| 21. |
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| 22. |
- Bensby, Thomas, et al.
(författare)
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Exploring the Milky Way stellar disk A detailed elemental abundance study of 714 F and G dwarf stars in the solar neighbourhood
- 2014
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 562
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Tidskriftsartikel (refereegranskat)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 analysis using equivalent widths and one-dimensional, plane-parallel model atmospheres calculated under the assumption of local thermodynamical equilibrium (LTE). The spectra have high resolution (R = 40 000-110 000) and high signal-to-noise)S/V = 150-300) and were obtained with the FEROS spectrograph on the ESO 1.5 in and 2.2 in telescopes, the SOFIN and PIES spectrographs on the Nordic Optical Telescope, the LIVES spectrograph on the E50 Very Large Telescope, the HARPS spectrograph on the ESO 3.6 m telescope, and the MIKE spectrograph on the Magellan Clay telescope. The abundances from individual Fe I lines were were corrected for non-LTE effects in every step of the analysis. Results. We present stellar parameters, stellar ages, kinematical parameters, orbital parameters, and detailed elemental abundances for 0, Na, Mg, Al, Si, Ca, Ti, Cr, Fe, Ni, Zn, Y. and Ba for 714 nearby 12 and G dwarf stars. Our data show that there is an old and a-enhanced disk population, and a younger and less a-enhanced disk population. While they overlap greatly in metallicity between 0.7 < [Fe/HI] less than or similar to +0.1, they show a bimodal distribution in [alpha/Fe]. This bimodality becomes even clearer if stars where stellar parameters and abundances show larger uncertainties (T-eff less than or similar to 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. In addition, we find that the a-enhanced population has orbital parameters placing the stellar birthplaces in the inner Galactic disk while the loss-alpha stars mainly come from the outer Galactic disk, fully consistent with the recent claims of a short scale-length for the alpha-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 presents chemical or age signatures that could suggest 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 have also discovered that a standard 1D. LTE analysis, utilising ionisation and excitation balance of Fe I and Fen 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 stars and more evolved stars appear to be unaffected.
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| 23. |
- Bensby, Thomas, et al.
(författare)
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OGLE-2009-BLG-076S: THE MOST METAL-POOR DWARF STAR IN THE GALACTIC BULGE
- 2009
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Ingår i: Astrophysical Journal. - 0004-637X. ; 699:2, s. 174-177
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Tidskriftsartikel (refereegranskat)abstract
- Measurements based on a large number of red giant stars suggest a broad metallicity distribution function (MDF) for the Galactic bulge, centered on [Fe/H] approximate to -0.1. However, recently, a new opportunity emerged to utilize temporary flux amplification (by factors of similar to 100 or more) of faint dwarf stars in the Bulge which are gravitationally lensed, making them observable with high-resolution spectrographs during a short observational window. Surprisingly, of the first six stars measured, five have [Fe/H] > +0.30, suggesting a highly skewed MDF, inconsistent with observations of giant stars. Here we present a detailed elemental abundance analysis of OGLE-2009-BLG-076S, based on a high-resolution spectrum obtained with the UVES spectrograph at the ESO Very Large Telescope. Our results indicate it is the most metal-poor dwarf star in the Bulge yet observed, with [Fe/H] = -0.76. Our results argue against a strong selection effect disfavoring metal-poor microlensed stars. It is possible that small number statistics is responsible for the giant/dwarf Bulge MDF discrepancy. Should this discrepancy survive when larger numbers of Bulge dwarf stars (soon to be available) are analyzed, it may require modification of our understanding of either Bulge formation models, or the behavior of metal-rich giant stars.
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| 24. |
- Bensby, Thomas, et al.
(författare)
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Oxygen in the Galactic Thin and Thick Disks
- 2003
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Ingår i: CNO in the universe. (ASP Conference Series ; Vol. 304).. - 1583811532 ; 304, s. 175-178
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Konferensbidrag (refereegranskat)
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| 25. |
- Bensby, Thomas, et al.
(författare)
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Oxygen trends in the Galactic thin and thick disks
- 2004
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 415:1, s. 155-170
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Tidskriftsartikel (refereegranskat)abstract
- We present oxygen abundances for 72 F and G dwarf stars in the solarneighbourhood. Using the kinematics of the stars we divide them into twosub-samples with space velocities that are typical for the thick andthin disks, respectively. The metallicities of the stars range from[Fe/H] ≈ -0.9 to +0.4 and we use the derived oxygen abundances of thestars to: (1) perform a differential study of the oxygen trends in thethin and the thick disk; (2) to follow the trend of oxygen in the thindisk to the highest metallicities. We analyze the forbidden oxygen linesat 6300 Å and 6363 Å as well as the (NLTE afflicted) tripletlines around 7774 Å. For the forbidden line at 6300 Å wehave spectra of very high S/N (>400) and resolution (R ≳ 215000). This has enabled a very accurate modeling of the oxygen line andthe blending Ni lines. The high internal accuracy in our determinationof the oxygen abundances from this line is reflected in the very tighttrends we find for oxygen relative to iron. From these abundances we areable to draw the following major conclusions: (i) That the [O/Fe] trendat super-solar [Fe/H] continues downward which is in concordance withmodels of Galactic chemical evolution. This is not seen in previousstudies as it has not been possible to take the blending Ni lines in theforbidden oxygen line at 6300 Å properly into account; (ii) Thatthe oxygen trends in the thin and the thick disks are distinctlydifferent. This confirms and extends previous studies of the otherα-elements; (iii) That oxygen does not follow Mg at super-solarmetallicities; (iv) We also provide an empirical NLTE correction for theinfrared O I triplet that could be used for dwarf star spectra with aS/N such that only the triplet lines can be analyzed well, e.g. stars atlarge distances; (v) Finally, we find that Gratton et al. (1999)overestimate the NLTE corrections for the permitted oxygen triplet linesat ∼7774 Å for the parameter space that our stars span.Based on observations collected at the European Southern Observatory, LaSilla and Paranal, Chile, Proposals #65.L-0019, 67.B-0108, and69.B-0277.The full Table 4 is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?/A+A/415/155
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| 26. |
- Bensby, Thomas, et al.
(författare)
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Signatures of an Intermediate-Age Metal-Rich Bulge Population
- 2012
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Ingår i: Galactic Archaeology: Near-Field Cosmology and the Formation of the Milky Way (Astronomical Society of the Pacific Conference Series). - 9781583817988 ; 458, s. 203-204
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Konferensbidrag (refereegranskat)abstract
- We have determined detailed elemental abundances and stellar ages for a sample of now 38 microlensed dwarf and subgiant stars in the Galactic bulge. Stars with sub-solar metallicities are all old and have enhanced alpha-element abundances - very similar to what is seen for local thick disk stars. The metal-rich stars on the other hand show a wide variety of stellar ages, ranging from 3-4 Gyr to 12 Gyr, and an average around 7-8 Gyr. The existence of young and metal-rich stars are in conflict with recent photometric studies of the bulge which claim that the bulge only contains old stars.
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| 27. |
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| 28. |
- Bensby, T., et al.
(författare)
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The Nature of the Metal-Rich Thick Disk
- 2007
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Ingår i: ASP Conference Proceedings. - 9781583813096 ; 374, s. 181-186
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Konferensbidrag (refereegranskat)abstract
- We have traced the Galactic thick disk to its, to date, highest metallicities. Based on high-resolution spectroscopic observations of 187 F and G dwarf stars that kinematically can be associated either with the thin disk (60 stars) or with the thick disk (127 stars), we find that the thick disk stars reach at least solar metallicities, and maybe even higher. This finding is independent of the U_LSR, V_LSR and W_LSR velocities of the stars.
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| 29. |
- Bensby, Thomas, et al.
(författare)
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The origin and chemical evolution of carbon in the Galactic thin and thick discs
- 2006
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Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 1365-2966 .- 0035-8711. ; 367:3, s. 1181-1193
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Tidskriftsartikel (refereegranskat)abstract
- In order to trace the origin and evolution of carbon in the Galactic disc, we have determined carbon abundances in 51 nearby F and G dwarf stars. The sample is divided into two kinematically distinct subsamples with 35 and 16 stars that are representative of the Galactic thin and thick discs, respectively. The analysis is based on spectral synthesis of the forbidden [C I] line at 872.7 nm using spectra of very high resolution (R approximate to 220 000) and high signal-to-noise ratio (S/N greater than or similar to 300) that were obtained with the Coude Echelle Spectrograph (CES) spectrograph by the European Southern Observatory (ESO) 3.6-m telescope at La Silla in Chile. We find that [C/Fe] versus [Fe/H] trends for the thin and thick discs are totally merged and flat for subsolar metallicities. The thin disc that extends to higher metallicities than the thick disc shows a shallow decline in [C/Fe] from [Fe/H] approximate to 0 and up to [Fe/H]approximate to+0.4. The [C/O] versus [O/H] trends are well separated between the two discs (due to differences in the oxygen abundances) and bear a great resemblance to the [Fe/O] versus [O/H] trends. Our interpretation of our abundance trends is that the sources that are responsible for the carbon enrichment in the Galactic thin and thick discs have operated on a time-scale very similar to those that are responsible for the Fe and Y enrichment [i.e. SN Ia and asymptotic giant branch (AGB) stars, respectively]. We further note that there exist other observational data in the literature that favour massive stars as the main sources for carbon. In order to match our carbon trends, we believe that the carbon yields from massive stars then must be very dependent on metallicity for the C, Fe and Y trends to be so finely tuned in the two disc populations. Such metallicity-dependent yields are no longer supported by the new stellar models in the recent literature. For the Galaxy, we hence conclude that the carbon enrichment at metallicities typical of the disc is mainly due to low- and intermediate-mass stars, while massive stars are still the main carbon contributor at low metallicities (halo and metal-poor thick disc).
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| 30. |
- Bergemann, M., et al.
(författare)
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The Gaia-ESO Survey : radial metallicity gradients and age-metallicity relation of stars in the Milky Way disk
- 2014
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 565, s. A89-
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Tidskriftsartikel (refereegranskat)abstract
- We study the relationship between age, metallicity, and alpha-enhancement of FGK stars in the Galactic disk. The results are based upon the analysis of high-resolution UVES spectra from the Gaia-ESO large stellar survey. We explore the limitations of the observed dataset, i.e. the accuracy of stellar parameters and the selection effects that are caused by the photometric target preselection. We find that the colour and magnitude cuts in the survey suppress old metal-rich stars and young metal-poor stars. This suppression may be as high as 97% in some regions of the age-metallicity relationship. The dataset consists of 144 stars with a wide range of ages from 0.5 Gyr to 13.5 Gyr, Galactocentric distances from 6 kpc to 9.5 kpc, and vertical distances from the plane 0 < vertical bar Z vertical bar < 1.5 kpc. On this basis, we find that i) the observed age-metallicity relation is nearly flat in the range of ages between 0 Gyr and 8 Gyr; ii) at ages older than 9 Gyr, we see a decrease in [Fe/H] and a clear absence of metal-rich stars; this cannot be explained by the survey selection functions; iii) there is a significant scatter of [Fe/H] at any age; and iv) [Mg/Fe] increases with age, but the dispersion of [Mg/Fe] at ages > 9 Gyr is not as small as advocated by some other studies. In agreement with earlier work, we find that radial abundance gradients change as a function of vertical distance from the plane. The [Mg/Fe] gradient steepens and becomes negative. In addition, we show that the inner disk is not only more alpha-rich compared to the outer disk, but also older, as traced independently by the ages and Mg abundances of stars.
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