δ¹³⁴Ba₁₃₆ (top row), δ¹³⁷Ba₁₃₆ (middle row), and δ¹³⁸Ba₁₃₆ (bottom row) vs δ¹³⁵Ba₁₃₆ from mainstream SiC grain data (dark grey circles with 1σ error bars from Savina et al. 2003; Barzyk 2007; Marhas et al. 2007; Liu et al. 2014, 2015, 2017a,b, 2019, 2022; Stephan et al. 2018, 2019) compared to the surface evolution of Set 1 and Set 2 stellar models of solar (left panels) and of double-solar metallicity (right panels) of different masses (see legend). Circles (Set 1) and triangles (Set 2) on the lines represent the composition after each TDU after the envelope becomes C-rich. The dashed black lines represent the solar composition (δ = 0 by definition).