FastChem Cond: Equilibrium chemistry with condensation and rainout for cool planetary and stellar environments

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 object. 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.Comment: submitted to MNRAS, code available at https://github.com/exoclime/FastChe

FastChem 2 : an improved computer program to determine the gas-phase chemical equilibrium composition for arbitrary element distributions

The computation of complex neutral/ionized chemical equilibrium compositions is invaluable to obtain scientific insights of, for example, the atmospheres of extrasolar planets and cool stars. We present FASTCHEM 2 , a new version of the established semi-analytical thermochemical equilibrium code FASTCHEM. Whereas the original version is limited to atmospheres containing a significant amount of hydrogen, FASTCHEM 2 is also applicable to chemical mixtures dominated by any other species, such as CO2 or N2. The new C++ code and an optional PYTHON module are publicly available under the GPLv3 license. The program is backward compatible so that the previous version can be easily substituted. We updated the thermochemical data base by adding HNC, FeH, TiH, Ca−, and some organic molecules. In total 523 species are now in the thermochemical data base including 28 chemical elements. The user can reduce the total number of species to, for example, increase the computation performance or can add further species if the thermochemical data are available. The program is validated against its previous version and extensively tested over an extended pressure–temperature grid with pressures ranging from 10−13 up to 103bar and temperatures between 100 and 6000K⁠. FASTCHEM 2 is successfully applied to a number of different scenarios including nitrogen-, carbon-, and oxygen-dominated atmospheres and test cases without hydrogen and helium. Averaged over the extended pressure–temperature grid FASTCHEM 2 is up to 50 times faster than the previous version and is also applicable to situations not treatable with version 1

FastChem: A computer program for efficient complex chemical equilibrium calculations in the neutral/ionized gas phase with applications to stellar and planetary atmospheres

For the calculation of complex neutral/ionized gas-phase chemical equilibria, we present a semi-analytical, versatile, and efficient computer program, called FastChem. The applied method is based on the solution of a system of coupled non-linear (and linear) algebraic equations, namely the law of mass action and the element conservation equations including charge balance, in many variables. Specifically, the system of equations is decomposed into a set of coupled nonlinear equations in one variable each, which are solved analytically whenever feasible to reduce computation time. Notably, the electron density is determined by using the method of Nelder and Mead at low temperatures. The program is written in object-oriented C++ which makes it easy to couple the code with other programs, although a stand-alone version is provided. FastChem can be used in parallel or sequentially and is available under the GNU General Public License version 3 at https://github.com/exoclime/FastChem together with several sample applications. The code has been successfully validated against previous studies and its convergence behaviour has been tested even for extreme physical parameter ranges down to 100K100K and up to 1000bar1000bar⁠. FastChem converges stable and robust in even most demanding chemical situations, which posed sometimes extreme challenges for previous algorithms

Revisiting the inner boundary of the Habitable Zone: Criteria and Feedback processes

The Habitable Zone (HZ) is generally defined as the orbital region around a star, in which life-supporting (habitable) planets can exist. Taking into account that liquid water is a fundamental requirement for life as we know it, the HZ boundaries depend on conditions, such as the amount of stellar insolation, which may limit the potential existence of liquid water on the planetary surface. We investigate two scientific key questions concerning the inner boundary of the HZ: First, which are the criteria important for the determination of the inner boundary of the HZ in general (i.e. critical point of water, water loss limit, runaway greenhouse effect), and second, where is the inner boundary of the HZ located in the Solar System. We address these questions by applying a one-dimensional radiative-convective atmospheric model to an Earth-like planet orbiting the Sun. The model includes updated treatments of the radiative transfer for the thermal infrared and solar wavelengths to account for absorption by water vapor at high temperatures and pressures. Our calculations are performed self-consistently taking into account the feedback between surface heating, water evaporation and energy transport in the atmosphere. The inner boundary of the HZ is evaluated in terms of key criteria for the Solar System. The results of our calculation of the inner boundary of the HZ for the Solar System are comparable to previous studies for the water loss limit and the critical point of water criterion. However, we find that the runaway greenhouse is not the limiting process for habitability for the particular example of the Earth-like planet orbiting the Sun. The critical point of water is reached before the entire ocean is evaporated. We show that a radiation limit of the outgoing infrared flux is a necessary but not sufficient condition for the runaway greenhouse effect. This is in contrast to results of previous studies. Rather the radiation limit is a phenomenon occurring in water dominated convective atmospheres and is not generally linked to the occurrence of a runaway greenhouse process

Chemical pathway analysis of the Martian atmosphere: CO_2-formation pathways

The chemical composition of a planetary atmosphere plays an important role for atmospheric structure, stability, and evolution. Potentially complex interactions between chemical species do not often allow for an easy understanding of the underlying chemical mechanisms governing the atmospheric composition. In particular, trace species can affect the abundance of major species by acting in catalytic cycles. On Mars, such cycles even control the abundance of its main atmospheric constituent CO_2. The identification of catalytic cycles (or more generally chemical pathways) by hand is quite demanding. Hence, the application of computer algorithms is beneficial in order to analyze complex chemical reaction networks. Here, we have performed the first automated quantified chemical pathways analysis of the Martian atmosphere with respect to CO_2-production in a given reaction system. For this, we applied the Pathway Analysis Program (PAP) to output data from the Caltech/JPL photochemical Mars model. All dominant chemical pathways directly related to the global CO_2-production have been quantified as a function of height up to 86 km. We quantitatively show that CO_2-production is dominated by chemical pathways involving HO_x and O_x. In addition, we find that NO_x in combination with HO_x and O_x exhibits a non-negligible contribution to CO_2-production, especially in Mars’ lower atmosphere. This study reveals that only a small number of chemical pathways contribute significantly to the atmospheric abundance of CO_2 on Mars; their contributions to CO_2-production vary considerably with altitude. This analysis also endorses the importance of transport processes in governing CO_2-stability in the Martian atmosphere. Lastly, we identify a previously unknown chemical pathway involving HO_x, O_x, and HO_2-photodissociation, contributing 8% towards global CO_2-production by chemical pathways using recommended up-to-date values for reaction rate coefficients