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| 2. |
- Adén, Daniel, et al.
(författare)
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A photometric and spectroscopic study of the new dwarf spheroidal galaxy in Hercules. Metallicity, velocities and a clean list of RGB members
- 2009
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 506:3, s. 1147-1168
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Tidskriftsartikel (refereegranskat)abstract
- Our aim is to provide as clean and as complete a sample as possible of red giant branch stars that are members of the Hercules dSph galaxy. With this sample we explore the velocity dispersion and the metallicity of the system. Stromgren photometry and multi-fibre spectroscopy are combined to provide information about the evolutionary state of the stars (via the Stromgren c_1 index) and their radial velocities. Based on this information we have selected a clean sample of red giant branch stars, and show that foreground contamination by Milky Way dwarf stars can greatly distort the results. Our final sample consists of 28 red giant branch stars in the Hercules dSph galaxy. Based on these stars we find a mean photometric metallicity of -2.35 dex which is consistent with previous studies. We find evidence for an abundance spread. Using those stars for which we have determined radial velocities we find a systemic velocity of 45.2 km/s with a dispersion of 3.72 km/s, this is lower than values found in the literature. Furthermore we identify the horizontal branch and estimate the mean magnitude of the horizontal branch of the Hercules dSph galaxy to be V_0=21.17, which corresponds to a distance of 147 kpc. We have shown that a proper cleaning of the sample results in a smaller value for the velocity dispersion of the system. This has implications for galaxy properties derived from such velocity dispersions.
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| 3. |
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| 4. |
- Arnadottir, Anna, et al.
(författare)
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The ability of intermediate-band Stromgren photometry to correctly identify dwarf, subgiant, and giant stars and provide stellar metallicities and surface gravities
- 2010
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 521
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Tidskriftsartikel (refereegranskat)abstract
- Context. Several large scale photometric and spectroscopic surveys are being undertaken to provide a more detailed picture of the Milky Way. Given the necessity of generalisation in the determination of, e.g., stellar parameters when tens and hundred of thousands of stars are considered it remains important to provide independent, detailed studies to verify the methods used in the surveys. Aims. Our first aim is to critically evaluate available calibrations for deriving [M/H] from Stromgren photometry. Secondly, we develop the standard sequences for dwarf stars to reflect their inherent metallicity dependence. Finally, we test how well metallicities derived from ugriz photometry reproduce metallicities derived from the well-tested system of Stromgren photometry. Methods. We evaluate available metallicity calibrations based on Stromgren uvby photometry for dwarf stars using a catalogue of stars with both uvby photometry and spectroscopically determined iron abundances ([Fe/H]). The catalogue was created for this project. Using this catalogue, we also evaluate available calibrations that determine log g. A larger catalogue, in which metallicity is determined directly from uvby photometry, is used to trace metallicity-dependent standard sequences for dwarf stars. We also perform comparisons, for both dwarf and giant stars, of metallicities derived from ugriz photometry with metallicities derived from Stromgren photometry. Results. We provide a homogenised catalogue of 451 dwarf stars with 0.3 < (b - y)(0) < 1.0. All stars in the catalogue have uvby photometry and [Fe/H] determined from spectra with high resolution and high signal-to-noise ratios (S/N). Using this catalogue, we test how well various photometric metallicity calibrations reproduce the spectroscopically determined [Fe/H]. Using the preferred metallicity calibration for dwarf stars, we derive new standard sequences in the c(1,0) versus (b - y)(0) plane and in the c(1,0) versus (v - y)(0) plane for dwarf stars with 0.40 < (b - y)(0) < 0.95 and 1.10 < (v - y)(0) < 2.38. Conclusions. We recommend the calibrations of Ramirez & Melendez (2005) in deriving metallicities from Stromgren photometry and find that intermediate band photometry, such as Stromgren photometry, more accurately than broad band photometry reproduces spectroscopically determined [Fe/H]. Stromgren photometry is also better at differentiating between dwarf and giant stars. We conclude that additional investigations of the differences between metallicities derived from ugriz photometry and intermediate-band photometry, such as Stromgren photometry, are required.
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| 5. |
- Bensby, Thomas, et al.
(författare)
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A Differential Study of the Oxygen Abundances in the Galactic Thin and Thick Disks
- 2004
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Ingår i: Carnegie Observatories Astrophysics Series, Origin and Evolution of the Elements. ; 4
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Konferensbidrag (refereegranskat)abstract
- First results from a study into the abundance trends of oxygen in the Galactic thin and thick disks are presented. Oxygen abundances for 63nearby F and G dwarf stars, based on very high resolution spectra (R∼215 000) and high signal-to-noise (S/N>400) of the faint forbidden oxygen line at 6300Å, have been determined. Our findings can be summarized as follows: 1) at [Fe/H]<0 the oxygen trends in thethin and thick disk are smooth and distinct, indicating their different origins, 2) [O/Fe] for the thick disk stars show a turn-over at[Fe/H]∼-0.35, indicating the peak of the enrichment from SNe type Ia to the interstellar medium, 3) the thin disk stars show a shallow decrease going from [Fe/H] ∼-0.7 to the highest metallicities with no apparent turn-over present indicating a more quiet star formation history, and 4) [O/Fe] continues to decrease at [Fe/H] > 0 without showing the leveling out that previously has been seen.
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| 6. |
- Bensby, Thomas, et al.
(författare)
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A possible age-metallicity relation in the Galactic thick disk?
- 2004
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 1432-0746 .- 0004-6361. ; 421, s. 969-976
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Tidskriftsartikel (refereegranskat)abstract
- A sample of 229 nearby thick disk stars has been used to investigate theexistence of an age-metallicity relation (AMR) in the Galactic thickdisk. The results indicate that that there is indeed an age-metallicityrelation present in the thick disk. By dividing the stellar sample intosub-groups, separated by 0.1 dex in metallicity, we show that the medianage decreases by about 5-7 Gyr when going from [Fe/H] ≈ -0.8 to[Fe/H] ≈ -0.1. Combining our results with our newly publishedα-element trends for a local sample of thick disk stars that showsignatures from supernovae type Ia (SN Ia), we draw the conclusion thatthe time-scale for the peak of the SN Ia rate is of the order of 3-4 Gyrin the thick disk. The tentative evidence for a thick disk AMR that wepresent here also has implications for the thick disk formationscenario; star-formation must have been an ongoing process for severalbillion years. This appears to strengthen the hypothesis that the thickdisk originated from a merger event with a companion galaxy that puffedup a pre-existing thin disk.Based on data obtained with the Hipparcos satellite.
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| 7. |
- Bensby, Thomas, et al.
(författare)
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alpha-, r-, and s-process element trends in the Galactic thin and thick disks
- 2005
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 1432-0746 .- 0004-6361. ; 433, s. 185-203
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Tidskriftsartikel (refereegranskat)abstract
- 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 today's thick disk) in a pre-existing old thin disk is themost likely formation scenario for the Galactic thick disk.
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| 8. |
- Bensby, Thomas, et al.
(författare)
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Elemental abundance trends in the Galactic thin and thick disks as traced by nearby F and G dwarf stars
- 2003
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 410:2, s. 527-551
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Forskningsöversikt (refereegranskat)abstract
- Based on spectra from F and G dwarf stars, we present elementalabundance trends in the Galactic thin and thick disks in the metallicityregime -0.8< [Fe/H < +0.4. Our findings can besummarized as follows. 1) Both the thin and the thick disks show smoothand distinct abundance trends that, at sub-solar metallicities, areclearly separated. 2) For the alpha -elements the thick disk showssignatures of chemical enrichment from SNe type Ia. 3) The age of thethick disk sample is in the mean older than the thin disk sample. 4)Kinematically, there exist thick disk stars with super-solarmetallicities. Based on these findings, together with other constraintsfrom the literature, we discuss different formation scenarios for thethick disk. We suggest that the currently most likely formation scenariois a violent merger event or a close encounter with a companion galaxy.Based on kinematics the stellar sample was selected to contain starswith high probabilities of belonging either to the thin or to the thickGalactic disk. The total number of stars are 66 of which 21 belong tothe thick disk and 45 to the thin disk. The analysis is based onhigh-resolution spectra with high signal-to-noise (R 48 000 and S/Ngtrsim 150, respectively) recorded with the FEROS spectrograph on LaSilla, Chile. Abundances have been determined for four alpha -elements(Mg, Si, Ca, and Ti), for four even-nuclei iron peak elements (Cr, Fe,Ni, and Zn), and for the light elements Na and Al, from equivalent widthmeasurements of 30 000 spectral lines. An extensive investigation ofthe atomic parameters, log gf-values in particular, have been performedin order to achieve abundances that are trustworthy. Noteworthy is thatwe find for Ti good agreement between the abundances from Ti I and TiIi. Our solar Ti abundances are in concordance with the standardmeteoritic Ti abundanceBased on observations collected at the European Southern Observatory, LaSilla, Chile, Proposals #65.L-0019(B) and 67.B-0108(B).Full Tables eftab:linelist and eftab:abundances are onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/410/527
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| 9. |
- 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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| 10. |
- 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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