Context. Most of the mass in protoplanetary disks is in the form of gas. The study of the gas
and its diagnostics is of fundamental importance in order to achieve a detailed description of
the thermal and chemical structure of the disk. Both radiation from the central star (from
optical to X-ray wavelengths) and viscous accretion are the main sources of energy,
dominating disk physics and chemistry in its early stages. This is the environment in which
the first phases of planet formation will proceed. Aims. We investigate how stellar and disk�…

Context. Planets are thought to eventually form from the mostly gaseous (~ 99% of the mass)
disks around young stars. The density structure and chemical composition of protoplanetary
disks are affected by the incident radiation field at optical, far-ultraviolet (FUV), and X-ray
wavelengths, as well as by the dust properties. Aims. The effect of FUV and X-rays on the
disk structure and the gas chemical composition are investigated. This work forms the basis
of a second paper, which discusses the impact on diagnostic lines of, eg, C+, O, H 2 O, and�…