Chemical abundance constraints on white dwarfs as halo dark matter

We examine the chemical abundance constraints on a population of white dwar fs in the halo of our Galaxy. We are motivated by microlensing experiments that have reported evidence for massive compact halo objects (MACHOs) in th e halo of our Galaxy, with an estimated mass of 0.1-1 M-.; the only convent ional dark astrophysical candidates for objects in this mass range are whit e dwarfs. However, our work constrains white dwarfs in the halo regardless of what the MACHOs are. Further motivation for our work comes from the rece nt 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 b e ejected as the white dwarfs are formed. This material would bear the sign atures of nucleosynthesis processing and contain abundance patterns that ca n be used to constrain white dwarf production scenarios. Using both analyti cal 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 s tellar model, significant nitrogen is produced rather than carbon. The comb ined constraints from carbon and nitrogen give Ohm(Wd)h less than or simila r to 2 x 10(-4) from comparison with the low abundances of these elements m easured in the Ly alpha forest. We note, however, that these results are su bject to uncertainties regarding the nucleosynthetic yields of low-metallic ity stars. We thus investigate additional constraints from the light elemen ts 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) le ss than or similar to 0.003 and for a white dwarf progenitor initial mass f unction sharply peaked at low mass (2 M,). Finally, we consider a Galactic wind, which is required to remove the ejecta accompanying white dwarf produ ction from the galaxy. We show that such a wind can be driven by Type Ia su pernovae arising from the white dwarfs themselves but find that these super novae 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-metalli city MACHO progenitors are absent or completely unprocessed or (2) the proc essed ejecta remain as hot (greater than or similar to 20.3 keV) gas that i s segregated from all observable neutral material to a precision of greater than or similar to 99%. Aside from these loopholes, we conclude that abund ance constraints exclude white dwarfs as MACHOs.