Element Ratios and the Formation of the Stellar Halo

It is well-established that the vast majority of metal-poor Galactic halo stars shows evidence for enrichment by solely massive stars. Recent observations have identified more varied behavior in the pattern of elemental abundances measured for metal-rich, [Fe/H]∼ −1 dex, halo stars, with both high and low values of the ratio of [α/Fe]. The low values are most naturally due to the incorporation of iron from Type Ia supernovae. These ‘low-alpha’ halo stars have been interpreted as being accreted from dwarf galaxies. However, these stars are on very high-energy radial orbits, that plunge into the Galactic center, from the outer regions of the stellar halo. We demonstrate here that known dwarf galaxies could not reproduce the observations, which require the proposed parent dwarf to have a very high mean density to provide ‘low-alpha’ stars on orbits of such low values of periGalactic distance. Rather, the observations are consistent with fragmentation of a gaseous proto-halo into transient star-forming regions, some of which are sufficiently dense to survive close to the Galactic center, and self-enrich to relatively high metallicities. Those that probe the outer halo sustain star formation long enough to incorporate the ejecta from Type Ia supernovae. Further, we point out that the values of the ‘Type II plateau’ in the element ratios for disk stars and for halo stars are equal. This implies that the (massive) stars that enriched the early disk and halo had the same IMF. However, the bulk of the halo stars with [Fe/H]∼ −1 dex that have been observed have significantly lower values of [α/Fe] than does a typical disk star of that metallicity. Thus there is a discontinuity in the chemical enrichment history between the halo and disk, as probed by these samples, consistent with previous inferences based on angular momentum considerations. Subject headings: Galaxy: halo — Galaxy: formation — Galaxy : evolution — Galaxy — abundances