  In warm (equilibrium temperature <1000 K) gas giant exoplanet atmospheres,
the observation of trace species in abundances deviating from thermochemical
equilibrium predictions could be used as an indicator of disequilibrium
chemical processes, such as photochemistry. To predict which compounds could be
used as such tracers, it is therefore essential to study how photochemical
processes affect their abundances. For this purpose, we investigated
experimentally the efficiency of the photochemical formation of hydrocarbons in
gas mixtures representative of warm gas giant atmospheres as a function of the
gas temperature at millibar pressures. We find that, compared to thermal
reactions alone, photochemistry efficiently promotes, under the studied
conditions, the formation of hydrocarbons, with the detection of acetylene,
ethane, and propane, as well as carbon monoxide. Therefore, our results confirm
the importance of photochemistry in exoplanet atmospheres as a disequilibrium
process. Ethane is the major hydrocarbon formed in our experiments, in apparent
contradiction with the prediction by thermophotochemical models that acetylene
should be the main hydrocarbon product. We also observe an evolution of the
hydrocarbon production efficiency as a function of the temperature, a behavior
not reproduced by a 0D thermophotochemical model. Additional studies are
necessary to definitively understand the origin of the differences between the
experimental and modeling results and to infer the importance of our results
for understanding hydrocarbon formation in warm gas giant exoplanet
atmospheres. Finally, our work demonstrates the importance of experimental
studies together with modeling studies to accurately interpret, understand, and
predict observations of exoplanet atmospheres.
