1,450 research outputs found
Abundances in the Galactic bulge: results from planetary nebulae and giant stars
Our understanding of the chemical evolution of the Galactic bulge requires
the determination of abundances in large samples of giant stars and planetary
nebulae (PNe). We discuss PNe abundances in the Galactic bulge and compare
these results with those presented in the literature for giant stars. We
present the largest, high-quality data-set available for PNe in the direction
of the Galactic bulge (inner-disk/bulge). For comparison purposes, we also
consider a sample of PNe in the Large Magellanic Cloud (LMC). We derive the
element abundances in a consistent way for all the PNe studied. By comparing
the abundances for the bulge, inner-disk, and LMC, we identify elements that
have not been modified during the evolution of the PN progenitor and can be
used to trace the bulge chemical enrichment history. We then compare the PN
abundances with abundances of bulge field giant. At the metallicity of the
bulge, we find that the abundances of O and Ne are close to the values for the
interstellar medium at the time of the PN progenitor formation, and hence these
elements can be used as tracers of the bulge chemical evolution, in the same
way as S and Ar, which are not expected to be affected by nucleosynthetic
processes during the evolution of the PN progenitors. The PN oxygen abundance
distribution is shifted to lower values by 0.3 dex with respect to the
distribution given by giants. A similar shift appears to occur for Ne and S. We
discuss possible reasons for this PNe-giant discrepancy and conclude that this
is probably due to systematic errors in the abundance derivations in either
giants or PNe (or both). We issue an important warning concerning the use of
absolute abundances in chemical evolution studies.Comment: 23 pages, 15 figures, 16 pages of online material, A&A in pres
Nonlinear dispersion relation in anharmonic periodic mass-spring and mass-in-mass systems
The study of wave propagation in chains of anharmonic periodic systems is of
fundamental importance to understand the response of dynamical absorbers of
vibrations and acoustic metamaterials working in nonlinear regime. Here, we
derive an analytical nonlinear dispersion relation for periodic chains of
anharmonic mass-spring and mass-in-mass systems resulting from considering the
hypothesis of weak anharmonic energy and a periodic distribution function as
ansatz of a general solution of the nonlinear equations of motion. Numerical
simulations show that this expression is valid for anharmonic potential energy
up to 50% of the harmonic one. This work provides a simple tool to design and
study nonlinear dynamics for a class of seismic metamaterials.Comment: 18 pages, 5 figure
The effects of stellar winds of fast-rotating massive stars in the earliest phases of the chemical enrichment of the Galaxy
We use the growing data sets of very-metal-poor stars to study the impact of
stellar winds of fast rotating massive stars on the chemical enrichment of the
early Galaxy. We use an inhomogeneous chemical evolution model for the Galactic
halo to predict both the mean trend and scatter of C/O and N/O. In one set of
models, we assume that massive stars enrich the interstellar medium during both
the stellar wind and supernovae phases. In the second set, we consider that in
the earliest phases (Z <10^-8), stars with masses above 40 Msun only enrich the
interstellar medium via stellar winds, collapsing directly into black holes. We
predict a larger scatter in the C/O and N/O ratios at low metallicities when
allowing the more massive fast-rotating stars to contribute to the chemical
enrichment only via stellar winds. The latter assumption, combined with the
stochasticity in the star formation process in the primordial Galactic halo can
explain the wide spread observed in the N/O and C/O ratios in normal
very-metal-poor stars. For chemical elements with stellar yields that depend
strongly on initial mass (and rotation) such as C, N, and neutron capture
elements, within the range of massive stars, a large scatter is expected once
the stochastic enrichment of the early interstellar medium is taken into
account. We also find that stellar winds of fast rotators mixed with
interstellar medium gas are not enough to explain the large CNO enhancements
found in most of the carbon-enhanced very-metal-poor stars. In particular, this
is the case of the most metal-poor star known to date, HE 1327-2326, for which
our models predict lower N enhancements than observed when assuming a mixture
of stellar winds and interstellar medium. We suggest that these carbon-enhanced
very metal-poor stars were formed from almost pure stellar wind material,
without dilution with the pristine interstellar medium.Comment: 10 pages, 7 figures, accepted for publication in A&
Guideline on management of the acute asthma attack in children by Italian Society of Pediatrics.
BACKGROUND: Acute asthma attack is a frequent condition in children. It is one of the most common reasons for emergency department (ED) visit and hospitalization. Appropriate care is fundamental, considering both the high prevalence of asthma in children, and its life-threatening risks. Italian Society of Pediatrics recently issued a guideline on the management of acute asthma attack in children over age 2, in ambulatory and emergency department settings. METHODS: The Grading of Recommendations Assessment, Development, and Evaluation (GRADE) methodology was adopted. A literature search was performed using the Cochrane Library and Medline/PubMed databases, retrieving studies in English or Italian and including children over age 2 year. RESULTS: Inhaled Ă2 agonists are the first line drugs for acute asthma attack in children. Ipratropium bromide should be added in moderate/severe attacks. Early use of systemic steroids is associated with reduced risk of ED visits and hospitalization. High doses of inhaled steroids should not replace systemic steroids. Aminophylline use should be avoided in mild/moderate attacks. Weak evidence supports its use in life-threatening attacks. Epinephrine should not be used in the treatment of acute asthma for its lower cost / benefit ratio, compared to ÎČ2 agonists. Intravenous magnesium solphate could be used in children with severe attacks and/or forced expiratory volume1 (FEV1) lower than 60% predicted, unresponsive to initial inhaled therapy. Heliox could be administered in life-threatening attacks. Leukotriene receptor antagonists are not recommended. CONCLUSIONS: This Guideline is expected to be a useful resource in managing acute asthma attacks in children over age 2
Chemical evolution of the Milky Way: the origin of phosphorus
Context. Recently, for the first time the abundance of P has been measured in
disk stars. This provides the opportunity of comparing the observed abundances
with predictions from theoretical models. Aims. We aim at predicting the
chemical evolution of P in the Milky Way and compare our results with the
observed P abundances in disk stars in order to put constraints on the P
nucleosynthesis. Methods. To do that we adopt the two-infall model of galactic
chemical evolution, which is a good model for the Milky Way, and compute the
evolution of the abundances of P and Fe. We adopt stellar yields for these
elements from different sources. The element P should have been formed mainly
in Type II supernovae. Finally, Fe is mainly produced by Type Ia supernovae.
Results. Our results confirm that to reproduce the observed trend of [P/Fe] vs.
[Fe/H] in disk stars, P is formed mainly in massive stars. However, none of the
available yields for P can reproduce the solar abundance of this element. In
other words, to reproduce the data one should assume that massive stars produce
more P than predicted by a factor of ~ 3. Conclusions. We conclude that all the
available yields of P from massive stars are largely underestimated and that
nucleosynthesis calculations should be revised. We also predict the [P/Fe]
expected in halo stars.Comment: Accepted for publication in A&A (minor changes with respect to the
submitted version
The Evolution of Carbon and Oxygen in the Bulge and Disk of the Milky Way
The evolution of C and O abundances in the Milky Way can impose strong
constraints on stellar nucleosynthesis and help understanding the formation and
evolution of our Galaxy. The aim is to review the measured C and O abundances
in the disk and bulge of the Galaxy and compare them with model predictions. We
adopt two successful chemical evolution models for the bulge and the disk,
which assume the same nucleosynthesis prescriptions but different histories of
star formation. The data show a clear distinction between the trend of [C/O] in
the thick and thin Galactic disks, while the thick disk and bulge trends are
indistinguishable with a large (>0.5 dex) increase in the C/O ratio in the
range from -0.1 to +0.4 dex for [O/H]. In our models we consider yields from
massive stars with and without the inclusion of metallicity-dependent stellar
winds. The observed increase in the [C/O] ratio with metallicity in the bulge
and thick disk lies between the predictions utilizing the mass-loss rates of
Maeder (1992) and those of Meynet & Maeder (2002). A model without
metallicity-dependent yields completely fails to match the observations. Thus,
the relative increase in carbon abundance at high metallicity appears to be due
to metallicity-dependent stellar winds in massive stars. These results also
explain the steep decline of the [O/Fe] ratio with [Fe/H] in the Galactic
bulge, while the [Mg/Fe] ratio is enhanced at all [Fe/H]. (abridged)Comment: 18 pages, 6 figures, submitted to Astronomy & Astrophysic
Abundance gradients in the Milky Way for alpha elements, Iron peak elements, Barium, Lanthanum and Europium
We model the abundance gradients in the disk of the Milky Way for several
chemical elements (O, Mg, Si, S, Ca, Sc, Ti, Co, V, Fe, Ni, Zn, Cu, Mn, Cr, Ba,
La and Eu), and compare our results with the most recent and homogeneous
observational data. We adopt a chemical evolution model able to well reproduce
the main properties of the solar vicinity. We compute, for the first time, the
abundance gradients for all the above mentioned elements in the galactocentric
distance range 4 - 22 kpc. The comparison with the observed data on Cepheids in
the galactocentric distance range 5-17 kpc gives a very good agreement for many
of the studied elements. In addition, we fit very well the data for the
evolution of Lanthanum in the solar vicinity for which we present results here
for the first time. We explore, also for the first time, the behaviour of the
abundance gradients at large galactocentric distances by comparing our results
with data relative to distant open clusters and red giants and select the best
chemical evolution model model on the basis of that. We find a very good fit to
the observed abundance gradients, as traced by Cepheids, for most of the
elements, thus confirming the validity of the inside-out scenario for the
formation of the Milky Way disk as well as the adopted nucleosynthesis
prescriptions.Comment: 11 pages, 9 figures, accepted for publication in A&
The impact of stellar rotation on the CNO abundance patterns in the Milky Way at low metallicities
We investigate the effect of new stellar models, which take rotation into
account, computed for very low metallicities on the chemical evolution of the
earliest phases of the Milky Way. We check the impact of these new stellar
yields on a model for the halo of the Milky Way that can reproduce the observed
halo metallicity distribution. In this way we try to better constrain the ISM
enrichment timescale, which was not done in our previous work. The stellar
models adopted in this work were computed under the assumption that the ratio
of the initial rotation velocity to the critical velocity of stars is roughly
constant with metallicity. This naturally leads to faster rotation at lower
metallicity, as metal poor stars are more compact than metal rich ones. We find
that the new Z = 10-8 stellar yields computed for large rotational velocities
have a tremendous impact on the interstellar medium nitrogen enrichment for
log(O/H)+12 < 7 (or [Fe/H]< -3). We show that upon the inclusion of the new
stellar calculations in a chemical evolution model for the galactic halo with
infall and outflow, both high N/O and C/O ratios are obtained in the very-metal
poor metallicity range in agreement with observations. Our results give further
support to the idea that stars at very low metallicities could have initial
rotational velocities of the order of 600-800kms-1. An important contribution
to N from AGB stars is still needed in order to explain the observations at
intermediate metallicities. One possibility is that AGB stars at very low
metallicities also rotate fast. This could be tested in the future, once
stellar evolution models for fast rotating AGB stars will be available.Comment: Contribution to Nuclei in the Cosmos IX (Proceedings of Science - 9
pages, 4 figs., accepted) - Version 2: one reference added in the caption of
Fig.
SPINSTARS at low metallicities
The main effect of axial rotation on the evolution of massive PopIII stars is
to trigger internal mixing processes which allow stars to produce significant
amounts of primary nitrogen 14 and carbon 13. Very metal poor massive stars
produce much more primary nitrogen than PopIII stars for a given initial mass
and rotation velocity. The very metal poor stars undergo strong mass loss
induced by rotation. One can distinguish two types of rotationnaly enhanced
stellar winds: 1) Rotationally mechanical winds occurs when the surface
velocity reaches the critical velocity at the equator, {\it i.e.} the velocity
at which the centrifugal acceleration is equal to the gravity; 2) Rotationally
radiatively line driven winds are a consequence of strong internal mixing which
brings large amounts of CNO elements at the surface. This enhances the opacity
and may trigger strong line driven winds. These effects are important for an
initial value of of 0.54 for a 60 M at
, {\it i.e.} for initial values of
higher than the one (0.4) corresponding to observations at solar .
These two effects, strong internal mixing leading to the synthesis of large
amounts of primary nitrogen and important mass losses induced by rotation,
occur for between about 10 and 0.001. For metallicities above 0.001
and for reasonable choice of the rotation velocities, internal mixing is no
longer efficient enough to trigger these effects.Comment: 5 pages, 4 figures, to be published in the conference proceedings of
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