Thomas Bensby

alpha-, r-, and s-process element trends in the Galactic thin and thick disks

From a detailed elemental abundance analysis of 102 F and G dwarf starswe present abundance trends in the Galactic thin and thick disks for 14elements (O, Na, Mg, Al, Si, Ca, Ti, Cr, Fe, Ni, Zn, Y, Ba, and Eu).Stellar parameters and elemental abundances (except for Y, Ba and Eu)for 66 of the 102 stars were presented in our previous studies (Bensbyet al. 2003, 2004a). The 36 stars that are new in this study extend andconfirm our previous results and allow us to draw further conclusionsregarding abundance trends. The s-process elements Y and Ba, and ther-element Eu have also been considered here for the whole sample for thefirst time. With this new larger sample we now have the followingresults: 1) Smooth and distinct trends that for the thin and thick disksare clearly separated; 2) The alpha-element trends for the thick diskshow typical signatures from the enrichment of SNIa; 3) The thick diskstellar sample is in the mean older than the thin disk stellar sample;4) The thick disk abundance trends are invariant with galactocentricradii (R_m); 5) The thick disk abundance trends appear to be invariantwith vertical distance (Z_max) from the Galactic plane. Adding furtherevidence from the literaure we argue that a merger/interacting scenariowith a companion galaxy to produce a kinematical heating of the stars(that make up todays thick disk) in a pre-existing old thin disk is themost likely formation scenario for the Galactic thick disk. (Less)

Tracing the Galactic Thick Disk to Solar Metallicities

We show that the Galactic thick disk reaches at least solar metallicities and that it experienced strong chemical enrichment during a period of ~3 Gyr, ending around 8-9 Gyr ago. This finding puts further constraints on the relation and interface between the thin and thick disks and their formation processes. Our results are based on a detailed elemental abundance analysis of 261 kinematically selected F and G dwarf stars in the solar neighborhood: 194 likely members of the thick disk and 67 likely members of the thin disk, in the range -1.3<~[Fe/H]<~+0.4. Based on data collected with the 6.5 m Magellan telescopes at the Las Campanas Observatory and with the Very Large Telescope at the European Southern Observatory (ESO proposal 72.B-0179). (Less)

The origin and chemical evolution of carbon in the Galactic thin and thick discs

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 kinemat- ically 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 ≈ 220 000) and high signal-to-noise ratio (S/N 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) ≈ 0 and up to (Fe/H) ≈+ 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 abun- dance 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).

Disentangling the Hercules stream

Using high-resolution spectra of nearby F and G dwarf stars, we have investigated the detailed abundance and age structure of the Hercules stream. We find that the stars in the stream have a wide range of stellar ages, metallicities, and element abundances. By comparing to existing samples of stars in the solar neighborhood with kinematics typical of the Galactic thin and thick disks, we find that the properties of the Hercules stream distinctly separate into the abundance and age trends of the two disks. Hence, we find it unlikely that the Hercules stream is a unique Galactic stellar population but rather is a mixture of thin and thick disk stars. This points toward a dynamical origin for the Hercules stream, probably caused by the Galactic bar.

Manganese trends in a sample of thin and thick disk stars - The origin of Mn

Context. Manganese is an iron-peak element and although the nucleosynthesis path that leads to its formation is fairly well understood, it remains unclear which objects, SN II and/or SN Ia, that contribute the majority of Mn to the interstellar medium. It also remains unclear to which extent the supernovae Mn yields depend on the metallicity of the progenitor star or not. Aims. By using a well studied and well defined sample of 95 dwarf stars we aim at further constraining the formation site(s) of Mn. Methods. We derive Mn abundances through spectral synthesis of four Mn I lines at 539.4, 549.2, 601.3, and 601.6 nm. Stellar parameters and data for oxygen are taken from Bensby et al. (2003, 2004, 2005). Results. When comparing our Mn abundances with O abundances for the same stars we find that the abundance trends in the stars with kinematics typical of the thick disk can be explained by metallicity dependent yields from SN II. We go on and combine our data for dwarf stars in the disks with data for dwarf and giant stars in the metal-poor thick disk and halo from the literature. We find that dwarf and giant stars show the same trends, which indicates that neither non-LTE nor evolutionary effects are a major concern for Mn. Furthermore, the [Mn/O] vs. [O/H] trend in the halo is flat. Conclusions. We conclude that the simplest interpretation of our data is that Mn is most likely produced in SN II and that the Mn yields for such SNae must be metallicity dependent. Contribution from SN Ia in the metal-rich thin disk can not, however, be excluded.

A GLIMPSE into the Nature of Galactic Mid-IR Excesses

We investigate the nature of the mid-IR excess for 31 intermediate-mass stars that exhibit an 8 � m excess in either theGalacticLegacyInfraredMid-PlaneSurveyExtraordinaireortheMid-CourseSpaceExperimentusinghigh-reso- lutionopticalspectratoidentifystarssurroundedbywarmcircumstellardust.Fromthesedatawedetermineprojected stellarrotationalvelocitiesandestimatestellareffectivetemperaturesforthesample.Weestimatestellaragesfromthese temperatures,parallacticdistances,andevolutionarymodels.UsingMIPS(24)measurementsandstellarparameterswe determine the nature of the infrared excess for 19 GLIMPSE stars. Wefind that 15 stars exhibit Hemission and four exhibitHabsorption.Assuming thatthemid-IR excessesarise incircumstellar disks,weusethe Hfluxestomodel and estimate the relative contributions of dust and free-free emission. Six stars exhibit Hfluxes that imply free-free emission can plausibly explain the infrared excess at (24). These stars are candidate classical Be stars. Nine stars ex- hibitHemission,buttheirHfluxesareinsufficienttoexplain theinfraredexcessesat(24),suggestingthepresence ofacircumstellar dustcomponent.After theremoval of thefree-free componentinthese sources,we determineprob- able disk dust temperatures of Tdisk 300 800 K and fractional infrared luminosities of LIR/L� 10 � 3 . These nine stars may be pre-main-sequence stars with transitional disks undergoing disk clearing. Three of the four sources showing Habsorption exhibit circumstellar disk temperatures 300-400 K, LIR/L� 10 � 3 , IR colors K �½ 24� 3:3 implying an op- tically thick outer disk and is a transition disk candidate. Subject headingg circumstellar matter — line: profiles — stars: emission-line, Be — stars: fundamental parameters — stars: rotation — techniques: spectroscopic

The Determination of Stellar Parameters of Giants in the Galactic Disks and Bulge

Here, we present our on-going work on the determination of stellar parameters of giants in the Galactic Disks and Bulge observed with UVES on the VLT. We present some preliminarily results.

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…)