Letter to the Editor

Detection of doubly-deuterated methanol in the solar-type protostar IRAS 16293-2422

¹ CESR CNRS-UPS, BP 4346, 31028 Toulouse Cedex 04, France
² Observatoire de Bordeaux, BP 89, 33270 Floirac, France
³ Laboratoire d'Astrophysique, Observatoire de Grenoble, BP 53, 38041 Grenoble Cedex 09, France
⁴ SRON, PO Box 800, 9700 AV Groningen, The Netherlands
⁵ Department of Physics, The Ohio State University, 174 W. 18th Ave. Columbus, OH 43210-1106, USA
⁶ College of Natural Sciences, Dakota State University, Madison, SD 57042-1799, USA
⁷ LERMA & FRE 2460 du CNRS, Observatoire de Paris, 61 Av. de l'Observatoire de Paris, 75014 Paris, France
⁸ Instituto de Astronomía, UNAM, Apdo Postal 72-3 (Xangari), 58089 Morelia, Michoacán, México

Corresponding author: B. Parise, Berengere.Parise@cesr.fr

Received: 28 June 2002
Accepted: 2 August 2002

Abstract

We report the first detection of doubly-deuterated methanol (CHD₂OH), as well as firm detections of the two singly-deuterated isotopomers of methanol (CH₂DOH and CH₃OD), towards the solar-type protostar IRAS 16293-2422. From the present multifrequency observations, we derive the following abundance ratios: , , . The total abundance of the deuterated forms of methanol is greater than that of its normal hydrogenated counterpart in the circumstellar material of IRAS 16293-2422, a circumstance not previously encountered. Formaldehyde, which is thought to be the chemical precursor of methanol, possesses a much lower fraction of deuterated isotopomers (~20%) with respect to the main isotopic form in IRAS 16293-2422. The observed fractionation of methanol and formaldehyde provides a severe challenge to both gas-phase and grain-surface models of deuteration. Two examples of the latter model are roughly in agreement with our observations of CHD₂OH and CH₂DOH if the accreting gas has a large (0.2–0.3) atomic D/H ratio. However, no gas-phase model predicts such a high atomic D/H ratio, and hence some key ingredient seems to be missing.