[Paper Review] Radiative hydrodynamics simulations of red supergiant stars. IV gray versus non-gray opacities

A. Chiavassa, B. Freytag+2 more|arXiv (Cornell University)|Sep 16, 2011

Stellar, planetary, and galactic studiesPhysics and Astronomy48 references63 citations

TL;DR

This study presents 3D radiative-hydrodynamics simulations of red supergiants using the CO5BOLD code with non-gray opacities, revealing a steeper thermal gradient and reduced temperature fluctuations compared to gray models. The non-gray treatment improves spectral energy distribution and interferometric radius precision, while empirical calibration shows micro- and macroturbulence in 1D models must be tuned to 3D results to match effective gravity and line profiles.

ABSTRACT

Red supergiants are massive evolved stars that contribute extensively to the chemical enrichment of our Galaxy. It has been shown that convection in those stars gives rise to large granules that cause surface inhomogeneities and shock waves in the photosphere. The understanding of their dynamics is crucial to unveil the unknown mass-loss mechanism, their chemical composition and stellar parameters. We present a new generation of red supergiants simulations with a more sophisticated opacity treatment done with 3D radiative- hydrodynamics CO5BOLD. In the code, the coupled equations of compressible hydrodynamics and non-local radiation transport are solved in the presence of a spherical potential. The stellar core is replaced by a special spherical inner boundary condition, where the gravitational potential is smoothed and the energy production by fusion is mimicked by a simply producing heat corresponding to the stellar luminosity. The post-processing radiative transfer code OPTIM3D is used to extract spectroscopic and interferometric observables. We show that the relaxation of the assumption of frequency-independent opacities shows a steeper mean thermal gradient in the optical thin region that affect strongly the atomic strengths and the spectral energy distribution. Moreover, the weaker temperature fluctuations reduce the incertitude on the radius determination with interferometry. We show that 1D models of red supergiants must include a turbulent velocity calibrated on 3D simulations to obtain the effective surface gravity that mimic the effect of turbulent pressure on the stellar atmosphere. We provide an empirical calibration of the ad-hoc micro- and macroturbulence parameters for 1D models using the 3D simulations: we find that there is not a clear distinction between the different macroturbulent profiles needed in 1D models to fit 3D synthetic lines.

Motivation & Objective

• Investigate the impact of non-gray opacities on the thermal structure and dynamics of red supergiant (RSG) atmospheres.
• Assess how frequency-dependent radiative transfer affects spectral line formation and interferometric observables compared to gray approximations.
• Calibrate micro- and macroturbulence parameters in 1D models to match the turbulent velocity fields from 3D simulations.
• Evaluate the influence of convective overshoot and boundary conditions on the surface inhomogeneities and energy transport in RSGs.
• Determine whether non-gray models better reproduce observed RSG properties such as effective temperature, luminosity, and radius.

Proposed method

• Perform 3D radiative-hydrodynamics simulations using the CO5BOLD code with a spherical gravitational potential and a heat source mimicking stellar luminosity.
• Implement multigroup frequency-dependent opacities (5 bins) to replace the traditional gray approximation in radiative transfer.
• Use the OPTIM3D post-processing code to compute synthetic spectra and interferometric visibilities from 3D simulation outputs.
• Compare 3D non-gray and gray simulations in terms of temperature structure, velocity fields, and surface inhomogeneities.
• Calibrate 1D model micro- and macroturbulence parameters by matching synthetic line profiles from 3D simulations.
• Analyze the effect of numerical resolution (255³ vs. 401³ grid points) on convective structure and convergence.

Experimental results

Research questions

• RQ1 How does the use of non-gray opacities affect the mean thermal gradient and temperature fluctuations in the outer layers of red supergiant atmospheres?
• RQ2 To what extent do non-gray simulations improve the agreement with observed spectral energy distributions and interferometric measurements compared to gray models?
• RQ3 What are the required micro- and macroturbulence values in 1D models to reproduce the effective surface gravity and line profiles from 3D non-gray simulations?
• RQ4 How do shock waves and convective motions differ in non-gray versus gray simulations, and what is their impact on surface intensity contrast?
• RQ5 Does increasing numerical resolution in 3D simulations lead to convergence in the number and size of convective granules?

Key findings

• Non-gray simulations exhibit a steeper mean thermal gradient in the optical thin region compared to gray models, altering molecular and line strengths.
• The 3D non-gray model produces a spectral energy distribution closer to 1D models than the 3D gray simulation, indicating that non-gray treatment is essential for accurate spectral synthesis.
• Temperature fluctuations are weaker in the non-gray model due to enhanced radiative heat exchange, reducing surface intensity contrast and improving the precision of interferometric radius determination.
• The microturbulence velocity in both gray and non-gray 3D models is similar, supporting the use of depth-independent microturbulence in 1D models as a reasonable approximation.
• Macroturbulence profiles in 1D models show no clear distinction for fitting 3D synthetic lines, but the non-gray model exhibits higher standard deviations in velocity fields, indicating more complex line profiles.
• The non-gray model shows stronger shocks in outer layers due to smaller pressure and density scale heights, leading to more structured velocity fields and line profiles.