102 research outputs found
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 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&
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
Observational Studies of the Chemical Evolution in the Galactic Thin and Thick Disks
The origin and evolution of the thin and thick disks in the Galaxy have been studied by means of detailed stellar abundances. High-resolution spectra of 102 F and G dwarf stars have been obtained with the spectrographs on the ESO 3.6-m and ESO 1.5-m telescopes on La Silla (Chile), the Very Large Telescope (VLT) on Paranal (Chile), and the Nordic Optical Telescope (NOT) on La Palma (Canary Islands, Spain). Abundances for 14 elements (O, Na, Mg, Al, Si, Ca, Ti, Cr, Fe, Ni, Zn, Y, Ba, and Eu) have been determined. The following results were found: 1) Both the thin and the thick disks show distinct and well-defined abundance trends at sub-solar metallicities; 2) The thick disk shows signatures of chemical enrichment from Type Ia Supernovae; 3) [O/Fe] in the thin disk continues to decrease linearly at super-solar metallicities; 4) The abundance trends we see in the thick disk are invariant with distance from the Galactic plane () and galactocentric radius (); 4) The thick disk sample is in the mean older than the thin disk sample. The thesis also includes an investigation of ages and metallicities in the thick disk. The results from this study are; 5) There is a possible age-metallicity relation present in the thick disk, 6) Star formation has been ongoing for several billion years in the thick disk. Based on these findings, together with other constraints from the literature, we discuss different formation scenarios for the thick disk. We suggest that the currently most likely formation scenario is a violent merger event or a close encounter between the Galaxy and a companion galaxy
The age of the Galaxy's thick disk
We discuss the age of the stellar disks in the solar neighbourhood. After
reviewing the various methods for age dating we discuss current estimates of
the age of both the thin and the thick disk. We present preliminary results for
kinematically-selected stars that belong to the thin as well as the thick disk.
All of these dwarf and sub-giant stars have been studied spectroscopically and
we have derived both elemental abundances as well as ages for them. A general
conclusion is that in the solar neighbourhood, on average, the thick disk is
older than the thin disk. However, we caution that the exclusion of stars with
effective temperatures around 6500 K might result in a biased view on the full
age distribution for the stars in the thick disk.Comment: 6 pages, contribution to IAU Symposium 258: The Ages of Stars, held
in Baltimore, USA, October 13-17, 200
Exploring the production and depletion of lithium in the Milky Way stellar disk
[ABRIDGED] We determine Li abundances for a well-studied sample of 714 F and
G dwarf, turn-off, and subgiant stars in the solar neighbourhood. The analysis
is based on line synthesis of the Li line at 6707 {\AA} in high-resolution and
high signal-to-noise ratio echelle spectra, obtained with the MIKE, FEROS,
SOFIN, UVES, and FIES spectrographs. The presented Li abundances are corrected
for non-LTE effects. Out of the sample of 714 stars we are able to determine Li
abundances for 420 stars and upper limits on the Li abundance for another 121
stars. 18 of the stars with well-determined Li abundances are listed as
exoplanet host stars. Our main finding is that there are no signatures of Li
production in the thick disk, but the Li abundance for stars of the same
effective temperature is independent of metallicity for stars that can be
associated with the Galactic thick disk. Significant Li production is however
seen in the thin disk, with a steady increase towards super-solar
metallicities. At the highest metallicities, however, around [Fe/H]~ +0.3, we
tentatively confirm the recent discovery that the Li abundances level out. We
hence contradict the recent finding in other studies that found that Li is also
produced in the thick disk. This is likely due to the chemically defined
selection criteria those studies used to define their thick disk samples. Age
criteria that we use here, produce a thick disk stellar sample that is much
less contaminated by thin disk stars, and hence more reliable abundance trends.
[ABRIDGED] A conclusion that can be drawn is that no significant Li production,
relative to the primordial abundance, took place during the first few billion
years of the Milky Way, an era coinciding with the formation and evolution of
the thick disk. [ABRIDGED]Comment: Accepted for publication in A&
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