Letter to the Editor
Evidence from stellar abundance ratios and kinematics
Department of Physics and Astronomy, University of Aarhus, 8000
Aarhus C, Denmark e-mail: firstname.lastname@example.org
2 Observatorio Astronómico Nacional, Universidad Nacional Autónoma de México, Apartado Postal 877, CP 22800 Ensenada, B.C., México e-mail: email@example.com
Accepted: 18 February 2010
Aims. Precise abundance ratios are determined for 94 dwarf stars with 5200 < Teff < 6300 K, -1.6 < [Fe/H] < -0.4, and distances D 335 pc. Most of them have halo kinematics, but 16 thick-disk stars are included.
Methods. Equivalent widths of atomic lines are measured from VLT/UVES and NOT/FIES spectra with resolutions R 55 000 and R 40 000, respectively. An LTE abundance analysis based on MARCS models is applied to derive precise differential abundance ratios of Na, Mg, Si, Ca, Ti, Cr, and Ni with respect to Fe.
Results. The halo stars fall into two populations, clearly separated in [ α/Fe] , where α refers to the average abundance of Mg, Si, Ca, and Ti. Differences in [Na/Fe] and [Ni/Fe] are also present with a remarkably clear correlation between these two abundance ratios.
Conclusions. The “high-α” stars may be ancient disk or bulge stars “heated” to halo kinematics by merging satellite galaxies or they could have formed as the first stars during the collapse of a proto-Galactic gas cloud. The kinematics of the “low-α” stars suggest that they have been accreted from dwarf galaxies, and that some of them may originate from the ω Cen progenitor galaxy.
Key words: stars: abundances / stars: kinematics and dynamics / Galaxy: halo / Galaxy: formation
Based on observations made with the Nordic Optical Telescope on La Palma, and on data from the European Southern Observatory ESO/ST-ECF Science Archive Facility.
Tables 3 and 4 are also available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (18.104.22.168) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/511/L10
Figures 5–8 and Tables 1–4 are only available in electronic form at http://www.aanda.org
© ESO, 2010