Abundances in a Large Sample of Stars in the LMC Disk

Luciana Pompeia, Vanessa Hill, Monique Spite ∗ a

aObservatoire de Paris-Meudon, 92125 Meudon Cedex, France

In the present project we aim to infer chemical abundances of stars pertaining to dif-ferent populations of the Large Magellanic Cloud (LMC). For this propose samples havebeen selected from three fields in the LMC: a field in the Inner Disk (with galactocentricradius of 2kpc); a field in the Outer Disk (with galactocentric radius of 7kpc) and a fieldin the Bar. In this paper chemical abundances of some α, iron-peak group and s-processelements are reported for 52 stars from the Inner Disk sample.

1. Introduction

The Large Magellanic Cloud (LMC), one of the nearest galaxies of the Milky Way(MW), is an irregular galaxy which shows a disk, a bar and a thick disk or a flattenedhalo. Due to the almost face on position of the disk of the LMC, stars from its variouscomponents can be distinguished. In the present project chemical abundances of sig-nificant samples (∼100) of stars of the different components of the LMC within a largeextent in metallicity are inferred. For this propose we selected targets based on metal-licities derived from the Ca II triplet and kinematics of stars from different fields in theLMC, sampling as evenly as possible the full metallicity range (particularly the metal-poor tail of the distribution). Here we present the results for the Inner Disk sample (withgalactocentric radius of 2 kpc) which was selected from the Ca II Triplet measurementsof Smecker-Hane et al. (SMH04).

2. Observations and Analysis

The observations were made at the VLT UT2 8m telescope at Paranal using theFLAMES/GIRAFFE multiobject spectrograph. We used the MEDUSA configurationwith the following setups: H14 λ638.0 - λ661.5 nm; H13 λ612.0 - λ639.8 nm and H11λ559.7 - λ584.0 nm. The setups were chosen in order to cover the maximum number ofkey elements (e.g. α and s process elements). The spectral resolution is of 22.000-29.00and the signal to noise ratio SN≈40. The line list and the atomic data were selected fromthe papers of Kraft et al. (1992), Sneden et al. 1991, Fulbright (2000) and Edvardsson etal. (1993). The radial velocities of the stars and the equivalent width (EW) of the lineswere automatically estimated by using the program daospec of Stetson (PB Stetson & E.Pancino, in preparation).

∗L.P. thanks CAPES (brazilian government) fellowship no 0606-03-0

Nuclear Physics A 758 (2005) 242c–245c

0375-9474/$ – see front matter © 2005 Elsevier B.V. All rights reserved.doi:10.1016/j.nuclphysa.2005.05.043

Spectroscopic stellar parameters were inferred from the abundance estimates from theEW. Effective temperatures were derived from the excitation equilibrium of the Fe1 lines(forcing no slope in the A(Fe1) vs. ξexc graphic); microturbulent velocities were inferreddemanding that lines of different EW give the same iron abundance; and gravities weredetermined by forcing the ionization equilibrium of Fe1 and Fe2. At the temperatureand gravity range of the present samples Teff and log g determinations are extremelycorrelated. The sample comprises 52 stars in the temperature range 3880K ≤ Teff ≤4490K with metallicities -1.76 ≤ [Fe/H] ≤s -0.31. Abundance errors vary from 0.03 dexto 0.23.

3. Abundance Determination

Abundances were derived from EW for 11 elements (in parenthesis the average numberof lines used in the analysis): Fe (45), Ni (7), Cr (4), V (11), Co (3), Si (3), Ca (10), Ti(7), Na (3), Y (2) and Zr (4). We also derived abundances by using line synthesis forthree elements: oxygen ([O I] 6300A); magnesium (λ6318 A) and lanthanium (λ6320 Ataking into account the HFS). In Figs. 1 and 2 the abundance distributions are shownfor the iron-peak group, s-elements, α-elements and Mg, O and Na. Our sample data arerepresented as filled circles and MW disk and halo data as open squares. There are alsoLMC data from globular clusters (Hill et al. 2000, filled squares) and the disk (Smith etal. 2003. filled triangles).

4. Discussion

As can be seen in Fig. 1, [Co/Fe] follows the solar values for almost all the metallicityrange with similar behavior compared to the Galactic Disk and Halo (GDH). [Cr/Fe],[V/Fe] and [Ni/Fe] have subsolar ratios for most stars; and show an underabundancerelative to the GDH in almost all stars; [V/Fe] and [Cr/Fe] show a high scatter. InFig. 1 we notice a high scatter in [Zr/Fe] ratios, probably due to the low number oflines; these ratios are also underabundant relative to the galactic samples with maybea trend of lower values for higher metallicities. [La/Fe] present supersolar ratios for allthe metallicity range, a behavior already observed for other samples of the LMC (see e.g.Hill 2004). Samples of the LMC show an overlap of the [O/Fe] ratios, with a decreasingtrend for higher metallicity stars. Compared to the GDH, the decreasing trend occoursat a lower metallicity. [Mg/Fe] show a decreasing trend for higher metallicity stars anda match with the Galactic trend with a higher scatter. [Na/Fe] ratios are subsolar andunderabundant relative to the GDH with apparently no trend with metallicity.

5. Conclusions

Some aspects can be outlined from the observed chemical distributions: LMC starsshow a definetelly different chemical pattern when compared to the GDH samples; mostelements show lower [X/Fe] ratios relative to the disk samples hinting for a higher contri-bution from SNe Ia, and probably a slower star formation history; the different patternobserved for Mg/Fe and O/Fe compared to those of the other α-elements indicates differ-ent nucleosynthetic regimes among these elements.

L. Pompéia et al. / Nuclear Physics A 758 (2005) 242c–245c 243c

[Fe/H]

-2 -1.5 -1 -0.5 0-1

0

1

[Fe/H]

-2 -1.5 -1 -0.5 0-1

0

1

-2 -1.5 -1 -0.5 0-1

0

1

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-2 -1.5 -1 -0.5 0-1

-0.5

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[Fe/H]

-2 -1.5 -1 -0.5 0-1.5

-1

-0.5

0

0.5

1

Figure 1. Abundances distributions for iron-peak elements (left) and for s-process elements(right). Data for our sample are depicted as filled circles and data for the MW disk and haloare shown as open squares. References for the MW data are: Fulbright (2000); Reddy et al.(2003); Prochaska et al. (2000); Burris et al. (2000); Johnson et al. (2002); Nissen et al. (2002)and Bensby et al. (2004).

[Fe/H]

-2 -1.5 -1 -0.5 0-1

-0.5

0

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-2 -1.5 -1 -0.5 0-1

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[Fe/H]

-2 -1.5 -1 -0.5 0-1

-0.5

0

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-2 -1.5 -1 -0.5 0

-1

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-2 -1.5 -1 -0.5 0-1

-0.5

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-2 -1.5 -1 -0.5 0-1

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[Fe/H]

Figure 2. Abundances distributions for α-elements and Na. Samples for the LMC are alsoincluded: data for globular clusters - Hill et al. 2000 (filled squares), and data for the LMC disk- Smith et al. 2002 (filled triangles).

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