2,337 research outputs found
Modelling the chemical evolution of the Galaxy halo
We study the chemical evolution and formation of the Galactic halo through
the analysis of its stellar metallicity distribution function and some key
elemental abundance patterns. Starting from the two-infall model for the
Galaxy, which predicts too few low-metallicity stars, we add a gas outflow
during the halo phase with a rate proportional to the star formation rate
through a free parameter, lambda. In addition, we consider a first generation
of massive zero-metal stars in this two-infall + outflow model adopting two
different top-heavy initial mass functions and specific population III yields.
The metallicity distribution function of halo stars, as predicted by the
two-infall + outflow model shows a good agreement with observations, when the
parameter lambda=14 and the time scale for the first infall, out of which the
halo formed, is not longer than 0.2 Gyr, a lower value than suggested
previously. Moreover, the abundance patterns [X/Fe] vs. [Fe/H] for C, N and
alpha-elements O, Mg, Si, S, Ca show a good agreement with the observational
data. If population III stars are included, under the assumption of different
initial mass functions, the overall agreement of the predicted stellar
metallicity distribution function with observational data is poorer than in the
case without population III. We conclude that it is fundamental to include both
a gas infall and outflow during the halo formation to explain the observed halo
metallicity distribution function, in the framework of a model assuming that
the stars in the inner halo formed mostly in situ. Moreover, we find that it
does not exist a satisfactory initial mass function for population III stars
which reproduces the observed halo metallicity distribution function. As a
consequence, there is no need for a first generation of only massive stars to
explain the evolution of the Galactic halo.Comment: Accepted for publication in A&A. 11 pages, 5 figure
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 chemical evolution of Manganese in different stellar systems
Aims. To model the chemical evolution of manganese relative to iron in three
different stellar systems: the solar neighbourhood, the Galactic bulge and the
Sagittarius dwarf spheroidal galaxy, and compare our results with the recent
and homogeneous observational data. Methods. We adopt three chemical evolution
models well able to reproduce the main properties of the solar vicinity, the
galactic Bulge and the Sagittarius dwarf spheroidal. Then, we compare different
stellar yields in order to identify the best set to match the observational
data in these systems. Results. We compute the evolution of manganese in the
three systems and we find that in order to reproduce simultaneously the [Mn/Fe]
versus [Fe/H] in the Galactic bulge, the solar neighbourhood and Sagittarius,
the type Ia SN Mn yield must be metallicity-dependent. Conclusions. We conclude
that the different histories of star formation in the three systems are not
enough to reproduce the different behaviour of the [Mn/Fe] ratio, unlike the
situation for [alpha/Fe]; rather, it is necessary to invoke
metallicity-dependent type Ia SN Mn yields, as originally suggested by
McWilliam, Rich & Smecker-Hane in 2003.Comment: 9 pages, 3 figures, submitted to A&
Chemical evolution of the Galactic bulge: different stellar populations and possible gradients
We compute the chemical evolution of the Galactic bulge to explain the
existence of two main stellar populations recently observed. After comparing
model results and observational data we suggest that the old more metal poor
stellar population formed very fast (on a timescale of 0.1-0.3 Gyr) by means of
an intense burst of star formation and an initial mass function flatter than in
the solar vicinity whereas the metal rich population formed on a longer
timescale (3 Gyr). We predict differences in the mean abundances of the two
populations (-0.52 dex for ) which can be interpreted as a metallicity
gradients. We also predict possible gradients for Fe, O, Mg, Si, S and Ba
between sub-populations inside the metal poor population itself (e.g. -0.145
dex for ). Finally, by means of a chemo-dynamical model following a
dissipational collapse, we predict a gradient inside 500 pc from the Galactic
center of -0.26 dex kpc^{-1} in Fe.Comment: 9 pages, 9 figures, accepted for publication in Section 5. of
Astronomy and Astrophysic
Observation of the stray field of thin film magnetic tips using electron holography
The stray field around thin film ferromagnetic tips employed for magnetic force microscopy has been revealed using electron holography. The experimental phase difference maps are in good agreement with simulations. Quantitative flux measurements of the leakage field are obtained
Detailed Chemical Evolution of Carina and Sagittarius Dwarf Spheroidal Galaxies
In order to verify the effects of the most recent data on the evolution of
Carina and Sagittarius Dwarf Spheroidal Galaxies (dSph) and to set tight
constraints on the main parameters of chemical evolution models, we study in
detail the chemical evolution of these galaxies through comparisons between the
new data and the predictions of a model, already tested to reproduce the main
observational constraints in dSphs. Several abundance ratios, such as
[/Fe], [Ba/Fe] and [Eu/Fe], and the metallicity distribution of stars
are compared to the predictions of our models adopting the observationally
derived star formation histories in these galaxies. These new comparisons
confirm our previously suggested scenario for the evolution of these galaxies,
and allow us to better fix the star formation and wind parameters. In
particular, for Carina the comparisons indicate that the best efficiency of
star formation is , that the best wind efficiency
parameter is = 5 (the wind rate is five times stronger than the star
formation rate), and that the star formation history, which produces the best
fit to the observed metallicity distribution of stars is characterized by
several episodes of activity. In the case of Sagittarius our results suggest
that and , again in agreement with our previous work.
Finally, we show new predictions for [N/Fe] and [C/Fe] ratios for the two
galaxies suggesting a scenario for Sagittarius very similar to the one of the
solar vicinity in the Milky Way, except for a slight decrease of [N/Fe] ratio
at high metallicities due to the galactic wind. For Carina we predict a larger
[N/Fe] ratio at low metallicities, reflecting the lower star formation
efficiency of this galaxy relative to Sagittarius and the Milky Way.Comment: 11 pages, 7 figures, accepted for publication in Asttronomy &
Astrophysic
The chemical evolution of Barium and Europium in the Milky Way
We compute the evolution of the abundances of barium and europium in the
Milky Way and we compare our results with the observed abundances from the
recent UVES Large Program "First Stars". We use a chemical evolution model
which already reproduces the majority of observational constraints. We confirm
that barium is a neutron capture element mainly produced in the low mass AGB
stars during the thermal-pulsing phase by the 13C neutron source, in a slow
neutron capture process. However, in order to reproduce the [Ba/Fe] vs. [Fe/H]
as well as the Ba solar abundance, we suggest that Ba should be also produced
as an r-process element by massive stars in the range 10-30 solar masses. On
the other hand, europium should be only an r-process element produced in the
same range of masses (10-30 solar masses), at variance with previous
suggestions indicating a smaller mass range for the Eu producers. As it is well
known, there is a large spread in the [Ba/Fe] and [Eu/Fe] ratios at low
metallicities, although smaller in the newest data. With our model we estimate
for both elements (Ba and Eu) the ranges for the r-process yields from massive
stars which better reproduce the trend of the data. We find that with the same
yields which are able to explain the observed trends, the large spread in the
[Ba/Fe] and [Eu/Fe] ratios cannot be explained even in the context of an
inhomogeneous models for the chemical evolution of our Galaxy. We therefore
derive the amount by which the yields should be modified to fully account for
the observed spread. We then discuss several possibilities to explain the size
of the spread. We finally suggest that the production ratio of [Ba/Eu] could be
almost constant in the massive stars.Comment: 14 pages, 17 figures, accepted for pubblication 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.
The Paradoxical Forces for the Classical Electromagnetic Lag Associated with the Aharonov-Bohm Phase Shift
The classical electromagnetic lag assocated with the Aharonov-Bohm phase
shift is obtained by using a Darwin-Lagrangian analysis similar to that given
by Coleman and Van Vleck to identify the puzzling forces of the Shockley-James
paradox. The classical forces cause changes in particle velocities and so
produce a relative lag leading to the same phase shift as predicted by Aharonov
and Bohm and observed in experiments. An experiment is proposed to test for
this lag aspect implied by the classical analysis but not present in the
currently-accepted quantum topological description of the phase shift.Comment: 8 pages, 3 figure
Impact of AGB Stars on the Chemical Evolution of Neutron-Capture Elements
In this review, we discuss the impact of s-process nucleosynthesis in asymptotic giant branch stars on the enrichment of heavy elements. We review the main steps made on this subject in the last 40 years and discuss the importance of modelling the evolution of the abundances of such elements in our Milky Way. From the comparison between model results and observations, we can impose strong constraints on stellar nucleosynthesis, as well as on the evolution of the Milky Way
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