2012ApJ...751....8K

We present updated calculations of stellar evolutionary sequences and detailed nucleosynthesis predictions for the brightest asymptotic giant branch (AGB) stars in the Galaxy with masses between 5 M_☉ and 9 M_☉, with an initial metallicity of Z = 0.02 ([Fe/H] = 0.14). In our previous studies we used the Vassiliadis & Wood mass-loss rate, which stays low until the pulsation period reaches 500 days after which point a superwind begins. Vassiliadis & Wood noted that for stars over 2.5 M_☉ the superwind should be delayed until P ~ 750 days at 5 M_☉. We calculate evolutionary sequences where we delay the onset of the superwind to pulsation periods of P ~ 700-800 days in models of M = 5, 6, and 7 M_☉. Post-processing nucleosynthesis calculations show that the 6 and 7 M_☉ models produce the most Rb, with [Rb/Fe] ~1 dex, close to the average of most of the Galactic Rb-rich stars ([Rb/Fe] ~1.4±0.8 dex). Changing the rate of the ²²Ne +α reactions results in variations of [Rb/Fe] as large as 0.5 dex in models with a delayed superwind. The largest enrichment in heavy elements is found for models that adopt the NACRE rate of the ²²Ne(α, n)²⁵Mg reaction. Using this rate allows us to best match the composition of most of the Rb-rich stars. A synthetic evolution algorithm is then used to remove the remaining envelope resulting in final [Rb/Fe] of ~1.4 dex although with C/O ratios >1. We conclude that delaying the superwind may account for the large Rb overabundances observed in the brightest metal-rich AGB stars.