PI: Bruce W. Carney, Univ. of North Carolina, email@example.com
Address: CB\# 3255, Chapel Hill, NC 27599-3255, USA
CoI: Inese I. Ivans, Univ. of Texas
CoI: Chris Sneden, Univ. of Texas
CoI: John B. Laird, Bowling Green State Univ.
Title: Formation & Evolution of the Galactic Halo: Refining the Timescale Resolution
Abstract: We propose to explore \em variations in the halo formation timescale with a higher timescale resolution than heretofore available from globular cluster color-magnitude diagrams or elemental abundance ratios. In the latter case, abundances of the ``(alpha)''-elements (oxygen, magnesium, silicon, calcium), relative to iron, are enhanced in metal-poor stars ([Fe/H < -1.0) by about 0.4 dex. If SNe Ia are responsible for the iron-rich material which begins to appear at higher metallicities, then the timescale for the halo to reach [Fe/H]=-1 was about 10^9 years. Variations in [(alpha)/Fe] then point to variations in the halo star formation timescale. But the appearance of s-process elements relative to r-process elements should provide a timescale resolution several times better than 10^9 years since the s-process elements from AGB stars are expected to appear within about 10^8 years, and in fact begin to appear at [Fe/H] ~ -2.5. We have already acquired high-resolution, high signal-to-noise echelle spectra at red wavelengths to study [(alpha)/Fe] \it vs. [Fe/H] for about 40 halo stars. Some have unusual [(alpha)/Fe] ratios, indicating variations of order 10^9 years in the star formation rate of metal- poor stars. We propose to extend this work to the blue spectral regions to derive s-process (Ba, Y, Sr) and r-process (Eu) abundances. In principle, our timescale resolution, manifested by differences in [r- process/s-process] ratios, will be comparable to a Galactic free-fall timescale.
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