  Atmospheres with a high C/O ratio are expected to contain an important
quantity of hydrocarbons, including heavy molecules (with more than 2 carbon
atoms). To study correctly these C-rich atmospheres, a chemical scheme adapted
to this composition is necessary. We have implemented a chemical scheme that
can describe the kinetics of species with up to 6 carbon atoms. This chemical
scheme has been developed with specialists of combustion and validated through
experiments on a wide range of T and P. This chemical network is available on
the online database KIDA. We have created a grid of 12 models to explore
different thermal profiles and C/O ratios. For each of them, we have compared
the chemical composition determined with a C0-C2 chemical scheme (species with
up to 2 carbon atoms) and with the C0-C6 scheme. We found no difference in the
results obtained with the two schemes when photolyses are not included in the
model, whatever the temperature of the atmosphere. In contrast, when there is
photochemistry, differences can appear in the upper atmosphere. These
differences are found for all the tested PT profiles in the case that the C/O
ratio is above 1. When the C/O ratio of the atmosphere is solar, differences
are only found at temperatures lower than 1000K. The differences linked to the
use of different chemical schemes do not have important influence on the
synthetic spectra. However, we have confirmed that C2H2 and HCN as possible
tracers of warm C-rich atmospheres. The use of this new chemical scheme is
mandatory to model atmospheres with a high C/O ratio and, in particular, if one
is interested in studying in details the photochemistry. If one is just
interested in the synthetic spectra, the use of a smaller scheme may be
sufficient.
