Magnesium Isotopes in Metal-poor Dwarfs: The Rise of AGB Stars and the Formation Timescale of the Galactic Halo

We have determined magnesium isotopic ratios ( 25, 26 Mg/Mg) in metal-poor (-2.6 ≤ [Fe/H] ≤ -1.3) halo dwarfs employing high signal-to-noise ratio (S/N = 90-280), high spectral resolution ( R = 10 5 ) Keck High Resolution Echelle Spectrometer (HIRES) spectra. Unlike previous claims of an important contribution from intermediate-mass asymptotic giant branch (AGB) stars at low metallicities, we find that the rise of the AGB contribution in the Galactic halo did not occur until intermediate metallicities ([Fe/H] ≳ -1.5).