FURTHER CONSTRAINTS ON WHITE-DWARF GALACTIC HALOS

The suggestion that roughly half the mass of the Galactic halo might b e in the form of white dwarfs, together with the limits on its mass fr action in faint red stars and on the initial metallicity of the Galact ic disk, would set strong constraints on the initial mass function (IM F) 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 f rom the halo white dwarf population. We find that, for most of the evo lutionary scenarios for the Type Ia supernova progenitor systems propo sed so far, constraints on the white dwarf mass fraction in the halo f rom the predicted production of iron would be extremely severe. When t he predicted iron excess is not so large, the exceedingly high Type la supernova rate predicted for the present time would also exclude a ma jor contribution of white dwarfs to the halo mass. The white dwarf con tribution, in all cases, should be below 5%-10%. Besides, for the IMFs considered, the duration of the halo burst should be shorter than 1 G yr in order to avoid too large a spread in the iron abundances of Popu lation II halo dwarfs, and the predicted halo [O/Fe] ratio would be at odds with observations.