Detection of H 2 D + : Measuring the Midplane Degree of Ionization in the Disks of DM Tauri and TW Hydrae

Abstract

We report the first detection of the ground transition of the ortho-H 2 D 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-H 2 D line originates in the outer disk midplane gas, which is cold and depleted of CO. Since H 2 D 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 , in TW Hya it is about , and in LkCa 15 it is less than about 10 10 7 # 10 4 # 10 2 #. This implies that the ionization in the disks is large enough for the magnetorotational instability to operate. 9 10 The required ionization rate, s 1 , in TW Hya is consistent, within the associated uncertainty, with normal 17 5 # 10 cosmic-ray irradiation, whereas in DM Tau it is a factor of 10 lower than the canonical value.