502 research outputs found
Abundance ratios of red giants in low mass ultra faint dwarf spheroidal galaxies
Low mass dwarf spheroidal galaxies are key objects for our understanding of
the chemical evolution of the pristine Universe and the Local Group of
galaxies. Abundance ratios in stars of these objects can be used to better
understand their star formation and chemical evolution. We report on the
analysis of a sample of 11 stars belonging to 5 different ultra faint dwarf
spheroidal galaxies (UfDSph) based on X-Shooter spectra obtained at the VLT.
Medium resolution spectra have been used to determine the detailed chemical
composition of their atmosphere. We performed a standard 1D LTE analysis to
compute the abundances.
Considering all the stars as representative of the same population of low
mass galaxies, we found that the [alpha/Fe] ratios vs [Fe/H] decreases as the
metallicity of the star increases in a way similar to what is found for the
population of stars belonging to dwarf spheroidal galaxies. The main difference
is that the solar [alpha/Fe] is reached at a much lower metallicity for the
UfDSph than the dwarf spheroidal galaxies.
We report for the first time the abundance of strontium in CVnI. The star we
analyzed in this galaxy has a very high [Sr/Fe] and a very low upper limit of
barium which makes it a star with an exceptionally high [Sr/Ba] ratio.
Our results seem to indicate that the galaxies which have produced the bulk
of their stars before the reionization (fossil galaxies) have lower [X/Fe]
ratios at a given metallicity than the galaxies that have experienced a
discontinuity in their star formation rate (quenching).Comment: 22 pages, 12 figures, submitted to A&
Energy and economic analysis of a residential Solar Organic Rankine plant
To answer the actual energy, water, economic, social and environmental challenges, renewable, distributed power plants need to
be developed. Among renewables, solar tri-generative power plants can be a solution where there is big low temperature
heating/cooling demand and small electricity demand, like many residential and industrial utilities. In this case, solar thermal
plants can produce thermal energy with low cost and high efficiency. The higher temperature heat not needed by the user can be
exploited via Organic Rankine Cycle to produce electrical energy and desalinized water via reverse osmosis. The present paper
analyses, via TRNSYS simulation, a system composed of 50 m2
of CPC solar thermal collectors, 3 m3
of thermal storage, a
synthetic heat transfer fluid, 3 kWe ORC, 8 kWth absorber, 200 l/h direct reverse osmosis desalination device. The system is able
to produce power, heating/cooling and fresh water needs for a residential house. Although system’s components are well known
technologies, the integration to a efficient and economic working system is still a challenge. Global energy and economic
analyses have been performed. Low temperature heating/cooling terminals allow to increase not only the use of thermal energy
but also the ORCand absorber efficiency. ORC-Absorber configuration and relative fluids and temperatures are central.
Government support and/or cost reduction of 30% are necessary to have positive NPV and acceptable PBT and IR
Experimental tests to recover the photovoltaic power by battery system
The uncertainty and variability of the Renewable Energy Sources (RES) power plants within the power grid is an open issue. The
present study focuses on the use of batteries to overcome the limitations associated with the photovoltaic inverter operation,
trying to maximize the global energy produced. A set of switches, was placed between a few photovoltaic modules and a
commercial inverter, capable to change configuration of the plant dynamically. Such system stores the power that the inverter is
not able to let into the grid inside batteries. At the base of this optimization, there is the achievement of two main configurations
in which the batteries and the photovoltaic modules are electrically connected in an appropriate manner as a function of inverter
efficiency and thus solar radiation. A control board and the relative program, to change the configuration, was designed and
implemented, based on the value of the measured radiation, current, batteries voltage, and calculated inverter efficiency. Finally
from the cost and impact analysis we can say that, today the technology of lithium batteries, for this application, is still too
expensive in comparison with lead-acid batteries
Detailed abundances in stars belonging to ultra-faint dwarf spheroidal galaxies
We report preliminary results concerning the detailed chemical composition of
metal poor stars belonging to close ultra-faint dwarf galaxies (hereafter
UfDSphs). The abundances have been determined thanks to spectra obtained with
X-Shooter, a high efficiency spectrograph installed on one of the ESO VLT
units. The sample of ultra-faint dwarf spheroidal stars have abundance ratios
slightly lower to what is measured in field halo star of the same
metallicity.We did not find extreme abundances in our Hercules stars as the one
found by Koch for his 2 Hercules stars. The synthesis of the neutron capture
elements Ba and Sr seems to originate from the same nucleosynthetic process in
operation during the early stages of the galactic evolution.Comment: 3 pages, 1 figure; OMEG11 conference (Tokyo, Nov 2011
Abundances of lithium, oxygen, and sodium in the turn-off stars of Galactic globular cluster 47 Tuc
We aim to determine abundances of Li, O and Na in a sample of of 110 turn-off
(TO) stars, in order to study the evolution of light elements in this cluster
and to put our results in perspective with observations of other globular and
open clusters, as well as with field stars. We use medium resolution spectra
obtained with the GIRAFFE spectrograph at the ESO 8.2m Kueyen VLT telescope and
use state of the art 1D model atmospheres and NLTE line transfer to determine
the abundances. We also employ CO5BOLD hydrodynamical simulations to assess the
impact of stellar granulation on the line formation and inferred abundances.
Our results confirm the existence of Na-O abundance anti-correlation and hint
towards a possible Li-O anti-correlation in the TO stars of 47 Tuc. We find no
convincing evidence supporting the existence of Li-Na correlation. The obtained
3D NLTE mean lithium abundance in a sample of 94 TO stars where Li lines were
detected reliably,
dex, appears to be significantly lower than what is observed in other globular
clusters. At the same time, star-to-star spread in Li abundance is also larger
than seen in other clusters. The highest Li abundance observed in 47 Tuc is
about 0.1 dex lower than the lowest Li abundance observed among the un-depleted
stars of the metal-poor open cluster NGC 2243. The lithium abundances in 47
Tuc, when put into context with observations in other clusters and field stars,
suggest that stars that are more metal-rich than [FeH] \sim -1.0 experience
significant lithium depletion during their lifetime on the main sequence, while
the more metal-poor stars do not. Rather strikingly, our results suggest that
initial lithium abundance with which the star was created may only depend on
its age (the younger the star, the higher its Li content) and not on its
metallicity.Comment: 24 pages, 13 figures; discussion and conclusions expanded. Accepted
for publication in A&
High Resolution HDS/SUBARU chemical abundances of the young stellar cluster Palomar 1
Context. Palomar\,1 is a peculiar globular cluster (GC). It is the youngest
Galactic GC and it has been tentatively associated to several of the
substructures recently discovered in the Milky Way (MW), including the Canis
Major (CMa) overdensity and the Galactic Anticenter Stellar Structure (GASS).
Aims. In order to provide further insights into its origin, we present the
first high resolution chemical abundance analysis for one red giant in Pal\,1.
Methods. We obtained high resolution (R=30000) spectra for one red giant star
in Pal\,1 using the High Dispersion Spectrograph (HDS) mounted at the SUBARU
telescope. We used ATLAS-9 model atmospheres coupled with the SYNTHE and WIDTH
calculation codes to derive chemical abundances from the measured line
equivalent widths of 18 among , Iron-peak, light and heavy elements.
Results. The Palomar~1 chemical pattern is broadly compatible to that of the MW
open clusters population and similar to disk stars. It is, instead, remarkably
different from that of the Sagittarius (Sgr) dwarf spheroidal galaxy.
Conclusions. If Pal\,1 association with either CMa or GASS will be confirmed,
this will imply that these systems had a chemical evolution similar to that of
the Galactic disk.Comment: 13 pages, 9 figures, 5 tables. Accepted for publication in Astronomy
and Astrophysics. Minor changes in the tex
On stellar limb darkening and exoplanetary transits
This paper examines how to compare stellar limb-darkening coefficients
evaluated from model atmospheres with those derived from photometry. Different
characterizations of a given model atmosphere can give quite different
numerical results (even for a given limb-darkening `law'), while light-curve
analyses yield limb-darkening coefficients that are dependent on system
geometry, and that are not directly comparable to any model-atmosphere
representation. These issues are examined in the context of exoplanetary
transits, which offer significant advantages over traditional binary-star
eclipsing systems in the study of stellar limb darkening. `Like for like'
comparisons between light-curve analyses and new model-atmosphere results,
mediated by synthetic photometry, are conducted for a small sample of stars.
Agreement between the resulting synthetic-photometry/atmosphere-model (SPAM)
limb-darkening coefficients and empirical values ranges from very good to quite
poor, even though the targets investigated show only a small dispersion in
fundamental stellar parameters.Comment: Accepted, MNRAS, 4/8/201
Evidence for the naphthalene cation in a region of the interstellar medium with anomalous microwave emission
We report high resolution spectroscopy of the moderately reddened (A=3)
early type star Cernis 52 located in a region of the Perseus molecular cloud
complex with anomalous microwave emission. In addition to the presence of the
most common diffuse interstellar bands (DIBs) we detect two new interstellar or
circumstellar bands coincident to within 0.01% in wavelength with the two
strongest bands of the naphthalene cation (CH) as measured in
gas-phase laboratory spectroscopy at low temperatures and find marginal
evidence for the third strongest band.
Assuming these features are caused by the naphthalene cation, from the
measured intensity and available oscillator strengths we find that 0.008 % of
the carbon in the cloud could be in the form of this molecule. We expect
hydrogen additions to cause hydronaphthalene cations to be abundant in the
cloud and to contribute via electric dipole radiation to the anomalous
microwave emission. The identification of new interstellar features consistent
with transitions of the simplest polycyclic aromatic hydrocarbon adds support
to the hypothesis that this type of molecules are the carriers of both diffuse
interstellar bands and anomalous microwave emission.Comment: Accepted for publication in The Astrophysical Journa
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