ON POPULATION-III STARS AS GRAVITATIONAL MICROLENSING OBJECTS IN THE GALACTIC-HALO

Recent gravitational microlensing experiments by Alcock et al. (1993) and by Aubourg et al. (1993) suggest that the ''dark matter'' in the Galactic halo is very low mass stars and brown dwarfs. If this is true, some of these objects, in particular those in the high-mass tail of the mass distribution, may be luminous enough to be visible. More spatially extended than Population II stars, these objects are expected to be first generation, ''Population III'' stars composed initially of ''primordial'' matter made in the big bang, although their surfaces may now be contaminated by metal-rich gas which they have accreted over their long lives. By utilizing the fact that stellar models of primordial composition evolve into nitrogen-rich carbon stars despite surface polution by accretion, we show that it is possible to probe the intrinsic nature of these lens objects. Using currently available data from a deep survey of the Galactic halo, we estimate the number of potentially observable Population III stars and argue that our estimates are consistent with extant observations.