| Description |
We seek to identify stars accreted into the Milky Way which are remnants of its merging with smaller galaxies or star clusters. In our approach, we selected stars from candidate moving clusters in the Milky Way's Halo. These stars were grouped by similar stellar characteristics: orbital energy, orbital angular momentum and overall chemical enrichment (Allen et al, 2006). High-resolution spectra were gathered for ten stars split between two candidate moving clusters. Photon absorption from element excitations in the stars' atmospheres leave visible features on their spectra. Through direct measurements of these features, we can derive composition and characterize each star by a "signature" abundance of elements. The abundance of a-elements (Mg, Si, Ca and Ti) are of particular interest because they trace the initial mass function and the star formation history of a star's birthplace. Stars in the Milky Way's Halo have a common enhancement of a-elements to iron ([a/Fe] ~ 0.4 dex; values are logarithmic and normalized to the Solar abundance pattern, e.g. [a/Fe]Sun = 0 dex). Some Halo stars are thought to have extragalactic origins by their distinct low a-to-iron ratio. From our ten observed stars, we report four with an abundance of a-elements common to the Halo (0.32 < [a/Fe] < 0.43 dex) and six with low a-to-iron ratios (-0.12 < [a/Fe] < 0.19 dex). Our effective yield (6:10) of low-a to total sample stars is significantly improved compared to previous selection criteria (e.g. Fulbright 2002 found 10:96 low-a stars to his Halo selection). Iron peak elements Ni and V are also reported here. Possible progenitor systems which could have accreted these stars do not likely include globular clusters our element abundances are consistent with dwarf or ultra-faint galaxies. Current models show that a few mergers during the Galaxy's formation likely contributed stars such as these to create the outer Halo of the Milky Way. |
