Rings and arcs around evolved stars

Abstract

Context. A fraction of planetary nebulae (PNe) presents ring-like features in their halos and outermost envelopes. These are thought to be the relic of the last mass loss gaps at the end of the asymptotic giant branch (AGB) phase, providing information on cyclic mass-loss modulations associated either with stellar pulsations or binary interactions.Aims. We aim at characterizing the spatial and physical properties of the ring-like features around IC 418, the iconic Spirograph Nebula.Methods. Deep Very Large Telescope (VLT) Multi Unit Spectroscopic Explorer (MUSE) integral field spectroscopic observations and sharp Hubble Space Telescope (HST) images have been investigated to search for the faintest arc-like features and to characterize their physical properties.Results. Up to 10 concentric arc-like features are detected, with a notable change in the inter-ring gap, which is smaller for the innermost rings. This is interpreted as evidence of the interaction of the expanding nebula with the arc-like features, which otherwise would expand slower. The nebula proper motion also affects the shape and gap between arc-like features.Conclusions. The extinction radial profile decreases outward, indicating the presence of dust. Otherwise the electronic temperature (T_{e) and density (ne) derived both from emission line radial profiles and integrated spectra extracted show values around 10 000 K and 8000 cm^{−3, respectively. Collectively these results indicate distinct differences between the inner nebula, which is denser and more extincted, and the ring-like structures.}}