MACHOs, White Dwarfs, and the Age of the Universe

(Abridged Abstract) A favored interpretation of recent microlensingmeasurements towards the Large Magellanic Cloud implies that a large fraction(i.e. 10--50%) of the mass of the galactic halo is composed of white dwarfs. Wecompare model white dwarf luminosity functions to the data from theobservational surveys in order to determine a lower bound on the age of anysubstantial white dwarf halo population (and hence possibly on the age of theUniverse). We compare various theoretical white dwarf luminosity functions, inwhich we vary hese three parameters, with the abovementioned survey results. From this comparison, we conclude that if white dwarfs do indeed constitutemore than 10% of the local halo mass density, then the Universe must be atleast 10 Gyr old for our most extreme allowed values of the parameters. When weuse cooling curves that account for chemical fractionation and more likelyvalues of the IMF and the bolometric correction, we find tighter limits: awhite dwarf MACHO fraction of 10% (30%) requires a minimum age of 14 Gyr (15.5Gyr). Our analysis also indicates that the halo white dwarfs almost certainlyhave helium-dominated atmospheres.Comment: Final version accepted for publication, straight TeX formate, 6 figs, 22 page