Spatial and spectral constraints on resolved mass loss of the massive post-red supergiant star IRAS 17163–3907 and its Fried Egg Nebula

¹ Joint ALMA Observatory, Alonso de Córdova 3107 Vitacura, Santiago, Chile
² ESO, Alonso de Córdova 3107, Vitacura, Casilla, 19001 Santiago, Chile
³ Gemini Observatory/NSF’s NOIRLab, Casilla 603, La Serena, Chile
⁴ Royal Observatory of Belgium, Ringlaan 3, 1180 Brussels, Belgium
⁵ Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544, USA
⁶ Armagh Observatory and Planetarium, College Hill, Armagh BT61 9DG, UK
⁷ Observatoire de la Côte d’Azur Nice, 96 Boulevard de l’Observatoire, 06300 Nice, France
⁸ Department of Space, Earth, and Environment, Chalmers University of Technology, Onsala Space Observatory, 43992 Onsala, Sweden
⁹ Jodrell Bank Centre for Astrophysics, Alan Turing Building, The University of Manchester, Oxford Road, Manchester M139PL, UK
¹⁰ IAASARS, National Observatory of Athens, I. Metaxa & Vas. Pavlou St., 15236 Penteli, Athens, Greece
¹¹ School of Physics & Astronomy, University of Leeds, Woodhouse Lane, LS2 9JT, Leeds, UK
¹² National Astronomical Observatory of Japan, Los Abedules 3085 Oficina 701 Vitacura, Santiago 763 0000, Chile
¹³ The Graduate University for Advanced Studies, SOKENDAI, Osawa 2-21-1 Mitaka, Tokyo postcode181-8588 Japan
¹⁴ National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA
¹⁵ Xinjiang Astronomical Observatory, Chinese Academy of Sciences, 150 Science 1-Street, Urumqi, Xinjiang 830011, China
¹⁶ Xinjiang Key Laboratory of Radio Astrophysics, 150 Science 1-Street, Urumqi, Xinjiang 830011, China
¹⁷ Nicolaus Copernicus Astronomical Centre, PAS, ul. Rabiańska 8, 87-100 cityToruń Poland

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 18 February 2026
Accepted: 12 March 2026

Abstract

Context. The fate of massive stars during the latest stages of their evolution is highly dependent on their mass-loss rate and geometry. The geometry of the mass-loss process can be inferred from the shape of the circumstellar material, which has a significant influence on the evolution of massive stars (between 25 and 40 M_⊙), i.e. type II SN progenitors. In this context, post-red supergiants (post-RSGs) offer an excellent opportunity to study mass-loss events.

Aims. We aim to investigate the mass-loss history, geometry, and physical conditions of the yellow hypergiant in a post-RSG stage, IRAS 17163−3907 (IRAS 17163, also known as the ‘Fried Egg’ nebula). We place it in context with another famous evolved massive star, the yellow hypergiant IRC+10420.

Methods. We combine M-band spectra of the source using high-resolution CRIRES+ spectroscopy, with VLTI/MATISSE mid-infrared interferometry in the L-band, and FORS2 optical spectropolarimetry to probe both the small-scale circumstellar structure and the large-scale dusty environment of IRAS 17163. The interferometric observables were analysed with simple geometric fitting and a more advanced parametric modelling using PMOIRED to extract the morphology of the hot inner shell that was previously reported via radiative transfer modelling.

Results. The CRIRES+ spectrum provides the first M-band coverage of IRAS 17163, revealing prominent low-excitation metal lines and hydrogen recombination features, but lacking the pronounced CO absorption seen in IRC+10420. The MATISSE observations reveal the first high angular scales of the source in the L-band and spatially resolve the Brα line-emitting region, which is a factor of two more extended than the continuum emission and hints at a marginally asymmetric and variable ionised wind. FORS2 spectropolarimetry shows intrinsic continuum polarisation and line effects in the Stokes Q parameter across Hα, pointing to deviations from perfect spherical symmetry also on larger scales. The interferometry reveals no evidence for a binary companion within the explored parameter space, indicating that the observed clumpy and time-variable mass loss is likely intrinsic to the star rather than companion-driven.

Conclusions. Our results demonstrate that IRAS 17163 hosts a dense, structured, and time-variable wind, coexisting with extended dusty shells. Comparison with IRC+10420 highlights the diversity among post-RSG yellow hypergiants, with IRAS 17163 showing an ionised environment without apparent molecular signatures. These findings emphasise the role of clumpy and near-symmetric mass loss in shaping the circumstellar medium of evolved massive stars, with implications for their subsequent evolution and core-collapse supernova progenitor properties.