Far-ultraviolet and X-ray irradiated protoplanetary disks:a grid of models II. Gas diagnostic line emission

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 parameters impact the fine-structure cooling lines [Ne II], [Ar II], [O I], [C II], and H2O rotational lines in the disk. These lines are potentially powerful diagnostics of the disk structure, and their modeling permits a thorough interpretation of the observations carried out with instrumental facilities such as Spitzer and Herschel.Methods. Following our earlier paper, we computed a grid of 240 disk models, in which the X-ray luminosity, UV-excess luminosity, minimum dust grain size, dust size distribution power law, and surface density distribution power law are systematically varied. We solve self-consistently for the disk vertical hydrostatic structure in every model and apply detailed line radiative transfer to calculate line fluxes and profiles for a series of well-known mid-and far-infrared cooling lines.Results. The [O I] 63 mu m line flux increases with increasing L-FUV when L-X <10(30) erg s(-1) and with increasing X-ray luminosity when L-X > 10(30) erg s(-1). While [C II] 157 mu m is mainly driven by L-FUV via C+ production, X-rays affect the line flux to a lesser extent. In addition, [Ne II] 12.8 mu m correlates with X-rays; the line profile emitted from the disk atmosphere shows a double-peaked component caused by emission in the static disk atmosphere, next to a high-velocity double-peaked component caused by emission in the very inner rim. Water transitions, depending on the disk region they arise from, show different slopes in correlation with the [O I] 63 mu m line.