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7. 3He

The primordial abundance of 3He has not been measured. This is most unfortunate, since it is nearly as sensitive as D to the baryon density during BBN. 3He is harder to measure than D because the difference in wavelength of 3He and 4He lines is smaller than for D, and the Lyman series lines of He II, main absorption lines of He in the IGM, are in the far ultraviolet at 228 - 304 Å which is hard to observe because of absorption in the Lyman continuum of H I at < 912 Å.

Rood, Steigman & Tinsley [127] argued that it was unlikely that 3He could be used to supplement cosmological information from D because low mass stars should make a lot of 3He , increasing the current ISM value to well above that in the pre-solar system ISM, and in potential conflict with observations at that time. This conflict has been confirmed. Measurements do show enhanced 3He in Planetary nebulae, as expected from the production in the associated low mass stars, but this is not reflected in the ISM as a whole. The pre-solar and current 3He abundances are similar [128], in contradiction with expectation [129], [130], for unknown reasons.

It was suggested ([131], see review by Hata et al. [132]) that the uncertainty over the amount of destruction of D could be circumvented using the sum of the abundances of D + 3He , since the destroyed D should become 3He , and 3He is relatively hard to destroy. The primordial D + 3He should then be leq the same sum observed today, as more 3He is made in stars over time. However, there are two problems with this scenario. First, the 3He should increase over time, which it does not, implying that some stars destroy 3He , and second, the 3He abundance should be about constant in the ISM today, which it appeared not to be in early data [133]. Hence, just prior to the measurement of D in quasars, most concluded that D + 3He in the Galaxy does not provide secure cosmological information [132], and summaries by [45], [10].

Balser et al. [134] report on a 14 year program to measure 3He in the Galactic H II regions. Using models for the gas density structure, they find an average 3He / H = 1.6 ± 0.5 x 10-5 for a sub-sample of seven simple nebulae. No variation is seen with Oxygen abundance over a factor of ten, and there is little scatter [135]. This value may represent the average in the ISM today, but it is not known how to use this to obtain primordial abundances.

These measurements are relevant to stellar nucleosynthesis and Galactic chemical evolution, and are consistent with a cosmological origin for the 3He , but we suggest that gas with much lower metal abundances will need to be observed to derive a secure primordial abundance for 3He .

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