95% C.I. upper limits of g_ae × g_aγ as function of ALP mass for Betelgeuse in the 20 − 600 keV energy range. The solid blue lines show the upper limit for each stellar model, assuming a representative value of B_T = 1.4 μG with the top blue line corresponding to t_cc = 1.55 × 10⁵ yr and the bottom blue line corresponding to t_cc = 3.6 yr. The constraints will scale with different B_T as in Eq. (4); the solid black line shows the upper limit for the most conservative (B_T = 0.4 μG and t_cc = 1.55 × 10⁵ yr) and the solid red line for the most optimistic cases (B_T = 3.0 μG and t_cc = 3.6 yr). For comparison, we also show the 95% C.I. upper limits of g_ae × g_aγ obtained from Xiao et al. (2022) for the most conservative and optimistic cases with dashed red lines.