The curious case of II Lup: a complex morphology revealed with SAM/NACO and ALMA

TL;DR

This study provides the first high-resolution infrared and sub-mm images of the carbon-rich AGB star II Lup, revealing a complex, non-spherical, spiral-shaped circumstellar envelope with clumps and structures extending up to 23 arcseconds.

Contribution

First imaging of II Lup's circumstellar environment in infrared and sub-mm wavelengths, unveiling its complex, non-spherical, spiral morphology with detailed structural features.

Findings

Detected non-spherical, spiral-shaped envelope extending up to 23 arcsec.

Identified clumps and hook-like structures in the circumstellar material.

Estimated orbital period of 128 years based on spiral arm spacing.

Abstract

We present the first-ever images of the circumstellar environment of the carbon-rich AGB star II Lup in the infrared and sub-mm wavelengths, and the discovery of the envelope's non-spherical morphology with the use of high-angular resolution imaging techniques with the sparse aperture masking mode on NACO/VLT (that enables diffraction limited resolution from a single telescope) and with ALMA. We have successfully recovered images in $Ks$ (2.18$\mu$m), $L'$ (3.80$\mu$m) and $M'$ (4.78$\mu$m), that revealed the non-spherical morphology of the circumstellar envelope around II Lup. The stellar surface of the AGB star is unresolved (i.e. $\leq30$ mas in $Ks$) however the detected structure extends up to 110 mas from the star in all filters. Clumps have been found in the $Ks$ maps, while at lower emission levels a hook-like structure appears to extend counter-clockwise from the south. At larger spatial scales, the circumstellar envelope extends up to approximately 23 arcsec, while its shape suggests a spiral at four different molecules, namely CO, SiO, CS and HC$_3$N, with an average arm spacing of 1.7 arcsec which would imply an orbital period of 128 years for a distance of 590pc.