The NeNa and MgAl Cycles

There are two cycles of reactions not significant for energy production, but which change some isotopic ratios, working above T6 > 25. These are the NeNa and MgAl cycles (Fig. 25.4). The NeNa chain starts from 20Ne. The 20Ne abundance is high enough not to be modified by the creation or destruction of the other much less-abundant isotopes (Appendix A.3). For these isotopes, the NeNA chain has the following effects for different T6 = T/(106 K) [17]:

- 21Ne: its abundance first increases with T up to a factor of ~ 6 at T6 = 35, then if T further increases it declines by a factor of 102 at T6 = 60.

- 22Ne: it disappears for T6 < 30 and recovers its initial value for T6 = 80.

- 22Na and 23Na: the isotope 22 is generally negligible with respect to 23Na. The abundance of 23Na may increase by nearly an order of magnitude at H exhaustion in the range of T6 = 25-60.

Fig. 25.3 Top: the log of the rates £ of nuclear energy generation in erg g s as a function of T in different stars on the ZAMS and for the present Sun. The kicks in the curves mark the edge of the convective cores. Outside of the cores, the CNO elements are not in equilibrium and this produces higher £. Bottom: the exponent v or £t as a function of T

Fig. 25.3 Top: the log of the rates £ of nuclear energy generation in erg g s as a function of T in different stars on the ZAMS and for the present Sun. The kicks in the curves mark the edge of the convective cores. Outside of the cores, the CNO elements are not in equilibrium and this produces higher £. Bottom: the exponent v or £t as a function of T

The main effect of the NeNa cycle is to produce some Na enhancements, a fact supported by observations in red giants and supergiants (see below). The much smaller rate (factor 10-2-10-3) of 23Na(p,y)24Mg with respect to 23Na(p,a)20Ne supports the view that the NeNa reactions form a cycle.

The MgAl cycle is, in principle, initiated by 24Mg, the most abundant isotope of the cycle. However, the reaction rate of 24Mg(p,y)25Al is very slow so that practically no 24Mg is destroyed except at T6 > 60 . 25Al rapidly disintegrates into 25Mg. Then, a (p,y) reaction leads to 26 Al, which exists in two forms, the long-lived (halflife t1/2 = 7.1 x 105 yr) ground state 26Alg and the short-lived (t1/2 = 6.35 s) iso-meric state 26Alm, the ground state being favored. These two states are generally

Fig. 25.4 The NeNa {left) and MgAl cycles (right)

treated as different elements. Reaction 25Mg(p,y)26Al has the highest rate (at least larger by 3 orders of magnitude compared to the destruction of 24Mg), thus the amount of radioactive 26Al depends mainly on the initial quantity of 25Mg. This reaction starts above T6 = 20 and becomes very efficient to create 26Al for T6 > 40.

The resulting 26Alg is a most important isotope, since the e+ disintegration of 26Alg and the subsequent annihilation of pairs e+e~ give rise to an observable y line at 1.8 MeV. For T < 40, the destruction of 26Alg occurs mainly through ¡5 decay. Above T = 40 (where lots of 26Alg is created), reaction 26Alg(p,y)27Si(,e+v)27Al comes into play in concurrence with the channel through 26Mg. The rate of the (p,y) destruction of 26Alg is uncertain, so that it affects the predictions of the y-ray production as well as the 27Al abundance. This abundance increases for T6 > 40, up to an order of magnitude at T6 = 70. However, the destruction rates of 27Al by 27Al(p,a)24Mg and 27Al(p,y)28Si are also uncertain, which may affect the cycling character of the MgAl cycle. Figure 26.18 shows the variety of conditions, in which H burning with the MgAl and NeNa cycles may occur.

25.1.5.1 Observational Consequences of the NeNa and MgAl Cycles

There are several astrophysical consequences of these two cycles, which we briefly mention. First, the 1.8 MeV y-ray emission observed by satellites HEAO-3, SMM and INTEGRAL comes from the decay of 26Alg. The total amount of 26Alg in the Galaxy is estimated to be of the order of 1.5-3 M0 and is mainly produced by massive OB stars. Measurements of isotopic ratios in meteorites and in dust grains likely of stellar origin indicate that 26Alg has decayed "in situ" starting from values compatible with stellar yields.

Globular cluster stars show two noticeable anticorrelations. The Na vs. O anti-correlation shows large [Na/Fe] excesses in stars with relative [O/Fe] deficiencies (the brackets indicate the excesses in log with respect to solar ratios). This anticor-relation results from the (p,y) reaction on 16O (cf. Fig. 25.1) which destroys 16O (making 14N), and from (p,y) on 22Ne in a partial NeNa cycle leading to 23Na (at T6 > 20). At T6 > 40, 23Na comes mainly from 20Ne in the full NeNa cycle. There is also an anticorrelation MgAl, it results from the 24Mg destruction and 27Al production in the MgAl cycle. These anticorrelations seem to result [156] from enrichments by the winds of massive rotating stars in previous star generations. These winds had the composition of matter processed by the CNO, NeNa and MgAl cycles.

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