IMPACT OF A REVISED²⁵Mg(p, γ)²⁶Al REACTION RATE ON THE OPERATION OF THE Mg-Al CYCLE

Abstract

Proton captures on Mg isotopes play an important role in the Mg-Al cycle\nactive in stellar H-burning regions. In particular, low-energy nuclear\nresonances in the $^{25}$Mg(p,$\\gamma$)$^{26}$Al reaction affect the production\nof radioactive $^{26}$Al$^{gs}$ as well as the resulting Mg/Al abundance ratio.\nReliable estimations of these quantities require precise measurements of the\nstrengths of low-energy resonances. Based on a new experimental study performed\nat LUNA, we provide revised rates of the $^{25}$Mg(p,$\\gamma$)$^{26}$Al$^{gs}$\nand the $^{25}$Mg(p,$\\gamma$)$^{26}$Al$^{m}$ reactions with corresponding\nuncertainties. In the temperature range 50 to 150 MK, the new recommended rate\nof the $^{26}$Al$^{m}$ production is up to 5 times higher than previously\nassumed. In addition, at T$=100$ MK, the revised total reaction rate is a\nfactor of 2 higher. Note that this is the range of temperature at which the\nMg-Al cycle operates in an H-burning zone. The effects of this revision are\ndiscussed. Due to the significantly larger $^{25}$Mg(p,$\\gamma$)$^{26}$Al$^{m}$\nrate, the estimated production of $^{26}$Al$^{gs}$ in H-burning regions is less\nefficient than previously obtained. As a result, the new rates should imply a\nsmaller contribution from Wolf-Rayet stars to the galactic $^{26}$Al budget.\nSimilarly, we show that the AGB extra-mixing scenario does not appear able to\nexplain the most extreme values of $^{26}$Al/$^{27}$Al, i.e. $>10^{-2}$, found\nin some O-rich presolar grains. Finally, the substantial increase of the total\nreaction rate makes the hypothesis of a self-pollution by massive AGBs a more\nrobust explanation for the Mg-Al anticorrelation observed in Globular-Cluster\nstars.\n