Two distinct halo populations in the solar neighborhood
Department of Physics and AstronomyUniversity of Aarhus, 8000 Aarhus C, Denmark
2 Observatorio Astronómico Nacional, Universidad Nacional Autónoma de México, Apartado Postal 877, C.P. 22800 Ensenada, B.C., Mexico
Received: 1 February 2011
Accepted: 16 March 2011
Context. Current models of galaxy formation predict that the Galactic halo was assembled hierarchically. By measuring abundance ratios in stars it may be possible to identify substructures in the halo resulting from this process.
Aims. A previous study of 94 dwarf stars with −1.6 < [Fe/H] < −0.4 in the solar neighborhood has revealed the existence of two distinct halo populations with a systematic difference in [α/Fe] at a given metallicity. In continuation of that work, abundances of Mn, Cu, Zn, Y, and Ba are determined for the same sample of stars.
Methods. Equivalent widths of atomic lines are measured from high resolution VLT/UVES and NOT/FIES spectra and used to derive abundance ratios from an LTE analysis based on MARCS model atmospheres. The analysis is made relative to two thick-disk stars, HD 22879 and HD 76932, such that very precise differential values are obtained.
Results. Systematic differences between the “high-α” and “low-α” halo populations are found for [Cu/Fe], [Zn/Fe], and [Ba/Y], whereas there is no significant difference in the case of [Mn/Fe]. At a given metallicity, [Cu/Fe] shows a large scatter that is closely correlated with a corresponding scatter in [Na/Fe] and [Ni/Fe].
Conclusions. The metallicity trends of [Cu/Fe], [Zn/Fe], and [Ba/Y] can be explained from existing nucleosynthesis calculations if the high-α stars formed in regions with such a high star formation rate that only massive stars and type II supernovae contributed to the chemical enrichment. The low-α stars, on the other hand, most likely originate from systems with a slower chemical evolution, characterized by additional enrichment from type Ia supernovae and low-mass AGB stars.
Key words: stars: atmospheres / stars: abundances / Galaxy: abundances / 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 (programs 65.L-0507, 67.D-0086, 67.D-0439, 68.D-0094, 68.B-0475, 69.D-0679, 70.D-0474, 71.B-0529, 72.B-0585, 76.B-0133 and 77.B-0507).
Tables 1, 2, and full Table 3 are also available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (126.96.36.199) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/530/A15
© ESO, 2011