Deuterated water in the solar-type protostars NGC 1333 IRAS 4A and IRAS 4B^⋆,⋆⋆

¹ Université de Toulouse, UPS-OMP, IRAP, Toulouse, France
e-mail: acoutens@nbi.dk
² CNRS, IRAP, 9 Av. Colonel Roche, BP 44346, 31028 Toulouse Cedex 4, France
³ Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen Ø., Denmark
⁴ Centre for Star and Planet Formation, Natural History Museum of Denmark, University of Copenhagen, Øster Voldgade 57, 1350 Copenhagen K., Denmark
⁵ LERMA, Observatoire de Paris, UMR 8112 CNRS/INSU, 61 Av. de l’Observatoire, 75014 Paris, France
⁶ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
⁷ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
⁸ Institut de Planétologie et d’Astrophysique de Grenoble (IPAG), UMR 5274, UJF-Grenoble 1/CNRS, 38041 Grenoble, France
⁹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
¹⁰ Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching, Germany
¹¹ California Institute of Technology, Infrared Processing and Analysis Center, Mail Code 100-22, Pasadena, CA 91125, USA
¹² Université de Bordeaux, Laboratoire d’Astrophysique de Bordeaux, 33000 Bordeaux, France
¹³ CNRS/INSU, UMR 5804, BP 89, 33271 Floirac Cedex, France
¹⁴ Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
¹⁵ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
¹⁶ University of Waterloo, Department of Physics and Astronomy, Waterloo, Ontario, Canada
¹⁷ NASA Postdoctoral Program Fellow, NASA Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20770, USA
¹⁸ SRON Netherlands Institute for Space Research, Landleven 12, 9747 AD Groningen, The Netherlands
¹⁹ Kapteyn Astronomical Institute, University of Groningen, 9700 AV Groningen, The Netherlands
²⁰ Department of Astronomy, University of Michigan, 500 Church Street, Ann Arbor, MI 48109-1042, USA

Received: 30 July 2013
Accepted: 21 October 2013

Abstract

Context. The measure of the water deuterium fractionation is a relevant tool for understanding mechanisms of water formation and evolution from the prestellar phase to the formation of planets and comets.

Aims. The aim of this paper is to study deuterated water in the solar-type protostars NGC 1333 IRAS 4A and IRAS 4B, to compare their HDO abundance distributions with other star-forming regions, and to constrain their HDO/H₂O abundance ratios.

Methods. Using the Herschel/HIFI instrument as well as ground-based telescopes, we observed several HDO lines covering a large excitation range (E_up/k = 22–168 K) towards these protostars and an outflow position. Non-local thermal equilibrium radiative transfer codes were then used to determine the HDO abundance profiles in these sources.

Results. The HDO fundamental line profiles show a very broad component, tracing the molecular outflows, in addition to a narrower emission component and a narrow absorbing component. In the protostellar envelope of NGC 1333 IRAS 4A, the HDO inner (T ≥ 100 K) and outer (T < 100 K) abundances with respect to H₂ are estimated with a 3σ uncertainty at 7.5_-3.0^+3.5 × 10^-9 and 1.2_-0.4^+0.4 × 10^-11, respectively, whereas in NGC 1333 IRAS 4B they are 1_-0.9^+1.8 × 10^-8 and 1.2_-0.4^+0.6 × 10^-10, respectively. Similarly to the low-mass protostar IRAS 16293-2422, an absorbing outer layer with an enhanced abundance of deuterated water is required to reproduce the absorbing components seen in the fundamental lines at 465 and 894 GHz in both sources. This water-rich layer is probably extended enough to encompass the two sources, as well as parts of the outflows. In the outflows emanating from NGC 1333 IRAS 4A, the HDO column density is estimated at about (2–4) × 10¹³ cm^-2, leading to an abundance of about (0.7–1.9) × 10^-9. An HDO/H₂O ratio between 7 × 10^-4 and 9 × 10^-2 is also derived in the outflows. In the warm inner regions of these two sources, we estimate the HDO/H₂O ratios at about 1 × 10^-4–4 × 10^-3. This ratio seems higher (a few %) in the cold envelope of IRAS 4A, whose possible origin is discussed in relation to formation processes of HDO and H₂O.

Conclusions. In low-mass protostars, the HDO outer abundances range in a small interval, between ~10^-11 and a few 10^-10. No clear trends are found between the HDO abundance and various source parameters (L_bol, L_smm, L_smm/L_bol, T_bol, L_bol^0.6/M_env). A tentative correlation is observed, however, between the ratio of the inner and outer abundances with the submillimeter luminosity.