Oxygen burning isotope ratios

For our first tree sequence [28-32] we measured D/H by reacting sawdust with uranium to produce H2, 99 percent quantitatively. For measurement of 180/160, we modified the method of Rittenberg and Pontecorvo [33] by carrying it out at very high temperatures, 99 percent quantitatively. The temperature must be 525 °C if it is lower, the reaction is not quantitative see the section on our chemistry later in this paper. To measure the stable isotope ratio in carbon, we burned sawdust to completion in oxygen. 

There are two burning cycles that involve the oxygen isotopes and these follow the same pattern as described for the H-shell. Both operate simultaneously but what effect each has on the envelope abundance depends on the branching ratio between the two 17 O + p reaction rates. Overall, there is a net decrease in the lsO and lsO abundances, along with an increase in the 170 abundance in the envelope. The decrease in the surface leO abundance is only significant in the most massive AGB models, depending on the convective model employed in the calculation [152, 104]. Hence the elemental oxygen abundances observed in AGB stars should be indicative of their initial abundances, except in the most massive objects. 

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