Astron. Astrophys. 347, 572-582 (1999)

4. The NeNa chain

The NeNa chain is illustrated in Fig. 5, while Fig. 6 displays some relevant NACRE reaction rates, and their, sometimes quite large, uncertainties. These affect in particular the proton captures by , and . In contrast, the rate may be considered as relatively well determined. Some of these rates may also deviate strongly from the CF88 proposed values.

Fig. 5. Same as Fig. 1, but for the NeNa and MgAl chains

Fig. 6. Same as Fig. 4, but for , , (left panel ) and , (right panel )

The NACRE rates are used to compute the abundances shown in Fig. 7. A slight alteration of the initial abundance is visible only for 50. However, an unnoticeable destruction is sufficient to lead to a significant increase of the abundance of the rare isotope through at . At higher temperatures, destroys . As a result, the abundance at H exhaustion is maximum when H burns in the approximate range. This conclusion may, however, be altered if the upper limit of the rate is adopted instead.

Fig. 7. Same as Fig. 2, but for the nuclides involved in the NeNa chain

The yield has raised much interest recently, following the discovery at the surface of globular cluster red giant stars of moderate sodium overabundances which correlate or anti-correlate with the amount of other elements (like C, N, O, Mg and Al) also involved in cold H burning (Denissenkov et al. 1998; Kraft et al. 1998, and references therein). This situation may be the signature of the dredge-up to the stellar surface of the ashes of the NeNa chain. The production results from , while and are responsible for its destruction, which can be substantial at . Unfortunately, our knowledge of these three reaction rates remains very poor, with uncertainties that can amount to factors of about 100 to in certain temperature ranges (see Fig. 6). As indicated in Fig. 7, this situation prevents an accurate prediction of the yields when . More precisely, the spread in the abundance at H exhaustion reaches a factor of 100 at these temperatures.

The possible cycling character of the NeNa chain is determined by the ratio of the rates of and of . Fig. 6 indicates that the former reaction is predicted to be faster than the latter one at only. In this case, the NeNa chain is indeed a cycle. However, at higher temperatures, an important leakage to the MgAl chain can be expected, unless future experiments confirm the lower bound of the uncertain rate.

© European Southern Observatory (ESO) 1999

Online publication: June 30, 1999
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