Mira Ceti, atypical archetype

TL;DR

This study analyzes ALMA observations of Mira Ceti's circumstellar envelope, revealing complex, episodic mass loss, confinement, and possible rotation within 50 au, challenging existing models of oxygen-rich AGB star winds.

Contribution

It provides new detailed analysis of molecular line emissions, especially $^{13}$CO(3-2), and highlights Mira Ceti's atypical wind features and episodic mass ejections.

Findings

Evidence for confinement and rotation within 50 au.

Mass loss occurs episodically with clumpy structures.

SiO emission linked to past ejections, not current wind generation.

Abstract

With the aim of unraveling the complexity of the morpho-kinematics of the circumstellar envelope (CSE) of Mira Ceti, we review, extend and in some cases revisit ALMA millimeter observations of the emission of the SiO(5-4) and CO(3-2) molecular lines. In addition, we present a detailed analysis of the optically thin $^{13}$CO(3-2) emission, which provides several important new results. In agreement with observations at infrared and visible wavelengths, we give evidence for confinement and probably rotation of a dense gas volume within $\sim$50 au from the star and for a large SiO line-width within $\sim$15 au. We show that the mass loss process is episodic and takes the form of clumps having a very low SiO/CO abundance ratio compared with similar oxygen-rich long period variables, probably a result of depletion on dust grains and photo-dissociation. We evaluate the mass loss rate associated with the main clumps and compare it with values obtained from single dish observations. We argue that the SiO emission observed in the south-western quadrant is not related to the mechanism of generation of the nascent wind but to a mass ejection that occurred eleven years before the observations. We remark that Mira Ceti is not a good archetype in terms of its wind: models aiming at describing the very complex gas-dust chemistry in action in the CSE of oxygen-rich AGB stars may find it difficult to account for its peculiar features, small variations in the parameters deciding when and where mass loss can proceed significantly.