AAS 201st Meeting, January, 2003
Session 27. Stars, Gas, and Dust in the Milky Way
Oral, Monday, January 6, 2003, 10:00-11:30am, 618-619

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[27.05] A New Population of Old Stars

I.I. Ivans (Caltech), C. Sneden (UT-Austin), C.R. James (Sam Houston), G.W. Preston (Carnegie), J.P. Fulbright (Carnegie,), P.A. Hoeflich (UT-Austin)

At what point in Galactic evolution did the earliest Type Ia supernovae start contributing their yields to the interstellar medium?

Most metal-poor halo stars exhibit very similar enhancements in their chemical abundance ratios of alpha-elements such as O, Mg, Si, Ca, and Ti with respect to Fe, and thus track the yields of massive-star nucleosynthesis that dominated the early Galactic halo ISM. Recent studies have identified stars that do not fit the usual halo trends. They possess instead low-alpha element abundances, possibly indicating that star formation histories varied from region to region. Evidence for further chemical substructure within the low-alpha star population will be reported at this meeting.

At [Fe/H] ~ -2, there exist a trio of stars that share similar abundance deficiencies in alpha-elements, as well as in the neutron-capture elements such as Sr and Ba. However, two of these stars also possess overabundances of Ti and iron-peak elements. The abundance differences in this trio are NOT subtle: in some cases the differences are over a factor of ten! Understanding the nucleosynthetic origins of the abundances in these stars does not lie in current Galactic chemical evolution models. Their elemental abundances suggest that these stars possibly witnessed, or owe their compositions to, the eariest supernovae Type Ia yields in the Milky Way.

We happily acknowledge support for this work from NASA through Hubble Fellowship grant HST-HF-01151.01-A from the STScI operated by the AURA under NASA contract NAS5-26555; The University of Texas at Austin; and McDonald Observatory.

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Bulletin of the American Astronomical Society, 34, #4
© 2002. The American Astronomical Soceity.