7 Conclusions

The continuum emission radiated by dust grains in the circumstellar environment of recently born stars can be used, when supplemented by a detailed radiative transfer analysis, to derive the physical properties of such dusty envelopes. Observational constraints in the form of both the SED and resolved radial brightness distributions are required for a successful model. For the protostellar object IRAS 16293-2422 it is possible to model the observational data using a single power-law distribution for the density structure. However, the molecular line profiles suggest that parts of the envelope are in a state of collapse. A dynamical age of $\sim$1- $3\times10^4$ yr is derived with an annual mass accretion rate of roughly $4\times10^{-5}~M_{\odot}$ yr^-1.

Once the physical structure of IRAS 16293-2422 is known, it is possible to determine its chemical properties through detailed radiative transfer modelling of the observed molecular line emission. The abundances and quality of the fits for molecules like CO and CS further strengthen the adopted physical model. While the emission from some molecules is well reproduced assuming a constant fractional abundance throughout the envelope, other species require a steep abundance gradient to be introduced at typically 90 K. The presence of such a jump for molecules like H₂CO and CH₃OH is interpreted as evidence of thermal evaporation of ices in the inner dense and hot regions of the envelope. Other molecules like HC₃N, CH₃CN and several sulfur-bearing molecules also require such drastic jumps in their abundance distributions. In high mass protostars, these molecules are typically cited as evidence of a rapid gas-phase chemistry initiated by the evaporation of the ices. The "hot core'' region in IRAS 16293-2422 is, however, very small, only $\sim$150 AU in radius and comparable to the size of the circumstellar disk(s). This large change in physical size and the organized velocity field surrounding low mass protostars lead to chemical time scales that only a small fraction of the dynamical age. Thus, it may be difficult for a full "hot core'' chemistry leading to complex species such as CH₃OCH₃ to develop in class 0 objects, even though the appropriate physical conditions are present. Alternative scenarios in which ices are liberated by grain-grain collisions in turbulent shear zones associated with the outflows need to be tested by higher angular resolution observations.

Whatever their precise origin, the molecules located in the inner envelope of IRAS 16293-2422 can be incorporated into the growing circumstellar disk(s) and become part of the material from which planetary bodies are formed. The molecular abundances derived here should provide an accurate reference point for comparison with the growing amount of data on protoplanetary disks and icy solar system objects such as comets.

Acknowledgements

The authors are grateful to R. Stark and S. Doty for useful discussions. The referee J. Hatchell is thanked for comments that helped to improve the paper. This research was supported by the Netherlands Organization for Scientific Research (NWO) grant 614.041.004, the Netherlands Research School for Astronomy (NOVA) and a NWO Spinoza grant. This article made use of data obtained through the JCMT archive as Guest User at the Canadian Astronomy Data Center, which is operated by the Dominion Astrophysical Observatory for the National Research Council of Canada's Herzberg Institute of Astrophysics.