[Paper Review] Radiative hydrodynamics simulations of red supergiant stars: II. simulations of convection on Betelgeuse match interferometric observations

A. Chiavassa, X. Haubois+5 more|HAL (Le Centre pour la Communication Scientifique Directe)|Mar 6, 2010

Stellar, planetary, and galactic studiesPhysics and Astronomy31 references72 citations

TL;DR

This study uses 3D radiative-hydrodynamics simulations with the CO5BOLD code to model convection on Betelgeuse, comparing synthetic interferometric visibility curves and closure phases to multi-wavelength observations from optical to H band. It provides robust evidence for a granulation pattern on Betelgeuse’s surface, identifying small-to-medium-scale (5–15 mas) and large (≈30 mas) convective cells, with H2O molecules playing a dominant role in shaping the first visibility null and small-scale structures.

ABSTRACT

Context. The red supergiant (RSG) Betelgeuse is an irregular variable star. Convection may play an important role in understanding this variability. Interferometric observations can be interpreted using sophisticated simulations of stellar convection. Aims. We compare the visibility curves and closure phases obtained from our 3D simulation of RSG convection with CO5BOLD to various interferometric observations of Betelgeuse from the optical to the H band in order to characterize and measure the convection pattern on this star. Methods. We use 3D radiative-hydrodynamics (RHD) simulation to compute intensity maps in different filters and we thus derive interferometric observables using the post-processing radiative transfer code OPTIM3D. The synthetic visibility curves and closure phases are compared to observations. Results. We provide a robust detection of the granulation pattern on the surface of Betelgeuse in the optical and in the H band based on excellent fits to the observed visibility points and closure phases. Moreover, we determine that the Betelgeuse surface in the H band is covered by small to medium scale (5-15 mas) convection-related surface structures and a large (30 mas) convective cell. In this spectral region, H2O molecules are the main absorbers and contribute to the small structures and to the position of the first null of the visibility curve (i.e. the apparent stellar radius).

Motivation & Objective

• To determine whether 3D radiative-hydrodynamics simulations can reproduce interferometric observations of Betelgeuse across optical and near-infrared wavelengths.
• To investigate the role of convection in shaping surface inhomogeneities and interferometric visibility curves and closure phases.
• To assess the contribution of molecular absorbers—particularly H2O, CO, and CN—to observed visibility features and apparent stellar radius.
• To validate the realism of RHD simulations by comparing synthetic observables to high-angular-resolution data from multiple instruments.
• To identify limitations in current models and guide future improvements, especially regarding non-grey opacities and radiation pressure.

Proposed method

• Employed 3D radiative-hydrodynamics (RHD) simulations using the CO5BOLD code to model convection in a red supergiant star.
• Generated synthetic intensity maps across multiple filters (optical to H band) from the RHD simulation outputs.
• Applied the post-processing radiative transfer code OPTIM3D to compute synthetic visibility curves and closure phases from the intensity maps.
• Compared the synthetic interferometric observables directly to observed data from instruments including WHT, COAST, VLT/NACO, and VLTI/AMBER.
• Used parametric models (e.g., circular disk with spots) from prior studies as benchmarks for qualitative comparison with simulation results.
• Analyzed the impact of molecular absorbers (TiO, H2O, CO, CN) on the visibility curve shape and surface contrast by examining spectral band effects.

Experimental results

Research questions

• RQ1 Can 3D RHD simulations reproduce the visibility curves and closure phases observed for Betelgeuse across the optical and H band?
• RQ2 What is the spatial scale and morphology of convective structures on Betelgeuse’s surface as inferred from interferometric data?
• RQ3 How do molecular absorbers such as H2O, CO, and TiO influence the visibility curve’s first null and the apparent stellar radius?
• RQ4 To what extent do observed surface brightness variations (e.g., hot spots) result from convective dynamics rather than other phenomena?
• RQ5 What improvements in simulation physics (e.g., non-grey opacities, radiation pressure) are needed to better match observations?

Key findings

• The 3D RHD simulations provide a robust detection of granulation on Betelgeuse’s surface, with excellent agreement between synthetic and observed visibility curves and closure phases.
• In the H band, the surface is covered by small-to-medium-scale granules (5–15 mas) and one large convective cell of approximately 30 mas in diameter.
• H2O molecules are the dominant absorber in the H band and are primarily responsible for the position of the first visibility null, which defines the apparent stellar radius.
• In the optical region, surface brightness variations reach up to 50 times contrast between bright and dark regions, driven by shock waves and non-radial pulsations in TiO-rich layers.
• The simulations show that molecular absorption (especially TiO) enhances surface contrast and that current models with grey opacities underpredict this effect.
• The inclusion of non-grey opacities and radiation pressure in future models is expected to improve agreement with observations, particularly in the TiO band regions.