Studies of Mass Loss in Pulsating Asymptotic Giant Branch Stars

Asymptotic Giant Branch stars are the final actively fusing stage of stellar evolution. These stars undergo mass loss, ejecting their envelopes through a pulsation and dust-driven wind. This recycles material back into the interstellar medium as well as adding in new materials such as nitrogen, heavy metals, and cosmic dust.

Characterizing mass loss in these stars in terms of their physical properties has historically been difficult. Recent surveys have led to large sets of reliable pulsation period (P) data for extragalactic stars, but there is still significant difficulty in measuring the luminosities (L), and to a lesser extent mass-loss rates, accurately. This issue makes standard fitting approaches unreliable. We have developed a method using the distribution of these stars in LP-space to determine mass-loss rate formulas. These formulas are in agreement with models and known constraints on their evolution.

The large population of stars also provides an ideal test for stellar models. While modern models reproduce solar-composition stars, they struggle with low metallicity stars. We are examining grids of atmospheric models, to determine where the current limits of those models place us and what processes may need to be added to reach agreement with observations.