  Cool astrophysical objects, such as (exo)planets, brown dwarfs, or asymptotic
giant branch stars, can be strongly affected by condensation. Condensation does
not only directly affect the chemical composition of the gas phase by removing
elements but the condensed material also influences other chemical and physical
processes in these objects. This includes, for example, the formation of clouds
in planetary atmospheres and brown dwarfs or the dust-driven winds of evolved
stars. In this study we introduce FastChem Cond, a new version of the FastChem
equilibrium chemistry code that adds a treatment of equilibrium condensation.
Determining the equilibrium composition under the impact of condensation is
complicated by the fact that the number of condensates that can exist in
equilibrium with the gas phase is limited by a phase rule. However, this phase
rule does not directly provide information on which condensates are stable. As
a major advantage of FastChem Cond is able to automatically select the set
stable condensates satisfying the phase rule. Besides the normal equilibrium
condensation, FastChem Cond can also be used with the rainout approximation
that is commonly employed in atmospheres of brown dwarfs or (exo)planets.
FastChem Cond is available as open-source code, released under the GPLv3
licence. In addition to the C++ code, FastChem Cond also offers a Python
interface. Together with the code update we also add about 290 liquid and solid
condensate species to FastChem.
