ON THE NECESSITY OF COMPOSITION-DEPENDENT LOW-TEMPERATURE OPACITY IN MODELS OF METAL-POOR ASYMPTOTIC GIANT BRANCH STARS

Abstract

The vital importance of composition-dependent low-temperature opacity in low-mass ( M ⩽ 3 M ☉ ) asymptotic giant branch (AGB) stellar models of metallicity Z ⩾ 0.001 has recently been demonstrated. Its significance to more metal-poor, intermediate-mass ( M ⩾ 2.5 M ☉ ) models has yet to be investigated. We show that its inclusion in lower-metallicity models ([Fe/H] ⩽−2) is essential and that there exists no threshold metallicity below which composition-dependent molecular opacity may be neglected. We find it to be crucial in all intermediate-mass models investigated ([Fe/H] ⩽−2 and 2.5 ⩽ M / M ☉ ⩽ 5), because of the evolution of the surface chemistry, including the orders of magnitude increase in the abundance of molecule-forming species. Its effect on these models mirrors that previously reported for higher-metallicity models—increase in radius, decrease in T eff , faster mass loss, shorter thermally pulsing AGB lifetime, reduced enrichment in third dredge-up products (by a factor of 3–10), and an increase in the mass limit for hot bottom burning. We show that the evolution of low-metallicity models with composition-dependent low-temperature opacity is relatively independent of initial metal abundance because its contribution to the opacity is far outweighed by changes resulting from dredge-up. Our results imply a significant reduction in the expected number of nitrogen-enhanced metal-poor stars, which may help explain their observed paucity. We note that these findings are partially a product of the macrophysics adopted in our models, in particular, the Vassiliadis & Wood mass loss rate which is strongly dependent on radius.