Chemical abundance constraints on white dwarfs as halo dark matter

We examine the chemical abundance constraints on a population of white dwarfs in the halo of our Galaxy. We are motivated by microlensing experiments that have reported evidence for massive compact halo objects (MACHOs) in the halo of our Galaxy, with an estimated mass of 0.1-1 M-.; the only conventional dark astrophysical candidates for objects in this mass range are white dwarfs. However, our work constrains white dwarfs in the halo regardless of what the MACHOs are. Further motivation for our work comes from the recent claimed possible detection of a large population of white dwarfs in the Hubble Deep Field. We focus on the composition of the material that would be ejected as the white dwarfs are formed. This material would bear the signatures of nucleosynthesis processing and contain abundance patterns that can be used to constrain white dwarf production scenarios. Using both analytical and numerical chemical evolution models, we confirm previous work that very strong constraints come from Galactic Population II and extragalactic carbon abundances. We also point out that in some cases, depending on the stellar model, significant nitrogen is produced rather than carbon. The combined constraints from carbon and nitrogen give Ohm(Wd)h less than or similar to 2 x 10(-4) from comparison with the low abundances of these elements measured in the Ly alpha forest. We note, however, that these results are subject to uncertainties regarding the nucleosynthetic yields of low-metallicity stars. We thus investigate additional constraints from the light elements D and He-4, the nucleosynthesis of which is less uncertain. We find that these elements can be kept within observational limits only for Ohm(WD) less than or similar to 0.003 and for a white dwarf progenitor initial mass function sharply peaked at low mass (2 M,). Finally, we consider a Galactic wind, which is required to remove the ejecta accompanying white dwarf production from the galaxy. We show that such a wind can be driven by Type Ia supernovae arising from the white dwarfs themselves but find that these supernovae also lead to unacceptably large abundances of iron. The only ways we know of to avoid these constraints are that (1) the ejecta from low-metallicity MACHO progenitors are absent or completely unprocessed or (2) the processed ejecta remain as hot (greater than or similar to 20.3 keV) gas that is segregated from all observable neutral material to a precision of greater than or similar to 99%. Aside from these loopholes, we conclude that abundance constraints exclude white dwarfs as MACHOs.