[Paper Review] The rapidly evolving hypergiant IRC+10420: High-resolution bispectrum speckle-interferometry and dust-shell modelling

T. Bloêcker, Y. Y. Balega+4 more|arXiv (Cornell University)|Jun 29, 1999

Stellar, planetary, and galactic studies3 citations

TL;DR

This study presents the first high-resolution bispectrum speckle interferometry of the hypergiant star IRC+10420, achieving 73 mas resolution at 2.11 μm. Using radiative transfer modeling of the spectral energy distribution and visibility function, it finds that a two-component dust shell—featuring a density enhancement at 308R⋆ with a flatter density profile (x=1.7 vs. x=2)—best explains the data, indicating a recent, enhanced mass-loss phase 60–90 yr ago at d=5 kpc.

ABSTRACT

The hypergiant IRC+10420 is a unique object for the study of stellar evolution since it is the only object that is believed to be witnessed in its rapid transition from the red supergiant stage to the Wolf-Rayet phase. Its effective temperature has increased by 1000-2000K within only 20yr. We present the first speckle observations of IRC+10420 with 73mas resolution. A diffraction-limited 2.11 micron image was reconstructed from 6m telescope speckle data using the bispectrum speckle-interferometry method. The visibility function shows that the dust shell contributes 40% to the total flux and the unresolved central object 60%. Radiative transfer calculations have been performed to model both the spectral energy distribution and visibility function. The grain sizes, a, were found to be in accordance with a standard distribution function, n(a)~a^(-3.5), with 0.005 micron < a < 0.45 micron. The observed dust shell properties cannot be fitted by single-shell models but seem to require multiple components. At a certain distance we considered an enhancement over the assumed 1/r^x density distribution. The best model for both SED and visibility was found for a dust shell with a dust temperature of 1000K at its inner radius of 69Rstar. At a distance of 308Rstar the density was enhanced by a factor of 40 and and its density exponent was changed from x=2 to x=1.7. The shell's intensity distribution was found to be ring-like.The ring diameter is equal to the inner diameter of the hot shell (69mas). The diameter of the central star is 1mas. The two-component model can be interpreted in terms of a termination of an enhanced mass-loss phase roughly 60 to 90 yr (for d=5kpc) ago.

Motivation & Objective

• To investigate the physical structure of the rapidly evolving hypergiant IRC+10420, which is transitioning from red supergiant to Wolf-Rayet phase.
• To resolve the dust shell and central star using high-angular-resolution speckle interferometry at 2.11 μm.
• To model the spectral energy distribution (SED) and visibility function to constrain dust grain size distribution and shell geometry.
• To determine whether single-shell or multiple-shell models better fit the observed flux and visibility data.
• To infer the timing and nature of the recent mass-loss episode responsible for the observed dust shell structure.

Proposed method

• Bispectrum speckle interferometry was applied to 6m telescope data to reconstruct a diffraction-limited 2.11 μm image of IRC+10420 with 73 mas resolution.
• The visibility function was derived from the bispectrum to separate contributions from the unresolved central star and the extended dust shell.
• Radiative transfer models were used to fit both the observed SED and visibility function, assuming a power-law grain size distribution n(a) ∝ a^(-3.5) over 0.005–0.45 μm.
• Multiple dust shell components were tested, including modifications to the radial density profile (1/r^x) with a localized enhancement at 308R⋆.
• The best-fit model assumed an inner dust shell radius of 69R⋆ with a temperature of 1000 K, and a modified density exponent (x=1.7) in an enhanced-density region.
• The model was iteratively adjusted to match both the flux ratio (60% central star, 40% dust shell) and the angular diameter of the dust shell (69 mas).

Experimental results

Research questions

• RQ1 What is the spatial structure of the dust shell around IRC+10420 at sub-arcsecond resolution?
• RQ2 How do the flux contributions from the central star and the dust shell compare at 2.11 μm?
• RQ3 Can a single-shell dust model reproduce both the SED and visibility function of IRC+10420?
• RQ4 What is the radial density profile of the dust shell, and does it require a non-uniform distribution?
• RQ5 When did the enhanced mass-loss phase that formed the current dust shell occur?

Key findings

• The dust shell contributes 40% of the total flux at 2.11 μm, while the unresolved central star contributes 60%.
• The best-fit model requires a two-component dust shell, with a density enhancement by a factor of 40 at a distance of 308R⋆ from the star.
• The radial density profile changes from x=2 to x=1.7 in the enhanced region, indicating a flatter distribution.
• The inner dust shell has a radius of 69R⋆ and a temperature of 1000 K, consistent with the observed angular diameter of 69 mas.
• The central star's angular diameter is 1 mas, consistent with a compact, unresolved source.
• The enhanced mass-loss phase is inferred to have ended 60–90 years ago, assuming a distance of 5 kpc.