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

We report the detection in the circumstellar envelope IRC+10216 of mil limeter lines of the rare isotopomers (MgNC)-Mg-25 and (MgNC)-Mg-26, a s well as of a line at 234433 MHz, which could be the J = 7-6 transiti on of (AlF)-Al-26 (an alternate, although less likely identification w ould 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 wi th the solar system values (79.0:10.0:11.0), following Anders & Greves se 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 ma ny 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 lin es was made possible by ab-initio quantum mechanical calculations of t he 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 eq Q of (MgNC)-Mg-25 are determined from a fit of laboratory and astronom ical data (Table 1b).