Further constraints on white dwarf galactic halos

The suggestion that roughly half the mass of the Galactic halo might be in the form of white dwarfs, together with the limits on its mass fraction in faint red stars and on the initial metallicity of the Galactic disk, would set strong constraints on the initial mass function (IMF) of the halo. Particular IMFs have been proposed to cut off both the numbers of low-mass stars contributing to the infrared background and of high-mass stars that contribute to the growth of metallicity when they promptly explode as gravitational collapse (Type II and Type Ib/c) supernovae. Here we examine the further contribution to metallicity from Type Ia (thermonuclear) supernovae that would later be produced from the halo white dwarf population. We find that, for most of the evolutionary scenarios for the Type Ia supernova progenitor systems proposed so far, constraints on the white dwarf mass fraction in the halo from the predicted production of iron would be extremely severe. When the predicted iron excess is not so large, the exceedingly high Type la supernova rate predicted for the present time would also exclude a major contribution of white dwarfs to the halo mass. The white dwarf contribution, in all cases, should be below 5%-10%. Besides, for the IMFs considered, the duration of the halo burst should be shorter than 1 Gyr in order to avoid too large a spread in the iron abundances of Population II halo dwarfs, and the predicted halo [O/Fe] ratio would be at odds with observations.