Astronomy and Astrophysics, volume 464, 615-623 (2007/3-3)
Chemistry in disks. I. Deep search for N2H+ in the protoplanetary disks around LkCa 15, MWC 480, and DM Tauri.
DUTREY A., HENNING T., GUILLOTEAU S., SEMENOV D., PIETU V., SCHREYER K., BACMANN A., LAUNHARDT R., PETY J. and GUETH F.
Abstract (from CDS):
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.
stars: pre-main sequence - planetary systems: protoplanetary disks - submillimeter
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