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

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 �� × 3 �� (800 × 500 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 highmass 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.