95% C.I. upper limits of g_ae × g_aγ as function of ALP mass, m_a, for the combined analysis of all 18 stars for 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 solid purple line shows the upper limit for the most conservative (B_T = 0.4 μG and t_cc = 1.55 × 10⁵ yr) and the solid green line for the most optimistic cases (B_T = 3.0 μG and t_cc = 3.6 yr).