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

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 Brα 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.

Figures (17)

• Figure 1: CRIRES+ spectra comparison of IRAS 17163 (blue) and IRC+10420 (yellow), showing the Pfund$\beta$ line and CO R-branch transitions. The fluxes are normalised, and a telluric correction has been applied.
• Figure 2: Normalised and telluric corrected CRIRES+ spectra of IRAS 17163 (blue) and IRC+10420 (yellow) around the Brackett $\alpha$ line at $\sim$4.052 $\mu \textrm{m}$. The emission is significantly stronger in IRAS 17163 compared to IRC+10420, indicating a higher density of ionised gas in the inner wind region.
• Figure 3: Comparison of the Pfund $\gamma$ line profiles in IRAS 17163 and IRC+10420 before (top) and after (bottom) telluric correction. The uncorrected profile shows residual atmospheric features affecting both the continuum and line wings, while the corrected version provides a cleaner view of the intrinsic emission and absorption structure.
• Figure 4: uv coverage of the MATISSE observations. Each point corresponds to a projected baseline at a given wavelength. The plot includes all configurations used, illustrating the sampling of the (u,v) plane across different hour angles and baseline lengths. This coverage enables detailed image reconstruction and model fitting.
• Figure 5: Br$\alpha$ line profiles observed with MATISSE, corrected for heliocentric velocity shifts, and overplotted for all available epochs. This comparison highlights the temporal variability in the line emission, revealing changes in both the line shape and intensity. The profiles have been normalised to the continuum and aligned in velocity space to facilitate direct comparison.