259 research outputs found
Inhomogeneous Galactic halo: a possible explanation for the spread observed in s- and r- process elements
The considerable scatter of the s- and r-process elements observed in
low-metallicity stars, compared to the small star to star scatter observed for
the alpha elements, is an open question for the chemical evolution studies. We
have developed a stochastic chemical evolution model, in which the main
assumption is a random formation of new stars, subject to the condition that
the cumulative mass distribution follows a given initial mass function. With
our model we are able to reproduce the different features of alpha-elements and
s-and r-process elements. The reason for this resides in the random birth of
stellar masses coupled with the different stellar mass ranges from where
alpha-elements and s-and r-process elements originate. In particular, the sites
of production of the alpha elements are the whole range of the massive stars,
whereas the mass range of production for the s- and r-process elements has an
upper limit of 30 solar masses.Comment: 2 pages, 2 figures, proceedings of "From stars to galaxies", Venice
October 2006, to be published in the Astronomical Society of the Pacific
Conference Serie
Phosphorus Abundances in FGK Stars
We measured phosphorus abundances in 22 FGK dwarfs and giants that span
--0.55 [Fe/H] 0.2 using spectra obtained with the Phoenix high
resolution infrared spectrometer on the Kitt Peak National Observatory Mayall
4m telescope, the Gemini South Telescope, and the Arcturus spectral atlas. We
fit synthetic spectra to the P I feature at 10581 to determine abundances
for our sample. Our results are consistent with previously measured phosphorus
abundances; the average [P/Fe] ratio measured in [Fe/H] bins of 0.2 dex for our
stars are within 1 compared to averages from other IR
phosphorus studies. Our study provides more evidence that models of chemical
evolution using the results of theoretical yields are under producing
phosphorus compared to the observed abundances. Our data better fit a chemical
evolution model with phosphorus yields increased by a factor of 2.75 compared
to models with unadjusted yields. We also found average [P/Si] = 0.02
0.07 and [P/S] = 0.15 0.15 for our sample, showing no significant
deviations from the solar ratios for [P/Si] and [P/S] ratios.Comment: 11 pages, 5 figures, Accepted to Ap
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
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
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&
Lithium and beryllium in the Gaia-Enceladus galaxy
Data from Gaia DR2 and The Apache Point Observatory Galactic Evolution Experiment surveys revealed a relatively new component in the inner Galactic halo, which is likely the dynamical remnant of a disrupted dwarf galaxy named Gaia-Enceladus that collided with the Milky Way about 10 Gyr ago. This merging event offers an extraordinary opportunity to study chemical abundances of elements in a dwarf galaxy, since they are generally hampered in external galaxies. Here, we focus on 7Li and 9Be in dwarf stars that are out of reach even in Local Group galaxies. Searching in GALAH, Gaia-ESO survey and in literature, we found several existing 7Li abundance determinations of stars belonging to the Gaia-Enceladus galaxy. The 7Li abundances of stars at the low metallicity end overlap with those of the Galactic halo. These are effective extragalactic 7Li measurements, which suggest that the 7Li Spite plateau is universal, as is the cosmological 7Li problem. We found a 7Li-rich giant out of 101 stars, which suggests a small percentage similar to that of the Milky Way. We also collect 9Be abundance for a subsample of 25 Gaia-Enceladus stars from literature. Their abundances share the Galactic [Be/H] values at the low metallicity end but grow slower with [Fe/H] and show a reduced dispersion. This suggests that the scatter observed in the Milky Way could reflect the different 9Be evolution patterns of different stellar components that are mixed-up in the Galactic halo
Abundance Uncertainties Obtained With the PizBuin Framework For Monte Carlo Reaction Rate Variations
Uncertainties in nucleosynthesis models originating from uncertainties in
astrophysical reaction rates were estimated in a Monte Carlo variation
procedure. Thousands of rates were simultaneously varied within individual,
temperature-dependent errors to calculate their combined effect on final
abundances. After a presentation of the method, results from application to
three different nucleosynthesis processes are shown: the -process and
the s-process in massive stars, and the main s-process in AGB stars
(preliminary results). Thermal excitation of nuclei in the stellar plasma and
the combined action of several reactions increase the final uncertainties above
the level of the experimental errors. The total uncertainty, on the other hand,
remains within a factor of two even in processes involving a large number of
unmeasured rates, with some notable exceptions for nuclides whose production is
spread over several stellar layers and for s-process branchings.Comment: 8 pages, 4 figures; Proceedings of OMEG 2017, Daejeon, Korea, June
27-30, 2017; to appear in AIP Conf. Pro
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