346 research outputs found
Environmental Impact of Meals: How Big Is the Carbon Footprint in the School Canteens?
The inhabitants of the world are expected to grow by two billion in the next two decades; as population increases, food demand rises too, leading to more intensive resource exploitation and greater negative externalities related to food production. In this paper the environmental impact of meals provided in school canteens are analysed through the Life Cycle Assessment methodology, in order to evaluate the GHGs emissions released by food production. Meals, and not just individual foods, have been considered so as to include in the analysis the nutritional aspects on which meals are based. Results shows that meat, fish and dairy products are the most impacting in terms of greenhouse gas emissions, with values that shift from 31.7 and 24.1 kg CO2 eq for butter and veal, to 2.37 kg CO2 eq for the octopus, while vegetables, legumes, fruit and cereals are less carbon intensive (average of 3.71 kg CO2 eq for the considered vegetables). When the environmental impact is related to the food energy, the best option are first courses because they combine a low carbon footprint with a high energy content. The results of the work can be used both by the consumer, who can base the meal choice on environmental impact information, and by food services, who can adjust menus to achieve a more sustainable production
s-Processing in the Galactic Disk. I. Super-Solar Abundances of Y, Zr, La, Ce in Young Open Clusters
In a recent study, based on homogeneous barium abundance measurements in open
clusters, a trend of increasing [Ba/Fe] ratios for decreasing cluster age was
reported. We present here further abundance determinations, relative to four
other elements hav- ing important s-process contributions, with the aim of
investigating whether the growth found for [Ba/Fe] is or not indicative of a
general property, shared also by the other heavy elements formed by slow
neutron captures. In particular, we derived abundances for yttrium, zirconium,
lanthanum and cerium, using equivalent widths measurements and the MOOG code.
Our sample includes 19 open clusters of different ages, for which the spectra
were obtained at the ESO VLT telescope, using the UVES spectrometer. The growth
previously suggested for Ba is confirmed for all the elements analyzed in our
study. This fact implies significant changes in our views of the Galactic
chemical evolution for elements beyond iron. Our results necessarily require
that very low-mass AGB stars (M < 1.5M\odot) produce larger amounts of
s-process elements (hence acti- vate the 13 C-neutron source more effectively)
than previously expected. Their role in producing neutron-rich elements in the
Galactic disk has been so far underestimated and their evolution and
neutron-capture nucleosynthesis should now be reconsidered.Comment: ApJ accepte
Deep Mixing in Evolved Stars. II. Interpreting Li Abundances in RGB and AGB Stars
We reanalyze the problem of Li abundances in red giants of nearly solar
metallicity. After an outline of the problems affecting our knowledge of the Li
content in low-mass stars (M<3Mo), we discuss deep-mixing models for the RGB
stages suitable to account for the observed trends and for the correlated
variations of the carbon isotope ratio; we find that Li destruction in these
phases is limited to masses below about 2.3 Mo. Subsequently, we concentrate on
the final stages of evolution for both O-rich and C-rich AGB stars. Here, the
constraints on extra-mixing phenomena previously derived from heavier nuclei
(from C to Al), coupled to recent updates in stellar structure models
(including both the input physics and the set of reaction rates used), are
suitable to account for the observations of Li abundances below A(Li)= log
e(Li) = 1.5 (and sometimes more). Also their relations with other
nucleosynthesis signatures of AGB phases (like the abundance of F, the C/O and
12C/13C ratios) can be explained. This requires generally moderate efficiencies
(\dot M <= 0.3 - 0.5 x 10^-6 Mo/yr) for non-convective mass transport. At such
rates, slow extra-mixing does not modify remarkably Li abundances in early-AGB
phases; on the other hand, faster mixing encounters a physical limit in
destroying Li, set by the mixing velocity. Beyond this limit, Li starts to be
produced; therefore its destruction on the AGB is modest. Li is then
significantly produced by the third dredge up. We also show that effective
circulation episodes, while not destroying Li, would easily bring the 12C/13C
ratios to equilibrium, contrary to the evidence in most AGB stars, and would
burn F beyond the limits shown by C(N) giants. Hence, we do not confirm the
common idea that efficient extra-mixing drastically reduces the Li content of
C-stars with respect to K-M giants.Comment: 56 pages, 21 13 figures, ApJ submitte
On the origin of the Galactic thin and thick discs, their abundance gradients and the diagnostic potential of their abundance ratios
Using a semi-analytical model of the evolution of the Milky Way, we show how
secular evolution can create distinct overdensities in the phase space of
various properties (e.g. age vs metallicity or abundance ratios vs age)
corresponding to the thin and thick discs. In particular, we show how key
properties of the Solar vicinity can be obtained by secular evolution, with no
need for external or special events, like galaxy mergers or paucity in star
formation. This concerns the long established double-branch behaviour of
[alpha/Fe] vs metallicity and the recently found non-monotonic evolution of the
stellar abundance gradient, evaluated at the birth radii of stars. We extend
the discussion to other abundance ratios and we suggest a classification
scheme, based on the nature of the corresponding yields (primary vs secondary
or odd elements) and on the lifetimes of their sources (short-lived vs
long-lived ones). The latter property is critical in determining the single- or
double- branch behavior of an elementary abundance ratio in the Solar
neighborhood. We underline the high diagnostic potential of this finding, which
can help to separate clearly elements with sources evolving on different
timescales and help determining the site of e.g. the r-process(es). We define
the "abundance distance" between the thin and thick disc sequences as an
important element for such a separation. We also show how the inside-out
evolution of the Milky Way disc leads rather to a single-branch behavior in
other disc regions.Comment: 20 pages, 16 figures, to appear in MNRA
Abundances of neutron-capture elements in G 24-25. A halo-population CH subgiant
The differences between the neutron-capture element abundances of halo stars
are important to our understanding of the nucleosynthesis of elements heavier
than the iron group. We present a detailed abundance analysis of carbon and
twelve neutron-capture elements from Sr up to Pb for a peculiar halo star
G24-25 with [Fe/H] = -1.4 in order to probe its origin. The equivalent widths
of unblended lines are measured from high resolution NOT/FIES spectra and used
to derive abundances based on Kurucz model atmospheres. In the case of CH, Pr,
Eu, Gd, and Pb lines, the abundances are derived by fitting synthetic profiles
to the observed spectra. Abundance analyses are performed both relative to the
Sun and to a normal halo star G16-20 that has similar stellar parameters as
G24-25. We find that G24-25 is a halo subgiant star with an unseen component.
It has large overabundances of carbon and heavy s-process elements and mild
overabundances of Eu and light s-process elements. This abundance distribution
is consistent with that of a typical CH giant. The abundance pattern can be
explained by mass transfer from a former asymptotic giant branch component,
which is now a white dwarf.Comment: 11 pages, 9 figures, accepted for publication in A&
The s process in AGB stars as constrained by a large sample of barium stars
© ESO 2018. Context. Barium (Ba) stars are dwarf and giant stars enriched in elements heavier than iron produced by the slow neutron-capture process (s process). These stars belong to binary systems in which the primary star evolved through the asymptotic giant branch (AGB) phase. During this phase the primary star produced s-process elements and transferred them onto the secondary, which is now observed as a Ba star. Aims. We compare the largest homogeneous set of Ba giant star observations of the s-process elements Y, Zr, La, Ce, and Nd with AGB nucleosynthesis models to reach a better understanding of the s process in AGB stars. Methods. By considering the light-s (ls: Y and Zr) heavy-s (hs: La, Ce, and Nd) and elements individually, we computed for the first time quantitative error bars for the different hs-element to ls-element abundance ratios, and for each of the sample stars. We compared these ratios to low-mass AGB nucleosynthesis models. We excluded La from our analysis because the strong La lines in some of the sample stars cause an overestimation and unreliable abundance determination, as compared to the other observed hs-Type elements. Results. All the computed hs-Type to ls-Type element ratios show a clear trend of increasing with decreasing metallicity with a small spread (less than a factor of 3). This trend is predicted by low-mass AGB models in which 13C is the main neutron source. The comparison with rotating AGB models indicates the need for the presence of an angular momentum transport mechanism that should not transport chemical species, but significantly reduces the rotational speed of the core in the advanced stellar evolutionary stages. This is an independent confirmation of asteroseismology observations of the slow down of core rotation in giant stars, and of rotational velocities of white dwarfs lower than predicted by models without an extra angular momentum transport mechanism
Impact of a revised Mg(p,)Al reaction rate on the operation of the Mg-Al cycle
Proton captures on Mg isotopes play an important role in the Mg-Al cycle
active in stellar H-burning regions. In particular, low-energy nuclear
resonances in the Mg(p,)Al reaction affect the production
of radioactive Al as well as the resulting Mg/Al abundance ratio.
Reliable estimations of these quantities require precise measurements of the
strengths of low-energy resonances. Based on a new experimental study performed
at LUNA, we provide revised rates of the Mg(p,)Al
and the Mg(p,)Al reactions with corresponding
uncertainties. In the temperature range 50 to 150 MK, the new recommended rate
of the Al production is up to 5 times higher than previously
assumed. In addition, at T MK, the revised total reaction rate is a
factor of 2 higher. Note that this is the range of temperature at which the
Mg-Al cycle operates in an H-burning zone. The effects of this revision are
discussed. Due to the significantly larger Mg(p,)Al
rate, the estimated production of Al in H-burning regions is less
efficient than previously obtained. As a result, the new rates should imply a
smaller contribution from Wolf-Rayet stars to the galactic Al budget.
Similarly, we show that the AGB extra-mixing scenario does not appear able to
explain the most extreme values of Al/Al, i.e. , found
in some O-rich presolar grains. Finally, the substantial increase of the total
reaction rate makes the hypothesis of a self-pollution by massive AGBs a more
robust explanation for the Mg-Al anticorrelation observed in Globular-Cluster
stars
The temperature and chronology of heavy-element synthesis in low-mass stars
Roughly half of the heavy elements (atomic mass greater than that of iron)
are believed to be synthesized in the late evolutionary stages of stars with
masses between 0.8 and 8 solar masses. Deep inside the star, nuclei (mainly
iron) capture neutrons and progressively build up (through the
slow-neutron-capture process, or s-process) heavier elements that are
subsequently brought to the stellar surface by convection. Two neutron sources,
activated at distinct temperatures, have been proposed: 13C and 22Ne, each
releasing one neutron per alpha-particle (4He) captured. To explain the
measured stellar abundances, stellar evolution models invoking the 13C neutron
source (which operates at temperatures of about one hundred million kelvin) are
favoured. Isotopic ratios in primitive meteorites, however, reflecting
nucleosynthesis in the previous generations of stars that contributed material
to the Solar System, point to higher temperatures (more than three hundred
million kelvin), requiring at least a late activation of 22Ne. Here we report a
determination of the s-process temperature directly in evolved low-mass giant
stars, using zirconium and niobium abundances, independently of stellar
evolution models. The derived temperature supports 13C as the s-process neutron
source. The radioactive pair 93Zr-93Nb used to estimate the s-process
temperature also provides, together with the pair 99Tc-99Ru, chronometric
information on the time elapsed since the start of the s-process, which we
determine to be one million to three million years.Comment: 30 pages, 10 figure
The Most Metal-Poor Stars. II. Chemical Abundances of 190 Metal-Poor Stars Including 10 New Stars With [Fe/H] < -3.5
We present a homogeneous chemical abundance analysis of 16 elements in 190
metal-poor Galactic halo stars (38 program and 152 literature objects). The
sample includes 171 stars with [Fe/H] < -2.5, of which 86 are extremely metal
poor, [Fe/H] < -3.0. Our program stars include ten new objects with [Fe/H] <
-3.5. We identify a sample of "normal" metal-poor stars and measure the trends
between [X/Fe] and [Fe/H], as well as the dispersion about the mean trend for
this sample. Using this mean trend, we identify objects that are chemically
peculiar relative to "normal" stars at the same metallicity. These chemically
unusual stars include CEMP-no objects, one star with high [Si/Fe], another with
high [Ba/Sr], and one with unusually low [X/Fe] for all elements heavier than
Na. The Sr and Ba abundances indicate that there may be two nucleosynthetic
processes at lowest metallicity that are distinct from the main r-process.
Finally, for many elements, we find a significant trend between [X/Fe] versus
Teff which likely reflects non-LTE and/or 3D effects. Such trends demonstrate
that care must be exercised when using abundance measurements in metal-poor
stars to constrain chemical evolution and/or nucleosynthesis predictions.Comment: Accepted for publication in Ap
Population Synthesis of Binary Carbon-enhanced Metal-poor Stars
The carbon-enhanced metal-poor (CEMP) stars constitute approximately one
fifth of the metal-poor ([Fe/H] ~< -2) population but their origin is not well
understood. The most widely accepted formation scenario, invokes mass-transfer
of carbon-rich material from a thermally-pulsing asymptotic giant branch
(TPAGB) primary star to a less massive main-sequence companion which is seen
today. Recent studies explore the possibility that an initial mass function
biased toward intermediate-mass stars is required to reproduce the observed
CEMP fraction in stars with metallicity [Fe/H] < -2.5. These models also
implicitly predict a large number of nitrogen-enhanced metal-poor (NEMP) stars
which is not seen. We investigate whether the observed CEMP and NEMP to
extremely metal-poor (EMP) ratios can be explained without invoking a change in
the initial mass function.
We confirm earlier findings that with current detailed TPAGB models the large
observed CEMP fraction cannot be accounted for. We find that efficient third
dredge up in low-mass (less than 1.25Msun), low-metallicity stars may offer at
least a partial explanation to the large observed CEMP fraction while remaining
consistent with the small observed NEMP fraction.Comment: 20 pages, 23 figures, accepted for publication in A&
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