20,844 research outputs found
The connection between the Galactic halo and ancient Dwarf Satellites
We explore the hypothesis that the classical and ultra-faint dwarf spheroidal
satellites of the Milky Way have been the building blocks of the Galactic halo
by comparing their [O/Fe] and [Ba/Fe] versus [Fe/H] patterns with the ones
observed in Galactic halo stars. Oxygen abundances deviate substantially from
the observed abundances in the Galactic halo stars for [Fe/H] values larger
than -2 dex, while they overlap for lower metallicities. On the other hand, for
the [Ba/Fe] ratio the discrepancy is extended at all [Fe/H] values, suggesting
that the majority of stars in the halo are likely to have been formed in situ.
Therefore, we suggest that [Ba/Fe] ratios are a better diagnostic than [O/Fe]
ratios. Moreover, we show the effects of an enriched infall of gas with the
same chemical abundances as the matter ejected and/or stripped from dwarf
satellites of the Milky Way on the chemical evolution of the Galactic halo. We
find that the resulting chemical abundances of the halo stars depend on the
assumed infall time scale, and the presence of a threshold in the gas for star
formation.Comment: To appear in Proceeding of Science: Frontier Research in Astrophysics
- II 23-28 May 2016 Mondello (Palermo), Ital
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
The Chemical Evolution of the Milky Way: the Three Infall Model
We present a new chemical evolution model for the Galaxy that assumes three
main infall episodes of primordial gas for the formation of halo, thick and
thin disk, respectively. We compare our results with selected data taking into
account NLTE effects. The most important parameters of the model are (i) the
timescale for gas accretion, (ii) the efficiency of star formation and (iii) a
threshold in the gas density for the star formation process, for each Galactic
component. We find that, in order to best fit the features of the solar
neighbourhood, the halo and thick disk must form on short timescales (~0.2 and
~1.25 Gyr, respectively), while a longer timescale is required for the
thin-disk formation. The efficiency of star formation must be maximum (10
Gyr-1) during the thick-disk phase and minimum (1 Gyr-1) during the thin-disk
formation. Also the threshold gas density for star formation is suggested to be
different in the three Galactic components. Our main conclusion is that in the
framework of our model an independent episode of accretion of extragalactic
gas, which gives rise to a burst of star formation, is fundamental to explain
the formation of the thick disk. We discuss our results in comparison to
previous studies and in the framework of modern galaxy formation theories.Comment: 12 pages, 7 figures, accepted for publication in MNRA
Complete positivity and dissipative factorized dynamics
We show that any Hermiticity and trace preserving continuous semigroup
gamma_t in d dimensions is completely positive if and only if the semigroup
gamma_t otimes gamma_t is positivity preserving.Comment: 7 page
The effect of stellar migration on Galactic chemical evolution: a heuristic approach
In the last years, stellar migration in galactic discs has been the subject
of several investigations. However, its impact on the chemical evolution of the
Milky Way still needs to be fully quantified. In this paper, we aim at imposing
some constraints on the significance of this phenomenon by considering its
influence on the chemical evolution of the Milky Way thin disc. We do not
investigate the physical mechanisms underlying the migration of stars. Rather,
we introduce a simple, heuristic treatment of stellar migration in a detailed
chemical evolution model for the thin disc of the Milky Way, which already
includes radial gas flows and reproduces several observational constraints for
the solar vicinity and the whole Galactic disc. When stellar migration is
implemented according to the results of chemo-dynamical simulations by Minchev
et. al. (2013) and finite stellar velocities of 1 km s are taken into
account, the high-metallicity tail of the metallicity distribution function of
long-lived thin-disc stars is well reproduced. By exploring the velocity space,
we find that the migrating stars must travel with velocities in the range 0.5
-2 km s to properly reproduce the high-metallicity tail of the
metallicity distribution. We confirm previous findings by other authors that
the observed spread in the age-metallicity relation of solar neighbourhood
stars can be explained by the presence of stars which originated at different
Galactocentric distances, and we conclude that the chemical properties of stars
currently observed in the solar vicinity do suggest that stellar migration is
present to some extent.Comment: Accepted for publication by Ap
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