Cosmological Implications of the First Measurement of the Local ISM Abundance of 3He
Abstract
Deuterium plays a crucial role in testing big-bang nucleosynthesis. Its chemical evolution, while simple (it is burned to 3He), is intertwined with the more complicated evolution of 3He. Gloeckler \& Geiss' new measurement of the 3He abundance and the HST measurement of D, both in the local ISM today, can be compared to the pre-solar nebula abundances of D and 3He. Within the uncertainties, the sum of D + 3He relative to hydrogen is unchanged. This provides some validation of the cosmological utility of D + 3He, first suggested by Yang et al (1984), and further, indicates that over the past 4.5 Gyr there has been at most modest stellar production of 3He, in contradiction with stellar modeling, or modest stellar destruction of 3He, in contradiction with some ``solar spoons.'' While the earlier Galactic evolution of D + 3He cannot be constrained directly, it is expected to be dominated by massive stars, which deplete their 3He and produce metals. Based on the Galactic metallicity and the constancy of D + 3He over the past 4.5 Gyr, we derive a more empirically based lower bound to the cosmological baryon density; while not dramatically different from the original bound of Yang et al (1984) based on D + 3He, it alleviates some of the cosmic tension between the big-bang 4He abundance and those of D and 3He.
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