CO⁵BOLD is a novel radiation hydrodynamics code designed for modelling the convective layers and atmospheres of stars, including the Sun. The atmosphere of a star requires a complex modelling since the dynamics and state of the stellar gas are tightly coupled to the electromagnetic radiation. This involves a realistic equation of state including ionisation and formation and dissociation of molecules like, e.g., carbon monoxide (CO).

a 3-D radiation chemo-magneto-hydrodynamics code
CO⁵BOLD solves the time-dependent hydrodynamic equations coupled with the radiative transfer equation for a compressible plasma in a constant gravitational field in two or three spatial dimensions. A realistic equation of state is used. Operator splitting separates (magneto-)hydrodynamics, 3-D tensor viscosity, radiative transfer, and optional modules for the treatment of hydrogen ionisation, chemical reactions or dust formation.

(magneto-)hydrodynamics
The use of approximate Riemann solvers make the code suitable for handling shock waves that a ubiquitous phenomenon in the atmosphere of many stars. Simulations with and without magnetic fields are possible.

radiative transfer
Different modules are available for the radiative transfer. Next to a solver based on long characteristics there is also a method based on short characteristics. The different schemes have been developed with different flow geometries in mind: the long characteristics solver is optimised for a stellar atmosphere-like setup (``local box'') as used for solar applications, while the short characteristics method is optimised for a ``star-in-a-box'' global setup. This way CO⁵BOLD can be used not only for the simulation of rather small regions of solar and stellar surface convection but also for modelling a whole red supergiant.

hydrogen ionisation
The time-dependent treatment of hydrogen ionisation is an important step towards a realistic description of the upper atmospheric layers. This important feature is currently under development.

chemistry
Recently, the treatment of chemical reaction networks has been implemented. As first application the time-dependent formation and dissociation of carbon monoxide (CO) in the solar atmosphere including the radiative cooling of CO lines has been simulated.

dust
Dust formation can be optionally treated. This feature is used for simulations of giant stars.

more information
CO⁵BOLD User Manual