NUCLEOSYNTHESIS IN AGB STARS - OBSERVATION OF MG-25 AND MG-26 IN IRC+10216 AND POSSIBLE DETECTION OF AL-26

We report the detection in the circumstellar envelope IRC+10216 of millimeter lines of the rare isotopomers (MgNC)-Mg-25 and (MgNC)-Mg-26, as well as of a line at 234433 MHz, which could be the J = 7-6 transition of (AlF)-Al-26 (an alternate, although less likely identification would be the J = 9-8 transition of NaF). The derived Mg-24:Mg-25:Mg-26 isotopic abundance ratios (78 : 11 +/- 1 : 11 +/- 1) are consistent with the solar system values (79.0:10.0:11.0), following Anders & Grevesse 1989). According to new calculations of evolutionary models of 3 M. and 5 M. AGB stars, these ratios and the previously measured N, O and Si isotopic ratios imply that the central star had an initial mass 3 M. less than or equal to M*(,ini) <5 M. and has already experienced many 3(rd) dredge-up events. From this, it can be predicted that the Al-26/Al-27 isotopic ratio lies between 0.01 and 0.08; in fact, the value derived in the case that U234433 arises from (AlF)-Al-26 is Al-26/Al-27 = 0.04. The identification of the (MgNC)-Mg-25 and (MgNC)-Mg-26 lines was made possible by ab-initio quantum mechanical calculations of the molecule geometrical structure. It was confirmed through millimeter-wave laboratory measurements. The quantum mechanical calculations are briefly described and the laboratory results presented in some detail. The rotation constants B, D, H and the spin-rotation constant gamma of (MgNC)-Mg-25 and (MgNC)-Mg-26, and the hyperfine constants b and eqQ of (MgNC)-Mg-25 are determined from a fit of laboratory and astronomical data (Table 1b).