Fast ray-tracing algorithm for circumstellar structures (FRACS). II. Disc parameters of the B[e] supergiant CPD-57° 2874 from VLTI/MIDI data

TL;DR

This paper presents a fast, efficient ray-tracing radiative transfer algorithm (FRACS) used to analyze VLTI/MIDI data, enabling the first direct determination of physical parameters of the dusty circumstellar environment of a B[e] supergiant.

Contribution

The paper introduces FRACS, a rapid radiative transfer code optimized for interferometric data, and applies it to derive physical parameters of a B[e] supergiant's circumstellar environment from VLTI/MIDI observations.

Findings

Successfully determined dust radius, flux contributions, and disc inclination.

Demonstrated FRACS's efficiency with <10 seconds per model.

Provided one of the first direct physical parameter measurements of a B[e] supergiant's CSE.

Abstract

B[e] supergiants are luminous, massive post-main sequence stars exhibiting non-spherical winds, forbidden lines, and hot dust in a disc-like structure. The physical properties of their rich and complex circumstellar environment (CSE) are not well understood, partly because these CSE cannot be easily resolved at the large distances found for B[e] supergiants (typically $\ga 1$~kpc). From mid-IR spectro-interferometric observations obtained with VLTI/MIDI we seek to resolve and study the CSE of the Galactic B[e] supergiant CPD-57\degr\,2874. For a physical interpretation of the observables (visibilities and spectrum) we use our ray-tracing radiative transfer code (FRACS), which is optimised for thermal spectro-interferometric observations. Thanks to the short computing time required by FRACS ($<10$~s per monochromatic model), best-fit parameters and uncertainties for several physical quantities of CPD-57\degr\,2874 were obtained, such as inner dust radius, relative flux contribution of the central source and of the dusty CSE, dust temperature profile, and disc inclination. The analysis of VLTI/MIDI data with FRACS allowed one of the first direct determinations of physical parameters of the dusty CSE of a B[e] supergiant based on interferometric data and using a full model-fitting approach. In a larger context, the study of B[e] supergiants is important for a deeper understanding of the complex structure and evolution of hot, massive stars.