A&A 488, 959-968 (2008)
DOI: 10.1051/0004-6361:200809673

An interferometric study of the low-mass protostar IRAS 16293-2422: small scale organic chemistry

S. E. Bisschop¹, J. K. Jørgensen², T. L. Bourke³, S. Bottinelli¹, and E. F. van Dishoeck<sup>1, 4

1</sup>  Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
    e-mail: bisschop@mpifr-bonn.mpg.de
²  Argelander-Institut für Astronomie, University of Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
³  Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
⁴  Max-Planck-Institut für Extraterrestische Physik, Giessenbachstrasse 1, 85748 Garching, Germany

Received 28 February 2008 / Accepted 9 July 2008

Abstract
Aims. We investigate the chemical relations between complex organics based on their spatial distributions and excitation conditions in the low-mass young stellar objects IRAS 16293-2422 “A” and  “B”.
Methods. Interferometric observations with the Submillimeter Array have been performed at 5   (  AU) resolution revealing emission lines of HNCO, CH₃CN, CH₂CO, CH₃CHO and C₂H₅OH. Rotational temperatures are determined from rotational diagrams when a sufficient number of lines are detected.
Results. Compact emission is detected for all species studied here. For HNCO and CH₃CN it mostly arises from source “A”, CH₂CO and C₂H₅OH have comparable strength for both sources and CH₃CHO arises exclusively from source “B”. HNCO, CH₃CN and CH₃CHO have rotational temperatures >200 K implying that they arise from hot gas. The (u,v)-visibility data reveal that HNCO also has extended cold emission, which could not be previously determined through single dish data.
Conclusions. The relative abundances of the molecules studied here are very similar within factors of a few to those found in high-mass YSOs. This illustrates that the chemistry between high- and low-mass objects appears to be relatively similar and thus independent of luminosity and cloud mass. In contrast, bigger abundance differences are seen between the “A” and “B” source. For instance, the HNCO abundance relative to CH₃OH is ~4 times higher toward “A”, which may be due to a higher initial OCN^- ice abundances in source “A” compared to  “B”. Furthermore, not all oxygen-bearing species are co-existent, with CH₃CHO/CH₃OH an order of magnitude higher toward “B” than “A”. The different spatial behavior of CH₂CO and C₂H₅OH compared with CH₃CHO suggests that successive hydrogenation reactions on grain-surfaces are not sufficient to explain the observed gas phase abundance of the latter. Selective destruction of CH₃CHO may result in the anti-coincidence of these species in source “A”. These results illustrate the power of interferometric compared with single dish data in terms of testing chemical models.

Key words: astrochemistry -- line: identification -- methods: observational -- techniques: interferometric -- stars: formation

© ESO 2008

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