Synthesizing Spectra from 3D Radiation Hydrodynamic Models of Massive Stars Using Monte Carlo Radiation Transport

TL;DR

This paper uses 3D radiation hydrodynamic models combined with Monte Carlo radiation transport to synthesize stellar spectra, providing insights into turbulent broadening in massive star surfaces and matching observed spectral line broadening.

Contribution

It introduces a novel method of post-processing 3D RHD models with Monte Carlo radiation transport to generate spectra and analyze line broadening, advancing understanding of turbulence in massive stars.

Findings

Synthesized spectral lines match observed macroturbulent broadening.

The method captures viewing angle and temporal variations in spectral lines.

New techniques enable systematic studies of turbulence origins in stellar atmospheres.

Abstract

Observations indicate that turbulent motions are present on most massive star surfaces. Starting from the observed phenomena of spectral lines with widths much larger than thermal broadening (e.g. micro- and macroturbulence) to the detection of stochastic low-frequency variability (SLFV) in the Transiting Exoplanet Survey Satellite photometry, these stars clearly have large scale turbulent motions on their surfaces. The cause of this turbulence is debated, with near-surface convection zones, core internal gravity waves, and wind variability being proposed. Our 3D grey radiation hydrodynamic (RHD) models characterized the surfaces' convective dynamics driven by near-surface convection zones and provided a reasonable match to the observed SLFV in the most luminous massive stars. We now explore the complex emitting surfaces of these 3D RHD models, which strongly violate the 1D assumption of a plane parallel atmosphere. By post-processing the grey RHD models with the Monte Carlo radiation transport code SEDONA, we synthesize stellar spectra and extract information from the broadening of individual photospheric lines. The use of SEDONA enables the calculation of the viewing angle and temporal dependence of spectral absorption line profiles. Combining uncorrelated temporal snapshots together, we compare the broadening from the 3D RHD models' velocity fields to the thermal broadening of the extended emitting region, showing that our synthesized spectral lines closely resemble the observed macroturbulent broadening from similarly luminous stars. More generally, the new techniques we have developed will allow for systematic studies of the origin of turbulent velocity broadening from any future 3D simulations.