Chemistry in disks^*

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

¹ L3AB, Observatoire de Bordeaux, 2 rue de l'Observatoire, BP 89, 33270 Floirac, France e-mail: anne.dutrey@obs.u-bordeaux1.fr
² Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
³ IRAM, 300 rue de la Piscine, 38406 Saint-Martin-d'Hères, France
⁴ Astrophysikalisches Institut und Universitäts-Sternwarte, Schillergässchen 2-3, 07745 Jena, Germany

Received: 7 April 2006
Accepted: 21 November 2006

Abstract

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 line and applied a -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 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.