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

Abstract

ABSTRACT The morpho-kinematics of the circumstellar envelope of oxygen-rich AGB star R Leonis is probed using Atacama Large Millimeter/submillimeter Array observations of the emission of molecular lines, including in particular CO(2–1) and 29SiO(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 ∼6 arcsec and mean radial expansion velocity ∼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 SO2 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 description applies approximately beyond ∼10 stellar radii from the centre of the star but not at the smaller angular separations where the SO and SO2 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.