Spatial and spectral constraints on resolved mass-loss of the massive Post-RSG star IRAS 17163-3907 and its Fried Egg Nebula

TL;DR

This study investigates the mass-loss history, geometry, and physical conditions of the yellow hypergiant IRAS 17163-3907 using multi-wavelength high-resolution observations, revealing a dense, structured, and variable wind without evidence of a binary companion.

Contribution

It provides the first high-resolution spectral and interferometric analysis of IRAS 17163, demonstrating complex, clumpy mass loss and comparing it with another hypergiant, IRC+10420.

Findings

IRAS 17163 hosts a dense, structured, and time-variable wind.

No evidence of a binary companion within the explored parameter space.

The star exhibits a marginally asymmetric and variable ionised wind.

Abstract

The fate of massive stars during the latest stages of their evolution is highly dependent on their mass-loss rate and geometry. These processes have a significant influence on stars with masses between 25 and 40 Msun, i.e., type II SN progenitors. We aim to investigate the mass-loss history, geometry, and physical conditions of the yellow hypergiant in a post-RSG stage, IRAS 17163-3907. We place it in context with another famous yellow hypergiant, IRC+10420. We combine M-band high-resolution CRIRES+ spectroscopy with VLTI/MATISSE mid-infrared L-band interferometry, and FORS2 optical spectropolarimetry to probe both the small-scale circumstellar structure and the large-scale dusty environment of IRAS 17163. 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 Br{\alpha} line-emitting region, which hints at a marginally asymmetric and variable ionised wind. FORS2 spectropolarimetry points to deviations from perfect spherical symmetry also on larger scales. The data show 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. Our results demonstrate that IRAS 17163 hosts a dense, structured, and time-variable wind, coexisting with extended dusty shells. The comparison with IRC+10420 highlights diversity among post-RSG YHGs. 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.