Multi-scale molecular emission of the envelope of the O-rich AGB star R Leo

Abstract

Asymptotic Giant Branch stars (AGBs) display roughly spherically symmetric circumstellar envelopes (CSEs) composed of expanding molecular gas and dust. Nevertheless, several AGB stars have been found so far showing a set of concentric arcs or spiral-like structures (e.g., IRC+10216, CRL3068, R Scl). The formation of spirals can be explained by the effect of the orbital movement of binary systems on the ejected gas. They are easily noticeable if the binary systems are face-on but a different spatial orientation blurs the structures showing patterns that comprise many concentric arcs. However, the concentric arcs can be explained also by gas instabilities occurring in the winds of single AGB stars. The matter ejection process involve two mechanisms: (1) the stellar pulsation, that could include episodic (and anisotropic) matter ejection, and (2) the gas acceleration, based on dust and the related gas-phase chemistry. These mechanisms, along with the formation of arcs or spirals, could result in very complex structures difficult to understand and analyze. Hence, exploring the inner layers of the CSEs is crucial to understand the matter ejection and the structures in them. R Leo is a close O-rich low mass-loss rate AGB star where a complex structure in the maser SiO(v=1,J=2-1) has been detected (Cernicharo et al. 1994). This structure could be interpreted as a developing bipolar outflow or a rotating torus in the inner CSE. Additional asymmetries found at larger scales by Plez et al. (1994) suggest that this structure could be part of a larger one covering a significant fraction of the envelope. We present new interferometer molecular observations of R Leo taken at 1.2 mm with ALMA with a HPBW~0.025>. Short-spacings derived from on-the-fly observations acquired with the IRAM 30m telescope were added to recover the filtered emission. These data probe a complex structure that involves extended continuum and molecular emission which imply material infall onto the star. Lateral gas motions compatible with the presence of a torus-like structure have been also observed. At larger scales, the envelope also shows a remarkable red-shifted shell-like structure with a very bright spot in the CO(2-1) emission linked to the gas in the vicinity of the star. The rest of the envelope is still quite complex showing arcs and different elongations that suggest an irregular mass-loss. The maps also reveal significant asymmetries in the molecular abundance distributions.