Exploring the Milky Way stellar disk

Abstract

Aims. The aim of this paper is to explore and map the age and abundance structure of the stars in the nearby Galactic disk. Methods. We have conducted a high-resolution spectroscopic study of 714 F and G dwarf and subgiant stars in the Solar neighbourhood. The star sample has been kinematically selected to trace the Galactic thin and thick disks to their extremes, the metal-rich stellar halo, sub-structures in velocity space such as the Hercules stream and the Arcturus moving group, as well as stars that cannot (kinematically) be associated with either the thin disk or the thick disk. The determination of stellar parameters and elemental abundances is based on a standard 1-D LTE analysis using equivalent width measurements in high-resolution (R = 40 000 − 110 000) and high signal-to-noise (S /N = 150 − 300) spectra obtained with FEROS on the ESO 1.5-m and 2.2-m telescopes, SOFIN and FIES on the Nordic Optical Telescope, UVES on the ESO Very Large Telescope, HARPS on the ESO 3.6-m telescope, and MIKE on the Magellan Clay telescope. NLTE corrections for individual Fe i lines were employed in every step of the analysis. Results. We present stellar parameters, stellar ages, kinematical parameters, orbital parameters, and detailed elemental abundances for O, Na, Mg, Al, Si, Ca, Ti, Cr, Fe, Ni, Zn, Y, and Ba for 714 nearby F and G dwarf stars. Our data show that there is an old and α-enhanced disk population, and a younger and less α-enhanced disk population. While they overlap greatly in metallicity between −0.7 < [Fe/H] +0.1, they show a bimodal distribution in [α/Fe]. This bimodality becomes even clearer if stars where stellar parameters and abundances show larger uncertainties (T eff 5400 K) are discarded, showing that it is important to constrain the data set to a narrow range in the stellar parameters if small differences between stellar populations are to be revealed. We furthermore find that the α-enhanced population has orbital parameters placing the stellar birthplaces in the inner Galactic disk while the low-α stars mainly come from the outer Galactic disk, fully consistent with the recent claims of a short scale-length for the α-enhanced Galactic thick disk. We have also investigated the properties of the Hercules stream and the Arcturus moving group and find that neither of them present chemical or age signatures that could point to that they are disrupted clusters or extragalactic accretion remnants from ancient merger events. Instead, they are most likely dynamical features originating within the Galaxy. We furthermore have discovered that a standard 1-D, LTE analysis, utilising ionisation and excitation balance of Fe i and Fe ii lines produces a flat lower main sequence. As the exact cause for this effect is unclear we chose to apply an empirical correction. Turn-off, and more evolved, stars, appears to be un-affected.

The evolution of the Milky Way: new insights from open clusters

“…There is an indication that almost all the OCs older than 1 Gyr yield slightly more positive [α/Fe] ratios for both thin and thick disc samples than those represented by younger ages. A similar behaviour is observable for Galactic field stars where the older thin and thick disc field dwarfs from Bensby et al (2014) yield more positive [α/Fe] ratios than the younger field dwarfs with thin disc kinematics. The similarity in [α/Fe] between the thin and thick disc clusters with ages of 4 to 9 Gyr suggest that age cannot be the key variable, but different proportions of Type II to Type Ia supernovae products in clusters formed at dif- Adibekyan et al 2013;Bensby et al 2014).…”

“…A similar behaviour is observable for Galactic field stars where the older thin and thick disc field dwarfs from Bensby et al (2014) yield more positive [α/Fe] ratios than the younger field dwarfs with thin disc kinematics. The similarity in [α/Fe] between the thin and thick disc clusters with ages of 4 to 9 Gyr suggest that age cannot be the key variable, but different proportions of Type II to Type Ia supernovae products in clusters formed at dif- Adibekyan et al 2013;Bensby et al 2014). Although the origin of such objects is still in debate, Adibekyan et al (2011Adibekyan et al ( , 2013 have suggested a possible origin of these α-enhanced metal-rich stars in the inner Galaxy (bulge) followed by a subsequent migration to the solar vicinity.…”

A simple and general method for solving detailed chemical evolution with delayed production of iron and other chemical elements

“…2, since the scatter is still relatively high. Nevertheless, the work by Bensby et al (2014) claimed that thin and thick disc stars potentially exhibit two separate abundance trends, especially in the range −0.7 [Fe/H] −0.35 dex, expected by the authors also in the unpublished GAIA data.…”

CARMENES input catalogue of M dwarfs

“…In F-, G-, and K-type stars whose photospheric continua are well-defined in high-resolution spectra, stellar metallicity is computed through spectral synthesis (McWilliam 1990;Valenti & Piskunov 1996;González 1997;González Hernández et al 2004;Valenti & Fischer 2005;Recio-Blanco et al 2006) or measuring equivalent widths, especially of iron lines (Sousa et al 2008(Sousa et al , 2011Magrini et al 2010;Adibekyan et al 2012;Tabernero et al 2012;Bensby et al 2014). However, it is not possible to measure a photospheric continuum in M-type stars and, thus, their metallicity is studied through other techniques.…”