OCR Text |
Show 250 Dylan Gregersen EVIDENCE FOR EXTRAGALACTIC STARS WITHIN THE MILKY WAY HALO Dylan Gregersen (Inese I. Ivans) Department of Physics and Astronomy University of Utah honors college spring 2012 The history of our Galaxy's formation is still unknown, though evidence suggests that merg-ers with smaller stellar populations influenced the present size and structure of the Milky Way. Studying the kinematics, orbital motion and chemical composition of stars from past popula-tions can provide insight to their origins and constrain the formation history of the galaxy. We searched for stars with extragalactic origins by selecting stars from candidate moving clusters- Milky Way Halo stars grouped together by similar orbital energy, orbital angular momentum and overall chemical enrichment (Allen et al. 2007, Proc. IAU, 2: 405-413). We obtained high-resolution spectra for ten stars belonging to one of two candidate moving clusters. We derived the abun-dance of elements for each stellar atmosphere by measuring absorption features and fitting with model spectra. The abundance of alpha elements (Mg, Si, Ca and Ti) trace conditions of a star's initial environment. Stars with a low abundance of alpha elements are rare in the Halo, only found on the order of a few in a thousand. From our ten observed stars, we report five with an al-pha abundance common to the Halo (0.35< [alpha/Fe] < 0.44 dex) and five with low alpha-to-iron ratio (-0.12 < [alpha/Fe] < 0.17 dex). Our 50% effective yield of low-alpha to halo-normal-alpha stars marks a significant improvement over previous selection criteria. Additional abundances of iron-peak (Cr, Mn, Co, Ni and Cu), light (C and O), odd-Z (Na and Al), and rapid- and slow-neutron-capture process (Y, Ba, Eu and La) elements provide clues to past supernova enrichment and the progenitor stars. Linking chemical information to a star's kinematics and orbital parameters provides strong evidence for the extragalactic origins of candidate stars and contributes to our understanding of the Galaxy's formation. Values of element abundances are logarithmic and normalized to the Solar abundance pattern, e.g. [element/Fe]Sun = 0 dex. Stellar element abundances for this study's stars are plotted as circles, red for one candidate moving cluster and blue for the other. The abundance of singling ionized Ti are plotted with crosses; the relationship of elements in different ionization states acts as a constraint for determining atmospheric parameters (effective temperature, surface gravity, and micro-turbulence). Halo stars (from Fulbright, J.P. 2000, AJ, 120: 1841) are plotted for compar-ison as open circles, with high velocity stars colored purple. Stars in common between the two studies are connected by lines. Inese I. Ivans |