Abundances from High-Resolution Spectra of Kinematically Interesting Halo Stars

Abstract

There are stars in the halo of the Galaxy whose orbital properties aredistinctly different from the majority of the halo population. Onesuggestion to account for these kinematically peculiar stars is thatthey have been gravitationally subsumed by the Milky Way through thebreakup of nearby dwarf galaxies. One test of this hypothesis lies indetermining the chemical composition of the deviant halo stars relativeto that of normal (``native'') halo field stars. The possibly accretedstars are predicted to have different chemical enrichments due to aslower rate of star formation in nearby low-mass spheroidal or irregulargalaxies. We have obtained echelle spectra of metal-poor halo dwarfswith unusual orbital properties: from the outer halo, from the ``high''halo, and on retrograde orbits. Our spectra are at high spectralresolution (35,000-48,000) and high signal-to-noise ratios (median value140), primarily from the Keck I 10 m telescope with HIRES (53 stars),but also from the echelle spectrometer of the KPNO Mayall 4 m Telescope(three stars). The spectra cover a large spectral range: ~4000-6800{Å}. The strengths of approximately 9000 lines have been measured todetermine the stellar parameters (T_{eff}, logg, [Fe/H], and{ξ}) and the abundances of 10 elements, including Na, {α}-fusionproducts (Mg, Si, Ca, Ti), iron peak elements (Cr, Fe, Ni), and neutroncapture elements (Y, Ba). A model atmosphere was computed for each star,which was used to determine the composition under the assumption of LTE.The comparison of the abundances with the kinematic properties revealedno special trends, except that the ratio of the mean of the{α}-fusion elements to Fe is weakly correlated with stellarapogalactica in the sense that the outermost stars have lower ratios of[{α}/Fe] than those with smaller apogalactica. The hallmark of anaccreted halo star is presumed to be a deficiency (compared with normalstars) of the {α}-elements (e.g., O, Mg, Si, Ca, Ti) with respect toFe, a consequence of sporadic bursts of star formation within the dwarfgalaxies. However, we have found no new stars with such subsolar ratiosof [{α}/Fe]. For BD +80{\deg}245 we derive [{α}/Fe]=-0.22 inagreement with earlier results. For all four of our {α}-elementratios, [Mg/Fe], [Si/Fe], [Ca/Fe], and [Ti/Fe], we find high values atlow metallicities that decrease to near-solar values at highmetallicities. The slope of the mean [{α}/Fe] with [Fe/H] isapproximately -0.15. At a given metallicity, the outer halo stars havelower [{α}/Fe] than the inner halo stars. There is a spread in[{α}/Fe] beyond the individual measurement errors at both high andlow [Fe/H]. It appears that the ratios of [Na/Fe] and [Mg/Fe] increasetogether. The ratio of [Ni/Fe] is persistently mildly deficient relativeto solar. The ratio of [Ba/Fe] increases with [Fe/H]. The kinematicallypeculiar stars in our sample must have had their origins in localizedstar-forming regions far removed from the Galactic center. They do notcarry the chemical signature of an accreted population. It seems clearthat the chemical enrichment in our sample comes primarily from Type IIsupernovae with only a small component from Type Ia supernovae. Ourstars would have formed at most ~1 Gyr after the beginning of a starformation episode in the remote halo. The general conclusion extractedfrom these data is that the formation of the nascent Milky Way was notdominated by the late accretion of dwarf galaxies like the ones thatcurrently orbit the Galaxy. However, the assimilation of fragments earlyin the evolution of the Galaxy is a natural by-product of hierarchicalmodels of structure formation and can explain many properties of thehalo population.