Asymptotic giant branch stars at low metallicity: the challenging interplay between the mass-loss and molecular opacities

We investigate the main physical properties of low-metallicity asymptotic giant branch (AGB) stars with the aim of quantifying the uncertainties that presently affect the predicted chemical yields of these stars, associated with the mass-loss and description of molecular opacities. We find that above a threshold mass, M ~= 3.5Msolar for Z = 0.001, the results are little dependent on the opacity treatment, as long as the hot-bottom burning (HBB) prevents the surface C/O ratio from exceeding unity; the yields of these massive AGB stars are expected to be mostly determined by the efficiency of convection, with a relatively mild dependence on the mass-loss description. A much higher degree of uncertainty is associated with the yields of less-massive models, which critically depend on the adopted molecular opacities. An interval of masses exists, say, 2.0-3.0Msolar (the exact range depends on the mass-loss), in which the HBB may be even extinguished following the cooling produced by the opacity of C-bearing molecules. The yields of these stars are the most uncertain, the variation range being the largest (up to ~2dex) for the nitrogen and sodium yields. For very low mass models, not experiencing the HBB (M <= 1.5Msolar), the description of mass-loss and the treatment of the convective boundaries are crucial for the occurrence of the third dredge-up, with a sizeable impact on the CNO yields.