Letter to the Editor

High NH₂D/NH₃ ratios in protostellar cores

Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany

Corresponding author: hatchell@mpifr-bonn.mpg.de

Received: 20 June 2002
Accepted: 1 March 2003

Abstract

Observations of low mass protostars which probe small enough size scales to be within likely CO depletion regions show the highest [ NH₂D] /[ NH₃] ratios yet measured, of 4–33%. These molecular D/H ratios are higher than those measured on larger scales, showing that deuterium fractionation increases towards protostellar cores. As in cold clouds, such high ratios can be produced by gas-phase ion-molecule chemistry in the presence of depletion. Grain surface chemistry is less likely to explain the deuterium enhancement, as it would require higher fractionation in ices than current models predict. The link between accretion, depletion and high molecular deuterium fractionation is strongly supported.