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

Abstract

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.

Dense Molecular Environments of B[e] Supergiants and Yellow Hypergiants

“…The equivalent widths of the intense Na I lines from the YHG IRC +10420 [6] are about three times higher than for HD 179821, for which we measured values of −1.0670 ± 0.015 Å and −0.9231 ± 0.018 Å. Recent spatially resolved observations revealed that the Na I emission in IRC +10420 and Hen 3-1379 is confined within a compact spherical envelope around the star [50,123]. The emission lines in our spectrum are symmetric, and their wavelengths coincide with the systemic velocity of the star.…”

“…Na I emission is a clear indicator for circumstellar material. It has been reported from the NIR spectra of numerous evolved massive stars: (i) the YHGs ρ Cas, V509 Cas, Hen 3-1379 (the Fried Egg Nebula), IRC +10420 [70,123], and the IRC +10420 analog IRAS 18357-0604 [124], (ii) most of the B[e]SGs [21,29,125], and (iii) also many luminous blue variables [21,53]. The equivalent widths of the intense Na I lines from the YHG IRC +10420 [6] are about three times higher than for HD 179821, for which we measured values of −1.0670 ± 0.015 Å and −0.9231 ± 0.018 Å.…”

First spatially resolved Na i and He i transitions towards a massive young stellar object. Finding new tracers for the gaseous star/disc interface

“…The atomic sodium doublet emission (Na at 2.20 𝜇m) has been previously detected in various astrophysical environments (e.g., evolved massive stars; Hanson et al 1996;Koumpia et al 2022) including MYSOs (Porter et al 1998;Pomohaci et al 2017). Due to its low ionization potential (5.1 eV) and its co-existence in energetic environments with the hydrogen recombination line (Br𝛾), the physical process responsible for this emission has been under debate.…”

Observations of outflows of massive stars

“…Emission from magnetically aligned grains is the most likely origin of the observed continuum polarisation, which suggests a strong role of surface activity of a convective nature and magnetic fields in the star's recent mass-loss history. At milliarcsecond spatial resolution, the outflow produced by IRC +10420 has the shape of an hourglass nebula, possibly induced by high rotation and latitude dependent mass-loss (Koumpia et al 2022).…”