An attempt is made to resolve two conflicting constraints on the duration of the formation of the Galactic stellar halo: the few Gyr age differences in metal-poor stars and the approximately 0.1 Gyr timescale for the explosion and chemical enrichment of Type Ia SNe. The currently favored model of Type Ia SNe- mergers of binaries containing a carbon and oxygen white dwarf (CO WD) - is explored, and various efficiencies of the common envelope interaction are examined along with plausible restrictions on common envelope evolution. It is concluded that no region of parameter space of the CO-CO WD merger model can lead to explosions delayed from the birth of the binary by about 1 Gyr or more. Thus, CO-CO WD mergers cannot provide a consistent explanation for the element ratios and the age spread of halo stars, within the context of monolithic halo models. However, the conflict could be resolved by combining these SNe models with suitable hierarchical models of galaxy formation.
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Smecker-Hane et al. (1992) studied this question.