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

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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, [FORMULA] 25% of 49V has decayed then from Table 1 it is seen that [FORMULA] 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 [FORMULA] ([FORMULA] 11%) greater than the prediction for interstellar production plus a 1.1%[FORMULA]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 [FORMULA] 70% of 54Mn appears to have decayed (Duvernois, 1997; Lukasiak et al., 1997a) and this decay is attributed to [FORMULA]-decay into 54Fe with an estimated half-life [FORMULA] 1.2[FORMULA]106 yr. (This [FORMULA]-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 [FORMULA] (+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 [FORMULA] 20% to the decay of 54Mn along with a correspondingly longer [FORMULA]-decay lifetime [FORMULA] 1.8[FORMULA]106 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.

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© European Southern Observatory (ESO) 1998

Online publication: July 27, 1998