Unveiling the MRI through ionisation fraction of protoplanetary disks using ionisation tracers
The diversity of planetary systems depends on the physical and chemical structure of their formation sites, protoplanetary disks. It is generally accepted that the disk physical structure depends upon the degree of turbulent viscosity present in the disk. Turbulence transports angular momentum (and mass) and influences gas dispersion and dust coagulation. The magneto-rotational instability (MRI) has been suggested as the most promising source of turbulence. MHD simulations suggest that the MRI becomes active where the ionization fraction is higher than approximately 10^-13. In the disk molecular layer and midplane, HCO+ is expected to be the dominant cation and thus a robust tracer of the ionisation fraction. We have observed the J=1-0 transition of HCO+ and H13CO+ in three Herbig Ae stars with the Nobeyama 45m telescope to constrain the ionization fraction in the dense, cold regions of the disk, where UV photons cannot penetrate. We aim to determine whether the MRI is a viable mechanism for generating the required turbulence for efficient angular momentum transport and identify MRI active regions from the observational results.