Chemistry in disks I - Deep search for N$_2$H$^+$ in the protoplanetary disks around LkCa 15, MWC 480, and DM Tau - CNRS - Centre national de la recherche scientifique Accéder directement au contenu
Article Dans Une Revue Astronomy and Astrophysics - A&A Année : 2007

Chemistry in disks I - Deep search for N$_2$H$^+$ in the protoplanetary disks around LkCa 15, MWC 480, and DM Tau

Résumé

Aims: To constrain the ionization fraction in protoplanetary disks, we present new high-sensitivity interferometric observations of N$_2$H$^+$ in three disks surrounding DM Tau, LkCa 15, and MWC 480. Methods: We used the IRAM PdBI array to observe the N$_2$H$^+$ J=1-0 line and applied a $\chi^2$-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 \ndhp detections for LkCa 15 and DM Tau at the $5 \sigma$ 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 [N$_2$H$^+$/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 N$_2$H$^+$ 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.

Dates et versions

hal-00128299 , version 1 (31-01-2007)

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Anne Dutrey, Thomas Henning, S. Guilloteau, Dmitry Semenov, Vincent Piétu, et al.. Chemistry in disks I - Deep search for N$_2$H$^+$ in the protoplanetary disks around LkCa 15, MWC 480, and DM Tau. Astronomy and Astrophysics - A&A, 2007, 464 (2), pp.615-623. ⟨hal-00128299⟩

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