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¿10^-10, in TW Hya it is about 4¿10^-10, and in LkCa 15 it is less than about 2¿10^-9. This implies that the ionization in the disks is large enough for the magnetorotational instability to operate. The required ionization rate, 5¿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.