Observations of NH2D toward dark molecular clouds

The 1(11)(a)-1(01)(s) transition of NH2D at 110.1536 GHz was surveyed toward 16 cores of high ammonia abundance in 14 cold dark clouds and was detected toward eight cloud cores, L1448, B1, L1489N, L1551N, L1641N, L134N(S), rho Oph E, and L63, where L134N and L63 are not associated with infrared sources. The column densities of NH2D were compared with those of the main species NH3, previously determined for the corresponding sources, and the relative abundance, [NH2D]/[NH3], was found to be large, 0.025-0.18, which is among the highest deuterium fractionation so far observed. The relative abundance is remarkably larger than a predicted value by the latest gas-phase chemical model for dark clouds by Howe & Millar. The spectral line of NH2D was not detected in other dark cloud cores. These results suggest that a part of NH3 in dark clouds could originate from dust related reactions associated with energetic events.