Astronomy and Astrophysics, volume 551A, 69-69 (2013/3-1)
Neutral and ionised gas around the post-red supergiant IRC +10420 at AU size scales.
OUDMAIJER R.D. and DE WIT W.J.
Abstract (from CDS):
IRC+10420 is one of the few known massive stars in rapid transition from the red supergiant phase to the Wolf-Rayet or luminous blue variable phase. The star has an ionised wind and using the Brγ line we assess the mass-loss on spatial scales of ∼1AU. We present new VLT Interferometer AMBER data which are combined with all other AMBER data present in the literature. The final dataset covers a position angle range of ∼180° and baselines up to 110m. The spectrally dispersed visibilities, differential phases and line flux are conjointly analysed and modelled. We also present the first AMBER/FINITO observations which cover a larger wavelength range and allow us to observe the NaI doublet at 2.2µm. The data are complemented by X-Shooter data, which provide a higher spectral resolution view. The Brγ emission line and the NaI doublet are both spatially resolved. After correcting the AMBER data for the fact that the lines are not spectrally resolved, we find that Brγ traces a ring with a diameter of 4.18mas, in agreement with higher spectral resolution data. We consider a geometric model in which the Brγ emission emerges from the top and bottom rings of an hour-glass shaped structure, viewed almost pole-on. It provides satisfactory fits to most visibilities and differential phases. The fact that we detect line emission from a neutral metal like NaI within the ionised region, a very unusual occurrence, suggests the presence of a dense pseudo-photosphere. The ionised wind can be reproduced with a polar wind, which could well have the shape of an hour-glass. Closer in, the resolved NaI emission is found to occur on scales barely larger than the continuum. This fact and that many yellow hypergiants exhibit this comparatively rare emission hints at the presence of a ``Yellow'' or even ``White Wall'' in the Hertzsprung-Russell diagram, preventing them from visibly evolving to the blue.