Chemistry in disks. IX. Observations and modelling of HCO+ and DCO+ in DM Tauri

Aims: We study the deuteration and ionization structure of the DM Tau disk via interferometric observations and modelling of the key molecular ions, HCO+ and DCO+. Methods: The Plateau de Bure Array is used to observe DM Tau in lines of HCO+ (1-0), (3-2) and DCO+ (3-2) with a ~ 1.5'' angular and ~0.2 km s-1 spectral resolution. Using a power-law fitting approach the observed column densities profiles are derived and thus the isotopic ratio RD = DCO+/HCO+. Chemical modelling allowed an exploration of the sensitivity of HCO+ and DCO+ abundances to physical parameters out with temperature. A steady state approximation was employed to observationally constrain the ionization fraction x(e-). Results: Fitting of radiative transfer models suggests that there is a chemical hole in HCO+ and DCO+, extending up to 50 AU from the star. More work is required to discern the cause of this. The observed column densities of HCO+ and DCO+ at 100 AU were (9.8+0.3-0.7) × 1012 and (1.2 ± 0.7) × 1012 cm-2 respectively. Where both HCO+ and DCO+ were present, RD was found to increase radially from 0.1 at 50 AU to 0.2 at 450 AU. This behaviour was well reproduced by the chemical model. The X-ray luminosity of the central star, the interstellar UV and CO depletion were found to be the most important physical parameters controlling the abundances of HCO+ and DCO+. Differences in the vertical extent of HCO+ and DCO+ molecular layers resulted in different responses to changing physical parameters, manifesting as radial gradients in RD. The ionization fraction was found to be x(e-) ~ 10-7 in the molecular layer, comparable to the disk averaged value. Modelling shows that while HCO+ is the most dominant charged molecular ion in our disk model, atomic ions, such as C+, S+, H+, Na+ and Mg+, dominate the charge in both the molecular layer and disk atmosphere. Conclusions: A high value of RD is indicative of continued deuterium fractionation in a protoplanetary disk after pre/protostellar phases. Radial properties of RD can be employed to discern the importance of ionization from X-rays and UV, thus necessitating the need for more, high resolution observations of DCO+ and other deuterated species in disks. A steady-state approach commonly adopted for constraining ionization degree in prestellar cores is not applicable for disks where accurate determination of the ionization fraction in the molecular layer requires knowledge of the atomic ions present as molecular ions are relatively sparse.Based on observations carried out with the IRAM Plateau de Bure Interferometer. IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain)