Evidence for episodic and patchy mass ejection in the circumstellar envelope of AGB star R Leonis

TL;DR

This study uses ALMA observations to reveal episodic, patchy mass ejections and complex inhomogeneous structures in the circumstellar envelope of AGB star R Leonis, highlighting the role of convective activity.

Contribution

It provides the first detailed evidence of episodic, patchy mass ejection events and their morphology in R Leonis using high-resolution molecular line observations.

Findings

Detection of a broad, expanding shell from a mass-loss episode a few centuries ago.

Identification of inhomogeneous, patchy structures in the circumstellar envelope.

Evidence of multiple gas outflows influenced by stellar convection and granulation.

Abstract

The morpho-kinematics of the circumstellar envelope of oxygen-rich AGB star R Leonis is probed using ALMA (Atacama Large Millimeter/submillimeter Array) observations of the emission of molecular lines, including in particular CO(2-1) and $^{29}$SiO(5-4). Evidence is found for an episode of enhanced mass loss, a few centuries ago, that produced a broad expanding shell of mean radius $\sim$6 arcsec and mean radial expansion velocity $\sim$5.5 km s$^{-1}$ . Detailed scrutiny of its structure, as displayed by the emission of the CO(2-1) line, reveals strong inhomogeneity and patchy morphology. Evidence is also found, in particular from the morpho-kinematics of the emission of SiO, SO and SO$_2$ lines probing the close neighbourhood of the star, for distinct gas outflows covering broad solid angles in the south-eastern, south-western and north-western quadrants, suggesting significant contribution of the convective cell granulation in defining the pattern of mass ejection. A study of relative molecular abundances in these outflows suggests that a Local Thermal Equilibrium (LTE) description applies approximately beyond $\sim$10 stellar radii from the centre of the star but not at the smaller angular separations where the SO and SO$_2$ molecules are found to be confined. Near the stellar disc, masers of the vibrationally excited SiO lines are found to probe north-western parts of a layer of hot gas, consistent with the earlier observation of an asymmetric expanding shell within 1-2 stellar radii from the centre of the star. Globally, a picture dominated by episodic and patchy mass ejections is found to prevail.