ALBERT

Description: According to the current cosmological cold dark matter paradigm, the Galactic halo could have been the result of the assemblage of smaller structures. Here 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 [α/Fe] and [Ba/Fe] versus [Fe/H] patterns with the ones observed in Galactic halo stars. The α elements 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 [α/Fe] ratios. Moreover, for the first time we consider 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. In particular, in models with an infall time-scale for the halo around 0.8 Gyr coupled with a threshold in the surface gas density for the star formation (4 M pc –2 ), and the enriched infall from dwarf spheroidal satellites, the first halo stars formed show [Fe/H]〉–2.4 dex. In this case, to explain [α/Fe] data for stars with [Fe/H]〈–2.4 dex, we need stars formed in dSph systems.

Description: 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 disc, respectively. We compare our results with selected data taking into account NLTE effects. The most important parameters of the model are (i) the time-scale 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 disc must form on short time-scales (~0.2 and ~1.25 Gyr, respectively), while a longer time-scale is required for the thin-disc formation. The efficiency of star formation must be maximum (10 Gyr –1 ) during the thick-disc phase and minimum (1 Gyr –1 ) during the thin-disc 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 disc. We discuss our results in comparison to previous studies and in the framework of modern galaxy formation theories.

Description: We have explored the Eu production in the Milky Way by means of a very detailed chemical evolution model. In particular, we have assumed that Eu is formed in merging neutron star (or neutron star-black hole) binaries as well as in Type II supernovae. We have tested the effects of several important parameters influencing the production of Eu during the merging of two neutron stars, such as (i) the time-scale of coalescence, (ii) the Eu yields and (iii) the range of initial masses for the progenitors of the neutron stars. The yields of Eu from Type II supernovae are very uncertain, more than those from coalescing neutron stars, so we have explored several possibilities. We have compared our model results with the observed rate of coalescence of neutron stars, the solar Eu abundance, the [Eu/Fe] versus [Fe/H] relation in the solar vicinity and the [Eu/H] gradient along the Galactic disc. Our main results can be summarized as follows: (i) neutron star mergers can be entirely responsible for the production of Eu in the Galaxy if the coalescence time-scale is no longer than 1 Myr for the bulk of binary systems, the Eu yield is around 3 x 10 –7 M and the mass range of progenitors of neutron stars is 9–50 M ; (ii) both Type II supernovae and merging neutron stars can produce the right amount of Eu if the neutron star mergers produce 2 x 10 –7 M per system and Type II supernovae, with progenitors in the range 20–50 M , produce yields of Eu of the order of 10 –8 –10 –9 M ; (iii) either models with only neutron stars producing Eu or mixed ones can reproduce the observed Eu abundance gradient along the Galactic disc.