Hints of a rotating spiral structure in the innermost regions around IRC +10216

G. Quintana-Lacaci, J. Cernicharo, M. Agúndez, L. Velilla Prieto, A. Castro-Carrizo, N. Marcelino, C. Cabezas, I. Peña, J. L. Alonso, J. Zúñiga. 2016. Hints of a rotating spiral structure in the innermost regions around IRC +10216. Astrophysical Journal 818, 2, DOI: 10.3847/0004-637X/818/2/192

The Atacama Large Millimeter/submillimeter Array is allowing us to study the innermost regions of the circumstellar envelopes of evolved stars with unprecedented precision and sensitivity. Key processes in the ejection of matter and dust from these objects occur in their inner zones. In this work, we present sub-arcsecond interferometric maps of transitions of metal-bearing molecules toward the prototypical C-rich evolved star IRC + 10216. While Al-bearing molecules seem to be present as a roughly spherical shell, the molecular emission from the salts NaCl and KCl presents an elongation in the inner regions with a central minimum. In order to accurately analyze the emission from the NaCl rotational lines, we present new calculations of the collisional rates for this molecule based on new spectroscopic constants. The most plausible interpretation for the spatial distribution of the salts is a spiral with a NaCl mass of 0.08 M-circle dot. Alternatively, a torus of gas and dust would result in structures similar to those observed. From the torus scenario we derive a mass of similar to 1.1 x 10(-4) M-circle dot. In both cases, the spiral and the torus, the NaCl structure presents an inner minimum of 27 AU. In the case of the torus, the outer radius is 73 AU. The kinematics of both the spiral and the torus suggests that they are slowly expanding and rotating. Alternative explanations for the presence of the elongation are explored. The presence of these features only in KCl and NaCl might be a result of their comparatively high dipole moment with respect to the Al-bearing species.

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