Updated reaction rate of ${}^{25}\mathrm{Mg}(p,\gamma ){}^{26}\mathrm{Al}$ and its astrophysical implication

${}^{26}\mathrm{Al}$ with a half-life of $7.17\times {10}^5$ years is one of the most significant nuclides in $\gamma$-ray astronomy and presolar grains of meteorites. Its main production mechanism in the $\mathrm{H}$-burning MgAl cycle is the ${}^{25}\mathrm{Mg}(p,\gamma ){}^{26}\mathrm{Al}$ reaction. In the temperature region of 0.05–0.3 GK of astrophysical interest, the astrophysical ${}^{25}\mathrm{Mg}(p,\gamma ){}^{26}\mathrm{Al}$ reaction rate is dominated by the resonant capture of several low-energy resonances. In this work, we report the results of a complete experimental investigation of the $E_{\mathrm{c}.\mathrm{m}.}=92$, 130, and 189 keV resonances in the ${}^{25}\mathrm{Mg}(p,\gamma ){}^{26}\mathrm{Al}$ reaction with the Jinping Underground Nuclear Astrophysics Experimental Facility. The updated thermonuclear ${}^{25}\mathrm{Mg}(p,\gamma ){}^{26}\mathrm{Al}$ reaction rate is (32–39)% higher than that obtained at the Laboratory for Underground Nuclear Astrophysics around 0.07–0.09 GK, mainly due to the 32% enhancement of the 92-keV resonance strength. The astrophysical impact of our new rate on the ${}^{26}\mathrm{Al}$ yield in a 5 ${\mathrm{M}}_{\odot }$ low-metallicity asymptotic giant branch star is investigated, in which an increase of (45–79)% in the ${}^{26}\mathrm{Al}$ yield is found by adopting our new ${}^{25}\mathrm{Mg}(p,\gamma ){}^{26}\mathrm{Al}$ rates.

Figure 1

Yield curves for the 304-keV resonance at the beginning of the experiment (i.e., 0 C, blue square), and after 92 C (red circle), 220 C (green triangle), and 350 C (purple inverted triangle) proton beam bombardment, respectively.

Figure 2

$\gamma$-ray spectra of the 304-keV resonance (in blue circles). (a) Sum spectrum; (b) single spectrum. The red lines represent fitting results.

Figure 3

Yield curve measured for the 189-keV resonance.

Figure 4

$\gamma$-ray spectra of the 189-keV resonance (in blue circles). (a) Sum spectrum, where the shadowed area represents the background; (b) single spectrum. The line represents the fitting result using the Bayesian method, while the shadowed area represents the background.

Figure 5

The blue filled circles show the summing $\gamma$-ray spectrum of the 130-keV resonance. The brown shaded area represents the background measured with natural Mg targets, and the pink shaded area shows the Bayesian fitted contribution of the 130-keV resonance.

Figure 6

Posterior probability density function for the 130-keV resonance strength.

Figure 7

$\gamma$-ray spectra of the 92-keV resonance (in blue circles). (a) Sum spectrum after background subtraction; (b) single spectrum. The line represents the geant4 simulation using the primary $\gamma$-ray branching ratios extracted from the single spectrum with the Bayesian method.

Figure 8

The proportion of each major resonance in the total ${}^{25}\mathrm{Mg}(p,\gamma ){}^{26}\mathrm{Al}$ rate. The corresponding $\pm 1\sigma$ error bands are shown.

Figure 9

Comparison between the present total ${}^{25}\mathrm{Mg}(p,\gamma ){}^{26}\mathrm{Al}$ reaction rate and those of LUNA [53] (green line), Iliadis et al. [50] (blue line), and NACRE [29] (grey line). The shaded bands represent the corresponding $1\sigma$ uncertainties.

Figure 10

Evolution of the convective regions on the bottom of the convective envelope for the 5 ${\mathrm{M}}_{\odot }$ star of metallicity $Z=0.001$. The $x$ axis is evolution model number, which is a proxy for time. The green dashed areas correspond to convection. The yellow solid line indicates the hydrogen-depleted core, or helium core, where the hydrogen mass fraction is below 0.01 and the ${}^4\mathrm{He}$ mass fraction is above 0.1. The red dotted line indicates the same as the yellow solid line but with helium and carbon. The black dash dotted lines are the borders of the region where the temperature is between 0.07 GK and 0.09 GK. The teardrop-shaped pockets correspond to the flash-driven convective region.

Figure 11

The temperatures at the base of the convective envelope $T_{\mathrm{BCE}}$ (in blue line) and on the stellar surface $T_{\mathrm{eff}}$ (in red line) during the AGB phase. The 14th TP is zoomed in the inset.

Figure 12

The ${}^{26}\mathrm{Al}^{\mathrm{g}}$ yields (on the left $y$ axis) and the stellar mass (on the right $y$ axis) as a function of the star age on its AGB phase. The red dash-dotted line, blue dotted line, green solid line, and black dotted line represent the ${}^{26}\mathrm{Al}^{\mathrm{g}}$ yields obtained by using the present, LUNA [53], Iliadis et al. [50], and NACRE [29] rates for the ${}^{25}\mathrm{Mg}(p,\gamma ){}^{26}\mathrm{Al}$ reaction, respectively. The orange solid line represents the evolution of the star mass. The vertical dashed line indicates the end of the evolution where the mass of the convective envelope is less than 0.01 ${\mathrm{M}}_{\odot }$.