A surprisingly large asymmetric ejection from Mira A

TL;DR

This study investigates the asymmetric ejection of material from Mira A, revealing complex gas and dust dynamics, periodic ejection events, and variable illumination effects, advancing understanding of mass loss in AGB stars.

Contribution

It provides detailed observations of lobes around Mira A, suggesting periodic ejections and complex gas-dust interactions, which are novel insights into AGB star mass-loss mechanisms.

Findings

Gas mass in lobes is about 2 x 10^-5 solar masses.

Ejection events likely occur every 50-200 years.

Lobes show higher gas densities than expected from large-scale mass loss.

Abstract

Stars with masses between roughly 1 and 8~$M_\odot$ end their lives on the asymptotic giant branch (AGB), when intense mass loss takes place. The outflows are generally accepted to be driven by radiation pressure acting on dust grains that form in the dense extended atmospheres created by the action of convection and stellar pulsations. The complex physics underlying convection, stellar pulsations, and dust nucleation precludes predicting AGB mass loss from first principles. We investigated the evolution of two lobes observed to be expanding away from the AGB star Mira~A using images of polarized light obtained at six epochs using SPHERE on the VLT and of molecular emission at two epochs obtained with ALMA. While dust seems confined to the edges of the lobes, gas fills the lobes and displays higher densities than expected at the observed radii based on the large-scale mass-loss rate of Mira~A, with a total gas mass in the lobes of $\sim 2 \times 10^{-5}~M_\odot$. We find the expansion of the lobes to be consistent with both a constant velocity (ejection time in 2010 or 2011) or a decelerating expansion (ejection time in 2012). If ejection events with a similar magnitude happen periodically, we derive periods between 50 and 200~years to account for the mass-loss rate of Mira~A. This periodicity is uncertain because the average mass-loss rate of Mira A on larger scales is uncertain. We find abundances in the lobes of $\sim 1.5 \times 10^{-6}$ and $\sim 2.5 \times 10^{-6}$ for SO and SO$_2$, respectively, and of $2\times10^{-10}$, $6.5\times10^{-10}$, and $4\times10^{-7}$ for AlO, AlF, and PO. The strong variation in brightness of the different features identified in the polarized-light images is puzzling. We suggest that an asymmetric stellar radiation field preferentially illuminates specific regions of the circumstellar envelope at a given time, producing a lighthouse-like effect.