Neutral and ionised gas around the post-red supergiant IRC +10 420 at AU size scales ^⋆,⋆⋆

¹ School of Physics & Astronomy, University of Leeds, Woodhouse Lane, Leeds, LS2 9JT UK
e-mail: r.d.oudmaijer@leeds.ac.uk
² European Southern Observatory, 19001 Casilla, Santiago 19, Chile

Received: 7 August 2012
Accepted: 17 December 2012

Abstract

Context. IRC +10 420 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.

Aims. The star has an ionised wind and using the Brγ line we assess the mass-loss on spatial scales of ~1 AU.

Methods. 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 110 m. 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 Na i doublet at 2.2 μm. The data are complemented by X-Shooter data, which provide a higher spectral resolution view.

Results. The Brγ emission line and the Na i 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 4.18 mas, 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 Na i within the ionised region, a very unusual occurrence, suggests the presence of a dense pseudo-photosphere.

Conclusions. The ionised wind can be reproduced with a polar wind, which could well have the shape of an hour-glass. Closer in, the resolved Na i 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.