2007A&A...464..615D

To constrain the ionization fraction in protoplanetary disks, we present new high-sensitivity interferometric observations of N₂H⁺ in three disks surrounding DM Tau, LkCa 15, and MWC 480. We used the IRAM PdBI array to observe the N₂H⁺ J=1-0 line and applied a χ²-minimization technique to estimate corresponding column densities. These values are compared, together with HCO⁺ column densities, to results of a steady-state disk model with a vertical temperature gradient coupled to gas-grain chemistry. We report two N₂ H⁺ detections for LkCa 15 and DM Tau at the 5 σ level and an upper limit for MWC 480. The column density derived from the data for LkCa 15 is much lower than previously reported. The [N₂H⁺/HCO⁺] ratio is on the order of 0.02-0.03. So far, HCO⁺ remains the most abundant observed molecular ion in disks. All the observed values generally agree with the modelled column densities of disks at an evolutionary stage of a few million years (within the uncertainty limits), but the radial distribution of the molecules is not reproduced well. The low inferred concentration of N₂H⁺ in three disks around low-mass and intermediate-mass young stars implies that this ion is not a sensitive tracer of the overall disk ionization fraction.