Origins of the thick disk as traced by the alpha elements of metal-poor giant stars selected from RAVE




Ruchti, Gregory R
Fulbright, J.P.
Wyse, Rosemary F G
Gilmore, Gerard
Bienayme, O
Binney, J
Bland-Hawthorn, Joss
Campbell, R
Freeman, Kenneth
Gibson, Bradley K

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IOP Publishing


Theories of thick-disk formation can be differentiated by measurements of stellar elemental abundances. We have undertaken a study of metal-poor stars selected from the RAVE spectroscopic survey of bright stars to establish whether or not there is a significant population of metal-poor thick-disk stars ([Fe/H] ≲ -1.0) and to measure their elemental abundances. In this Letter, we present abundances of four α-elements (Mg, Si, Ca, and Ti) and iron for a subsample of 212 red giant branch and 31 red clump/horizontal branch stars from this study. We find that the [α/Fe] ratios are enhanced, implying that enrichment proceeded by purely core-collapse supernovae. This requires that star formation in each star-forming region had a short duration. The relative lack of scatter in the [α/Fe] ratios implies good mixing in the interstellar medium prior to star formation. In addition, the ratios resemble that of the halo, indicating that the halo and thick disk share a similar massive star initial mass function. We conclude that the α-enhancement of the metal-poor thick disk implies that direct accretion of stars from dwarf galaxies similar to surviving dwarf galaxies today did not play a major role in the formation of the thick disk.



Keywords: Galaxy: abundances; Galaxy: disk; Stars: abundances; Stars: late-type



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