DETECTION OF N2D+ IN A PROTOPLANETARY DISK

Observations of deuterium fractionation in the solar system, and in interstellar and circumstellar material, are commonly used to constrain the formation environment of volatiles. Toward protoplanetary disks, this approach has been limited by the small number of detected deuterated molecules, i.e., DCO+ and DCN. Based on ALMA Cycle 2 observations toward the disk around the T Tauri star AS 209, we report the first detection of N2D+ (J = 3-2) in a protoplanetary disk. These data are used together with previous Submillimeter Array observations of N2H+ (J = 3-2) to estimate a disk-averaged D/H ratio of 0.3-0.5, an order of magnitude higher than disk-averaged ratios previously derived for DCN/HCN and DCO+/HCO+ around other young stars. The high fractionation in N2H+ is consistent with model predictions. The presence of abundant N2D+ toward AS 209 also suggests that N2D+ and the N2D+/N2H+ ratio can be developed into effective probes of deuterium chemistry, kinematics, and ionization processes outside the CO snow line of disks.