gaining interest as alternative sources.This review
summarizes the analysis of polar lipids in dairy products including sample preparation (extraction and fractionation/isolation) and
analysis by GC or HPLC and the latest research works using ELSD, CAD, and MS detectors.info:eu-repo/semantics/publishedVersio
X-Shooter spectroscopy of young stellar objects in Lupus: Accretion properties of class II and transitional objects
We present the results of a study of the stellar and accretion properties of
the (almost) complete sample of class II and transitional YSOs in the Lupus I,
II, III and IV clouds, based on spectroscopic data acquired with the
VLT/X-Shooter spectrograph. Our study combines the dataset from our previous
work with new observations of 55 additional objects. We have investigated 92
YSO candidates in total, 11 of which have been definitely identified with giant
stars unrelated to Lupus. The stellar and accretion properties of the 81 bona
fide YSOs, which represent more than 90% of the whole class~II and transition
disc YSO population in the aforementioned Lupus clouds, have been homogeneously
and self-consistently derived, allowing for an unbiased study of accretion and
its relationship with stellar parameters. The accretion luminosity, Lacc,
increases with the stellar luminosity, Lstar, with an overall slope of ~1.6,
similar but with a smaller scatter than in previous studies. There is a
significant lack of strong accretors below Lstar~0.1Lsun, where Lacc is always
lower than 0.01Lstar. We argue that the Lacc-Lstar slope is not due to
observational biases, but is a true property of the Lupus YSOs. The
logMacc-logMstar correlation shows a statistically significant evidence of a
break, with a steeper relation for Mstar<0.2Msun and a flatter slope for higher
masses. The bimodality of the Macc-Mstar relation is confirmed with four
different evolutionary models used to derive the stellar mass. The bimodal
behaviour of the observed relationship supports the importance of modelling
self-gravity in the early evolution of the more massive discs, but other
processes, such as photo evaporation and planet formation during the YSO's
lifetime, may also lead to disc dispersal on different timescales depending on
the stellar mass. We also refined the empirical Lacc vs. Lline relationships.Comment: 43 pages, 22 figure
The Stellar Population of the Chamaeleon I Star-Forming Region
I present a new census of the stellar population in the Chamaeleon I
star-forming region. Using optical and near-IR photometry and followup
spectroscopy, I have discovered 50 new members of Chamaeleon I, expanding the
census of known members to 226 objects. Fourteen of these new members have
spectral types later than M6, which doubles the number of known members that
are likely to be substellar. I have estimated extinctions, luminosities, and
effective temperatures for the known members, used these data to construct an
H-R diagram for the cluster, and inferred individual masses and ages with the
theoretical evolutionary models of Baraffe and Chabrier. The distribution of
isochronal ages indicates that star formation began 3-4 and 5-6 Myr ago in the
southern and northern subclusters, respectively, and has continued to the
present time at a declining rate. The IMF in Chamaeleon I reaches a maximum at
a mass of 0.1-0.15 M_sun, and thus closely resembles the IMFs in IC 348 and the
Orion Nebula Cluster. In logarithmic units where the Salpeter slope is 1.35,
the IMF is roughly flat in the substellar regime and shows no indication of
reaching a minimum down to a completeness limit of 0.01 M_sun. The low-mass
stars are more widely distributed than members at other masses in the northern
subcluster, but this is not the case in the southern subcluster. Meanwhile, the
brown dwarfs have the same spatial distribution as the stars out to a radius of
3 deg (8.5 pc) from the center of Chamaeleon I
Cold Disks: Spitzer Spectroscopy of Disks around Young Stars with Large Gaps
We have identified four circumstellar disks with a deficit of dust emission
from their inner 15-50 AU. All four stars have F-G spectral type, and were
uncovered as part of the Spitzer Space Telescope ``Cores to Disks'' Legacy
Program Infrared Spectrograph (IRS) first look survey of ~100 pre-main sequence
stars. Modeling of the spectral energy distributions indicates a reduction in
dust density by factors of 100-1000 from disk radii between ~0.4 and 15-50 AU,
but with massive gas-rich disks at larger radii. This large contrast between
the inner and outer disk has led us to use the term `cold disks' to distinguish
these unusual systems. However, hot dust [0.02-0.2 Mmoon] is still present
close to the central star (R ~0.8 AU). We introduce the 30/13 micron, flux
density ratio as a new diagnostic for identifying cold disks. The mechanisms
for dust clearing over such large gaps are discussed. Though rare, cold disks
are likely in transition from an optically thick to an optically thin state,
and so offer excellent laboratories for the study of planet formation.Comment: 13 pages, 3 figures, accepted to ApJ
A Three Micron Survey of the Chamaeleon I Dark Cloud
We describe an L-band photometric survey of 0.5 square deg of the Cha I dark
cloud. The survey has a completeness limit of L < 11.0. Our survey detects 124
sources, including all known pre-main sequence stars with L < 11. The fraction
of sources with near-IR excess emission is 58% +- 4% for K = 9-11. Cha I
sources have bluer H-K and K-L colors than pre-main sequence stars in
Taurus-Auriga. These sources also have a strong correlation between EW(H-alpha)
and K-L. Stars with K-L 0.6
have strong H-alpha emission. Because many Cha I sources are heavily reddened,
this division between weak emission T Tauri stars and classical T Tauri stars
occurs at a redder K-L than in Taurus-Auriga.Comment: 12 pages of text, 4 figures, and 1 three page table of data modified
version adds reference and acknowledgemen
The role of marine reserves in achieving sustainable fisheries (One contribution of 15 to a Theme Issue 'Fisheries: a Future?')
Many fishery management tools currently in use have conservation value. They are designed to maintain stocks of commercially important species above target levels. However, their limitations are evident from continuing declines in fish stocks throughout the world. We make the case that to reverse fishery declines, safeguard marine life and sustain ecosystem processes, extensive marine reserves that are off limits to fishing must become part of the management strategy. Marine reserves should be incorporated into modern fishery management because they can achieve many things that conventional tools cannot. Only complete and permanent protection from fishing can protect the most sensitive habitats and vulnerable species. Only reserves will allow the development of natural, extended age structures of target species, maintain their genetic variability and prevent deleterious evolutionary change from the effects of fishing. Species with natural age structures will sustain higher rates of reproduction and will be more resilient to environmental variability. Higher stock levels maintained by reserves will provide insurance against management failure, including risk-prone quota setting, provided the broader conservation role of reserves is firmly established and legislatively protected. Fishery management measures outside protected areas are necessary to complement the protection offered by marine reserves, but cannot substitute for it
Intracluster stars in the Virgo cluster core
We have investigated the properties of the diffuse light in the Virgo cluster
core region, based on the detection of intracluster planetary nebulae (PNe) in
four fields. We eliminate the bias from misclassified faint continuum objects,
using improved Monte Carlo simulations, and the contaminations by high redshift
Ly galaxies, using the Ly luminosity function in blank fields.
Recent spectroscopic observations confirm that our photometric PN samples are
well-understood. We find that the diffuse stellar population in the Virgo core
region is inhomogeneous on scales of 30'-90': there exist significant
field-to-field variations in the number density of PNe and the inferred amount
of intracluster light, with some empty fields, some fields dominated by
extended Virgo galaxy halos, and some fields dominated by the true intracluster
component. There is no clear trend with distance from M87. The mean surface
luminosity density, its rms variation, and the mean surface brightness of
diffuse light in our 4 fields are L
arcmin, L arcmin, and
mag arcsec respectively. Our results indicate that
the Virgo cluster is a dynamically young environment, and that the intracluster
component is associated at least partially with local physical processes like
galaxy interactions or harassment. We also argue, based on kinematic evidence,
that the so-called 'over-luminous' PNe in the halo of M84 are dynamically
associated with this galaxy, and must thus be brighter than and part of a
different stellar population from the normal PN population in elliptical
galaxies.Comment: 31 pages, 6 figure. In press on the Astronomical Journa
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