Multiple shell ejections on a 100 yr timescale from a massive yellow hypergiant

TL;DR

This study investigates the circumstellar environment of the yellow hypergiant IRAS 17163-3907, revealing multiple shell ejections over a century, and explores mechanisms like pulsation and bi-stability jump driving its episodic mass loss.

Contribution

It provides high-resolution observations and a two-zone model to explain emission lines, and identifies at least three mass-loss episodes, proposing bi-stability jump as a key mechanism.

Findings

At least three mass-loss episodes in the past century.

A two-zone LTE model explains emission line sizes and strengths.

Evidence for a third hot inner shell around the star.

Abstract

This contribution focuses on a rare example of the class of post-Red Supergiants, IRAS 17163-3907, the central star of the Fried Egg nebula. In particular, we discuss some of our recently published results in detail. The inner parts of the circumstellar environment of this evolved massive star are probed at milli-arcsec resolution using VLTI's GRAVITY instrument operating in the K-band (2 $\mu$m), while larger, arcsecond, scales are probed by VISIR diffraction limited images around 10 $\mu$m, supplemented by a complete Spectral Energy Distribution. The spectro-interferometric data cover important diagnostic lines (Br$\gamma$, Na I), which we are able to constrain spatially. Both the presence and size of the Na I doublet in emission has been traditionally challenging to explain towards other objects of this class. In this study we show that a two-zone model in Local Thermal Equilibrium can reproduce both the observed sizes and strengths of the emission lines observed in the K-band, without the need of a pseudo-photosphere. In addition, we find evidence for the presence of a third hot inner shell, and demonstrate that the star has undergone at least three mass-loss episodes over roughly the past century. To explain the properties of the observed non-steady mass-loss we explore pulsation-driven and line-driven mass-loss and introduce the bi-stability jump as a possible underlying mechanism to explain mass-loss towards Yellow Hypergiants.