Tracing a decade of activity towards a yellow hypergiant. The spectral and spatial morphology of IRC+10420 at au scales

The fate of a massive star during the latest stages of its evolution is highly dependent on its mass-loss history and geometry, with the yellow hypergiants being key objects. We present near-IR interferometric observations of the famous yellow hypergiant IRC +10420 and blue spectra taken between 1994-2019. Our 2.2 μm GRAVITY/VLTI observations attain a spatial resolution of ∼5 stellar radii and spatially resolve the hot emission in the K-band tracing the gas via Na I doublet emission and the Brγ emission. Our geometric modelling reveals a compact neutral zone (Na I) which is slightly larger than the continuum but within an extended Brγ emitting region. Our study confirms an hour-glass geometry of the wind, but we find no signature of a companion at 7-800 au separations at the contrast limit of our observations (3.7 mag at 3σ) to explain this geometry. We report an evolution of the ejecta over 7 years, and constrain the opening angle of the hour-glass to be <10○. Lastly, we present the first blue optical spectra of IRC +10420 since 1994. The multi-epoch data indicate that the spectral type, and thus temperature, of the object has essentially remained constant during the intervening years. Therefore, the observed increase in temperature of 2000 K in less than two decades prior to 1994 is now halted. This suggests that this yellow hypergiant has ‘hit’ the White Wall in the HR-diagram preventing it from evolving blue-wards, and will likely undergo a major mass-loss event in the near future.