The Galactic thin and thick discs in the context of galaxy formation

We have obtained high-resolution spectra and carried out a detailed elemental abundance analysis for a new sample of 899 F and G dwarf stars in the Solar neighbourhood. The results allow us to, in a multi-dimensional space consisting of stellar ages, detailed elemental abundances, and full kinematic information for the stars, study and trace their respective origins. Here we briefly address selection criteria and discuss how to define a thick disc star. The results are discussed in the context of galaxy formation.Comment: Contributed talk at Chemical abundances in the Universe, connecting first stars to planets, Proceedings of the International Astronomical Union, IAU Symposium, Volume 265, K. Cunha, M. Spite and B. Barbuy, eds, Cambridge University Press, in pres

The star-formation history of the Milky Way Galaxy

The star-formation histories of the main stellar components of the Milky Way constrain critical aspects of galaxy formation and evolution. I discuss recent determinations of such histories, together with their interpretation in terms of theories of disk galaxy evolution.Comment: Invited review, IAU Symposium 258; 12 pages, 1 figur

The first chemical abundance analysis of K giants in the inner Galactic disc

The elemental abundance structure of the Galactic disc has been extensively studied in the solar neighbourhood using long-lived stars such as F and G dwarfs or K and M giants. These are stars whose atmospheres preserve the chemical composition of their natal gas clouds, and are hence excellent tracers of the chemical evolution of the Galaxy. As far as we are aware, there are no such studies of the inner Galactic disc, which hampers our ability to constrain and trace the origin and evolution of the Milky Way. Therefore, we aim in this study to establish the elemental abundance trend(s) of the disc(s) in the inner regions of the Galaxy. Based on equivalent width measurements in high-resolution spectra obtained with the MIKE spectrograph on the Magellan II telescope on Las Campanas in Chile, we determine elemental abundances for 44 K-type red giant stars in the inner Galactic disc, located at Galactocentric distances of 4-7\,kpc. The analysis method is identical to the one recently used on red giant stars in the Galactic bulge and in the nearby thin and thick discs, enabling us to perform a truly differential comparison of the different stellar populations. We present the first detailed elemental abundance study of a significant number of red giant stars in the inner Galactic disc. We find that these inner disc stars show the same type of chemical and kinematical dichotomy as the thin and thick discs show in the solar neighbourhood. The abundance trends of the inner disc agree very well with those of the nearby thick disc, and also to those of the Bulge. The chemical similarities between the Bulge and the Galactic thick disc stellar populations indicate that they have similar chemical histories, and any model trying to understand the formation and evolution of either of the two should preferably incorporate both of them.Comment: A&A Letters, accepte

The origin and evolution of r- and s-process elements in the Milky Way stellar disk

Knowledge of abundance ratios as functions of metallicity can lead to insights on the origin and evolution of our Galaxy and its stellar populations. We aim to trace the chemical evolution of the neutron-capture elements Sr, Zr, La, Ce, Nd, Sm, and Eu in the Milky Way stellar disk to constrain the formation sites of these elements as well as to probe the evolution of the Galactic thin and thick disks. Using spectra of high resolution and high signal-to-noise we determine Sr, Zr, La, Ce, Nd, Sm, and Eu abundances for a sample of 593 F and G dwarf stars in the Solar neighbourhood. We present abundance results for Sr, Zr, La, Ce, Nd, Sm and Eu. We find that Nd, Sm, and Eu show trends similar to what is observed for the alpha-elements when compared to [Fe/H]. [Sr/Fe] and [Zr/Fe] show decreasing abundance ratios for increasing metallicity, reaching sub-solar values at super-solar metallicities. [La/Fe] and [Ce/Fe] do not show any clear trend with metallicity. The rapid neutron-capture process is active early in the Galaxy, mainly in type II supernovae from stars in the mass range 8-10 M_sun. Eu is almost completely produced by r-process but Nd and Sm show similar trends to Eu even if their s-process component is higher. Sr and Zr show significant enrichment at low metallicity that requires extra r-process production, that probably is different from the classical r-process. Finally, La and Ce are mainly produced via s-process from AGB stars in mass range 2-4 M_sun. The trend of [X/Fe] with age found could be explained by considering that the decrease in [X/Fe] for the thick disk stars can be due to the decrease of type II supernovae with time meaning a reduced enrichment of r-process elements in the interstellar medium. In the thin disk the trends are flatter that probably is due to that the main production from s-process is balanced by Fe production from type Ia supernovae.Comment: 25 pages, published in A&

Carbon Abundances in the Galactic Thin and Thick Disks

Although carbon is, together with oxygen and nitrogen, one of the most important elements in the study of galactic chemical evolution its production sites are still poorly known and have been much debated (see e.g. Gustafsson et al. 1999; Chiappini et al. 2003). To trace the origin and evolution of carbon we have determined carbon abundances from the forbidden [C I] line at 8727 A and made comparisons to oxygen abundances from the forbidden [O I] line at 6300 A in a sample of 51 nearby F and G dwarf stars. These data and the fact that the forbidden [C I] and [O I] lines are very robust abundance indicators (they are essentially insensitive to deviations from LTE and uncertainties in the stellar parameters, see, e.g., Gustafsson et al. 1999; Asplund et al. 2005) enable us to very accurately measure the C/O ratio as well as individual C and O abundances. Our first results indicate that the time-scale for the main source that contribute to the carbon enrichment of the interstellar medium operate on the same time-scale as those that contribute to the iron enrichment (and can possibly be AGB stars...)Comment: 2 pages, to appear in Proceedings IAUS Symposium 228, From Li to U: Elemental Tracers of Early Cosmic Evolution (Paris, May 23- 27, 2005), eds. V. Hill, P. Francois, and F. Prima

The origin and evolution of the odd-Z iron-peak elements Sc, V, Mn, and Co in the Milky Way stellar disk

AIMS: The aim of this study is to investigate the origin and evolution of Sc, V, Mn, and Co for a homogeneous and statistically significant sample of stars probing the different populations of the Milky Way, in particular the thin and thick disks. METHODS: Using high-resolution spectra obtained with MIKE, FEROS, SOFIN, FIES, UVES and HARPS spectrographs, we determine Sc, V, Mn, and Co abundances for a large sample of F and G dwarfs in Solar neighbourhood. The method is based on spectral synthesis and using one-dimensional (1-D), plane-parallel, LTE model stellar atmospheres calculated with the MARCS 2012 code. NLTE corrections from literature were applied to Mn and Co. RESULTS: We find that the abundance trends derived for Sc (594 stars), V (466 stars), and Co (567 stars) are very similar to what has been observed for the alpha-elements in the thin and thick disks. On the contrary Mn (569 stars) is generally underabundant relative to the Sun (i.e. [Mn/Fe]<0) for [Fe/H]<0. Also, for Mn, when NLTE corrections are applied, the trend changes and is almost flat over the entire metallicity range that the stars in our sample span (-2<[Fe/H]<+0.4). The [Sc/Fe]-[Fe/H] abundance trends show a small separation between the thin and thick disks, while for V and Co they completely overlap. For Mn there is a small difference in [Mn/Fe] but only when NLTE corrections are used. Comparisons with Ti as a reference element show flat trends for all the elements except for Mn that show well separated [Mn/Ti]-[Ti/H] trends for the thin and thick disks. CONCLUSIONS: Sc and V present trends compatible with production from SNII events. In addition, Sc clearly shows a metallicity dependence for [Fe/H]<-1. Mn instead is produced in SNII events for [Fe/H]<-0.4 and then SNIa start to produce Mn. Finally, Co appears to be produced mainly in SNII with suggestion of enrichment from hypernovae at low metallicities.Comment: Accepted for publication in A&A, 19 page