Impact of a revised Mg-25(p, gamma)Al-26 Reaction Rate on the Operation of the Mg-Al Cycle

Impact of a revised Mg-25(p, gamma)Al-26 Reaction Rate on the Operation of the Mg-Al Cycle

Straniero, O.; Imbriani, G.; Strieder, F.; Bemmerer, D.; Broggini, C.; Caciolli, A.; Corvisiero, P.; Costantini, H.; Cristallo, S.; Dileva, A.; Formicola, A.; Elekes, Z.; Fulop, Z.; Gervino, G.; Guglielmetti, A.; Gustavino, C.; Gyurky, G.; Junker, M.; Lemut, A.; Limata, B.; Marta, M.; Mazzocchi, C.; Menegazzo, R.; Piersanti, L.; Prati, P.; Roca, V.; Rolfs, C.; Alvarez, C.; Somorjai, E.; Terrasi, F.; Trautvetter, H.

Proton captures on Mg isotopes play an important role in the Mg-Al cycle active in stellar H-burning regions. In particular, low-energy nuclear resonances in the Mg-25(p,gamma)Al-26 reaction affect the production of radioactive Al-26(gs) as well as the resulting Mg/Al abundance ratio. Reliable estimations of these quantities require precise measurements of the strengths of low-energy resonances. Based on a new experimental study performed at the Laboratory for Underground Nuclear Astrophysics, we provide revised rates of the Mg-25(p,gamma)Al-26(gs) and the Mg-25(p,gamma)Al-26(m) reactions with corresponding uncertainties. In the temperature range 50-150 MK, the new recommended rate of Al-26(m) production is up to five times higher than previously assumed. In addition, at T = 100 MK, the revised total reaction rate is a factor of two higher. Note that this is the range of temperature at which the Mg-Al cycle operates in a H-burning zone. The effects of this revision are discu!
ssed. Due to the significantly larger Mg-25(p,gamma)Al-26(m) rate, the estimated production of Al-26(gs) in H-burning regions is less efficient than previously obtained. As a result, the new rates should imply a smaller contribution from Wolf-Rayet stars to the galactic Al-26 budget. Similarly, we show that the asymptotic giant branch (AGB) extra-mixing scenario does not appear able to explain the most extreme values of Al-26/Al-27, i.e., > 10(-2), found in some O-rich presolar grains. Finally, the substantial increase of the total reaction rate makes the hypothesis of self-pollution by massive AGBs a more robust explanation for the Mg-Al anticorrelation observed in globular-cluster stars.

Keywords: globular clusters: general; nuclear reactions, nucleosynthesis, abundances; stars: AGB and post-AGB; stars: Wolf-Rayet

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Publ.-Id: 18473