Astron. Astrophys. 336, L61-L64 (1998)

4. Other K-capture isotopes

From Table 1 it is clear that in addition to ⁴⁹V and ⁵¹Cr there are several other K-capture isotopes that might be expected to decay. If, in fact, 25% of ⁴⁹V has decayed then from Table 1 it is seen that 38% of ⁵⁷Co should also decay. The Voyager results on the ⁵⁷Co abundance (Lukasiak et al., 1997b) are indeed lower than the prediction from secondary production only with no decay - but the data errors are large, the results being based on only 7 ⁵⁷Co events. This decay will contribute only a small increase to the much larger ⁵⁷Fe source abundance (although we note that Connell & Simpson, 1997, find a larger than expected source abundance of ⁵⁷Fe).

Connell & Simpson, 1997 find a finite source abundance of ⁵⁵Fe. For ⁵⁵Mn (the decay product of ⁵⁵Fe) the Voyager results (Lukasiak et al., 1997a) are 0.8 ( 11%) greater than the prediction for interstellar production plus a 1.1%⁵⁵Fe source abundance. Evidence for ⁵⁵Fe decay is thus less convincing than for ⁴⁹V and ⁵¹Cr and depends on the assumed ⁵⁵Mn source abundance.

The observation of the possible K-capture decay of ⁵⁴Mn into ⁵⁴Cr is complicated by the fact that overall roughly 70% of ⁵⁴Mn appears to have decayed (Duvernois, 1997; Lukasiak et al., 1997a) and this decay is attributed to -decay into ⁵⁴Fe with an estimated half-life 1.210⁶ yr. (This -decay lifetime has not yet been measured in the laboratory). If some of the observed ⁵⁴Mn decay is, in fact, due to K-capture decay then this portion of the decay would appear as an enhanced ⁵⁴Cr abundance. The Voyager abundance of ⁵⁴Cr is indeed measured to be 1.6 (+90%) above the propagation calculations. The excess ⁵⁴Cr/⁵⁶Fe fraction of 0.45% implied by the Voyager data would be consistent with a K-capture contribution 20% to the decay of ⁵⁴Mn along with a correspondingly longer -decay lifetime 1.810⁶ yr.

So overall the data on the other K-capture isotopes is suggestive of decay although not at the same statistical level as for ⁴⁹V and ⁵¹Cr. In particular, none of the other measurements are inconsistent with the observations reported here for ⁴⁹V and ⁵¹Cr.

© European Southern Observatory (ESO) 1998

Online publication: July 27, 1998
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