Optical/near-infrared observations of the Fried Egg Nebula: Multiple shell ejections on a 100 yr timescale from a massive yellow hypergiant

Abstract

Context. The fate of a massive star during the latest stages of its evolution is highly dependent on its mass-loss rate/geometry and therefore knowing the geometry of the circumstellar material close to the star and its surroundings is crucial. Aims. We aim to study the nature (i.e. geometry, rates) of mass-loss episodes. In this context, yellow hypergiants are great targets. Methods. We analyse a large set of optical/near-infrared data, in spectroscopic and photometric (X-shooter/VLT), spectropolarimetric (ISIS/WHT), and interferometric GRAVITY-AMBER/VLTI) modes, toward the yellow hypergiant IRAS 17163-3907. We present the first model-independent reconstructed images of IRAS 17163-3907 at these wavelengths at milli-arcsecond scales. Lastly, we apply a 2D radiative transfer model to fit the dereddened photometry and the radial profiles of published VISIR images at 8.59 {\mu}m, 11.85 {\mu}m and 12.81 {\mu}m simultaneously, adopting the revised Gaia distance (DR2). Results. The interferometric observables around 2 {\mu}m show that the Br{\gamma} emission is more extended and asymmetric than the Na i and the continuum emission. In addition to the two known shells surrounding IRAS 17163-3907 we report on the existence of a third hot inner shell with a maximum dynamical age of only 30 yr. Conclusions. The interpretation of the presence of Na i emission at closer distances to the star compared to Br{\gamma} has been a challenge in various studies. We argue that the presence of a pseudophotosphere is not needed, but it is rather an optical depth effect. The three observed distinct mass-loss episodes are characterised by different mass-loss rates and can inform the theories on mass-loss mechanisms, which is a topic still under debate. We discuss these in the context of photospheric pulsations and wind bi-stability mechanisms.