7,534 research outputs found
Extraordinary absorption of decorated undoped graphene
We theoretically study absorption by an undoped graphene layer decorated with
arrays of small particles. We discuss periodic and random arrays within a
common formalism, which predicts a maximum absorption of for suspended
graphene in both cases. The limits of weak and strong scatterers are
investigated and an unusual dependence on particle-graphene separation is found
and explained in terms of the effective number of contributing evanescent
diffraction orders of the array. Our results can be important to boost
absorption by single layer graphene due to its simple setup with potential
applications to light harvesting and photodetection based on energy (F\"orster)
rather than charge transfer.Comment: 5 pages, 3 figure
Water dynamics in a laurel montane cloud forest in the Garajonay National Park (Canary Islands, Spain)
International audienceField measurements from February 2003 to January 2004 in a humid (but dry in summer) crest heath wood-land (degraded laurel forest) in the National Park of Garajonay, Canary Islands (Spain), were combined to calculate water balance components. The water balance domain is at the surface of the catchment and is controlled by atmospheric processes and vegetation. This study found that annual water income (rainfall plus fog water) was 1440 mm year-1, half of which was occult (or fog) precipitation, while stand transpiration estimated from measurements of sap flow amounted, annually, to 40% of potential evapotranspiration calculated from measurements of meteorological variables. The positive role of crest laurel forests, which transpire less water than is incoming from rain and fog is emphasised. Keywords: laurel forest, fog interception, stand transpiration, sap flow, hydrological cycl
Are beryllium abundances anomalous in stars with giant planets?
In this paper we present beryllium (Be) abundances in a large sample of 41
extra-solar planet host stars, and for 29 stars without any known
planetary-mass companion, spanning a large range of effective temperatures. The
Be abundances were derived through spectral synthesis done in standard Local
Thermodynamic Equilibrium, using spectra obtained with various instruments. The
results seem to confirm that overall, planet-host stars have ``normal'' Be
abundances, although a small, but not significant, difference might be present.
This result is discussed, and we show that this difference is probably not due
to any stellar ``pollution'' events. In other words, our results support the
idea that the high-metal content of planet-host stars has, overall, a
``primordial'' origin. However, we also find a small subset of planet-host
late-F and early-G dwarfs that might have higher than average Be abundances.
The reason for the offset is not clear, and might be related either to the
engulfment of planetary material, to galactic chemical evolution effects, or to
stellar-mass differences for stars of similar temperature.Comment: 15 pages, 9 figures, accepted for publication in Astronomy &
Astrophysic
Light elements in stars with exoplanets
It is well known that stars orbited by giant planets have higher abundances
of heavy elements when compared with average field dwarfs. A number of studies
have also addressed the possibility that light element abundances are different
in these stars. In this paper we will review the present status of these
studies. The most significant trends will be discussed.Comment: 10 pages, 6 figures. Submitted to the proceedings of IAU symposium
268: Light elements in the universe
Beryllium anomalies in solar-type field stars
We present a study of beryllium (Be) abundances in a large sample of field
solar-type dwarfs and sub-giants spanning a large range of effective
temperatures. The analysis shows that Be is severely depleted for F stars, as
expected by the light-element depletion models. However, we also show that
Beryllium abundances decrease with decreasing temperature for stars cooler than
6000 K, a result that cannot be explained by current theoretical models
including rotational mixing, but that is, at least in part, expected from the
models that take into account internal wave physics. In particular, the light
element abundances of the coolest and youngest stars in our sample suggest that
Be, as well as lithium (Li), has already been burned early during their
evolution. Furthermore, we find strong evidence for the existence of a Be-gap
for solar-temperature stars. The analysis of Li and Be abundances in the
sub-giants of our sample also shows the presence of one case that has still
detectable amounts of Li, while Be is severely depleted. Finally, we compare
the derived Be abundances with Li abundances derived using the same set of
stellar parameters. This gives us the possibility to explore the temperatures
for which the onset of Li and Be depletion occurs.Comment: 16 pages, 13 figures, accepted for publication in Astronomy &
Astrophysic
Nitrogen abundances in Planet-harbouring stars
We present a detailed spectroscopic analysis of nitrogen abundances in 91
solar-type stars, 66 with and 25 without known planetary mass companions. All
comparison sample stars and 28 planet hosts were analysed by spectral synthesis
of the near-UV NH band at 3360 \AA observed at high resolution with the
VLT/UVES,while the near-IR NI 7468 \AA was measured in 31 objects. These two
abundance indicators are in good agreement. We found that nitrogen abundance
scales with that of iron in the metallicity range -0.6 <[Fe/H]< +0.4 with the
slope 1.08 \pm 0.05. Our results show that the bulk of nitrogen production at
high metallicities was coupled with iron. We found that the nitrogen abundance
distribution in stars with exoplanets is the high [Fe/H] extension of the curve
traced by the comparison sample of stars with no known planets. A comparison of
our nitrogen abundances with those available in the literature shows a good
agreement.Comment: 15 pages, 7 figures, Accepted for publication in A&
Beryllium abundances in stars hosting giant planets
We have derived beryllium abundances in a wide sample of stars hosting
planets, with spectral types in the range F7V-K0V, aimed at studying in detail
the effects of the presence of planets on the structure and evolution of the
associated stars. Predictions from current models are compared with the derived
abundances and suggestions are provided to explain the observed
inconsistencies. We show that while still not clear, the results suggest that
theoretical models may have to be revised for stars with Teff<5500K. On the
other hand, a comparison between planet host and non-planet host stars shows no
clear difference between both populations. Although preliminary, this result
favors a ``primordial'' origin for the metallicity ``excess'' observed for the
planetary host stars. Under this assumption, i.e. that there would be no
differences between stars with and without giant planets, the light element
depletion pattern of our sample of stars may also be used to further
investigate and constraint Li and Be depletion mechanisms.Comment: A&A in press -- accepted on the 22/02/2002 (11 pages, 6 figures
included
The Role of Probiotics and Prebiotics in the Prevention and Treatment of Obesity
Obesity is a global pandemic complex to treat due to its multifactorial pathogenesis—an
unhealthy lifestyle, neuronal and hormonal mechanisms, and genetic and epigenetic factors are
involved. Scientific evidence supports the idea that obesity and metabolic consequences are strongly
related to changes in both the function and composition of gut microbiota, which exert an essential role
in modulating energy metabolism. Modifications of gut microbiota composition have been associated
with variations in body weight and body mass index. Lifestyle modifications remain as primary
therapy for obesity and related metabolic disorders. New therapeutic strategies to treat/prevent
obesity have been proposed, based on pre- and/or probiotic modulation of gut microbiota to mimic
that found in healthy non-obese subjects. Based on human and animal studies, this review aimed
to discuss mechanisms through which gut microbiota could act as a key modifier of obesity and
related metabolic complications. Evidence from animal studies and human clinical trials suggesting
potential beneficial effects of prebiotic and various probiotic strains on those physical, biochemical,
and metabolic parameters related to obesity is presented. As a conclusion, a deeper knowledge
about pre-/probiotic mechanisms of action, in combination with adequately powered, randomized
controlled follow-up studies, will facilitate the clinical application and development of personalized
healthcare strategies.Supported by funds from European Union 7th FP KBBE.2013.2.2-02—MyNewGut Project (“Factors
influencing the human gut microbiome and its effect on the development of diet-related diseases and brain
development”, Grant Agreement 613979) and from Spanish Ministry of Economy and Competitiveness GD-Brain
Projects (SAF2015-69265-c2.2)
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