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arXiv · 2211.02207

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

D. T. Hoai·P. T. Nhung·M. N. Tan·P. Darriulat·P. N. Diep·N. B. Ngoc·T. T. Thai·P. Tuan-Anh
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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.

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D. T. Hoai, P. T. Nhung, M. N. Tan, P. Darriulat, P. N. Diep, N. B. Ngoc, T. T. Thai, P. Tuan-Anh. 2022-11-04. Evidence for episodic and patchy mass ejection in the circumstellar envelope of AGB star R Leonis. https://doi.org/10.1093/mnras%2Fstac3176

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