Modelling Gas in the Inner Circumstellar Environment of the AGB Star Mira

At the end of their lives low and intermediate mass stars reach the asymptotic giant branch (AGB), a phase in which copious mass loss takes place. In order to improve our understanding of the AGB phase, it is crucial to characterize the outflows of these stars. In this project, we investigate the distribution of the gas in the inner circumstellar envelope of Mira A. High-angular-resolution images obtained with ALMA allow us to study molecular line emission from gas very close to the star, revealing significant structure. We identify two expanding gas lobes that reach up to ~7.4 stellar radii, considerably beyond the extended atmosphere of the star. By modelling the 13CO, J=3-2 line, we constrain the density, temperature and velocity of the gas and find masses for the gas in the lobes on the order of 10^-5 solar masses. This is significantly larger than expected based on the observed mass-loss rate of Mira (~10^-7 solar masses per year). This implies that significant mass ejection takes place every few tens of years rather than in a more steady manner. Alternatively, we might be witnessing a very rare ejection process, but this scenario is less likely. The existence of a relatively close binary complicates the interpretation of the observations regarding single AGB stars. Nonetheless, our results indicate that determining how these gas lobes were created around Mira will help broaden our empirical understanding of the mass-loss process on the AGB.