Detection of H2D+:: Measuring the midplane degree of ionization in the disks of DM Tauri and TW Hydrae

We report the first detection of the ground transition of the ortho-H2D+ molecule toward one disk source, DM Tau, a tentative detection toward TW Hya, and an upper limit toward LkCa 15. The three observed sources possess young gas-rich disks with large CO depletion factors. We argue that the observed ground ortho-H2D+ line originates in the outer disk midplane gas, which is cold and depleted of CO. Since H2D+ is likely the most abundant ion, the measured line intensity allows to estimate the ionization degree in the midplane gas of the outer disk. In DM Tau the electron abundance is 7 x 10(-10), in TW Hya it is about 4 x 10(-10), and in LkCa 15 it is less than about 2 x This implies that the ionization in the disks is large enough for the magnetorotational instability to operate. The required ionization rate, 5 x 10(-17) s(-1), in TW Hya is consistent, within the associated uncertainty, with normal cosmic-ray irradiation, whereas in DM Tau it is a factor of 10 lower than the canonical value.