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

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, 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 (2.18μm), L΄ (3.80μm) and M΄ (4.78μm), that revealed the non-spherical morphology of the circumstellar envelope around II Lup. The stellar surface of the AGB star is unresolved (i.e. ≤30 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″, while its shape suggests a spiral at four different molecules, namely CO, SiO, CS and HC3N, with an average arm spacing of 1.7″ which would imply an orbital period of 128 years for a distance of 590pc.