Chemistry in disks. I. Deep search for N2H+ in the protoplanetary disks around LkCa 15, MWC 480, and DM Tauri

Aims: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.
Methods: We used the IRAM PdBI array to observe the N2H⁺ J=1-0 line and applied a χ^2-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.
Results: We report two N2H⁺ 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.
Conclusions: .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.

Based on observations carried out with the IRAM Plateau de Bure Interferometer. IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain). Research partially supported by PCMI, the French national program for the Physics and Chemistry of the Interstellar Medium.