First detection of ND in the solar-mass protostar IRAS16293-2422 [Letter]

Context. In the past decade, much progress has been made in characterising the processes leading to the enhanced deuteriumfractionation observed in the ISM and in particular in the cold, dense parts of star forming regions such as protostellar envelopes. Very high molecular D/H ratios have been found for saturated molecules and ions. However, little is known about the deuteriumfractionation in radicals, even though simple radicals often represent an intermediate stage in the formation of more complex, saturated molecules. The imidogenradical NH is such an intermediate species for the ammonia synthesis in the gas phase. Many of these light molecules however have their fundamental transitions in the submillimetre domain and their detection is hampered by the opacity of the atmosphere at these wavelengths. Herschel/HIFI represents a unique opportunity to study the deuteration and formation mechanisms of species not observable from the ground. Aims. We searched here for the deuterated radical ND in order to determine the deuteriumfractionation of imidogenand constrain the deuteration mechanism of this species. Methods. We observed the solar-massClass 0 protostarIRAS16293-2422 with the heterodyne instrument HIFI in Bands 1a (480–560 GHz), 3b (858–961 GHz), and 4a (949–1061 GHz) as part of the Herschel key programme CHESS (Chemical HErschel Survey of Star forming regions). Results. The deuterated form of the imidogenradical ND was detected and securely identified with 2 hyperfine component groups of its fundamental transition (N = 0–1) at 522.1 and 546.2 GHz, in absorption against the continuum background emitted from the nascent protostar. The 3 groups of hyperfine components of its hydrogenated counterpart NH were also detected in absorption. The absorption arises from the cold envelope, where many deuterated species have been shown to be abundant. The estimated column densities are ~2 × 1014 cm-2 for NH and ~ 1.3 × 1014 cm-2 for ND. We derive a very high deuteriumfractionation with an [ND]/[NH] ratio of between 30 and 100%. Conclusions. The deuteriumfractionation of imidogenis of the same order of magnitude as that in other molecules, which suggests that an efficient deuteriumfractionation mechanism is at play. We discuss two possible formation pathways for ND, by means of either the reaction of N+ with HD, or deuteron/proton exchange with NH.