摘要
Context. Much attention has been placed on the dust distribution in protostellar envelopes, but there are still many unanswered questions regarding the physico-chemical structure of the gas. Aims. Our aim is to start identifying the factors that determine the chemical structure of protostellar regions, by studying and comparing low-mass embedded systems in key molecular tracers. Methods. The cold and warm chemical structures of two embedded Class 0 systems, IRAS 16293-2422 and VLA 1623-2417 were characterized through interferometric observations. DCO + , N 2 H + , and N 2 D + were used to trace the spatial distribution and physics of the cold regions of the envelope, while c-C 3 H 2 and C 2 H from models of the chemistry are expected to trace the warm (UV-irradiated) regions. Results. The two sources show a number of striking similarities and differences. DCO + consistently traces the cold material at the disk-envelope interface, where gas and dust temperatures are lowered due to disk shadowing. N 2 H + and N 2 D + , also tracing cold gas, show low abundances toward VLA 1623-2417, but for IRAS 16293-2422, the distribution of N 2 D + is consistent with the same chemical models that reproduce DCO + . The two systems show different spatial distributions c-C 3 H 2 and C 2 H. For IRAS 16293-2422, c-C 3 H 2 traces the outflow cavity wall, while C 2 H is found in the envelope material but not the outflow cavity wall. In contrast, toward VLA 1623-2417 both molecules trace the outflow cavity wall. Finally, hot core molecules are abundantly observed toward IRAS 16293-2422 but not toward VLA 1623-2417. Conclusions. We identify temperature as one of the key factors in determining the chemical structure of protostars as seen in gaseous molecules. More luminous protostars, such as IRAS 16293-2422, will have chemical complexity out to larger distances than colder protostars, such as VLA 1623-2417. Additionally, disks in the embedded phase have a crucial role in controlling both the gas and dust temperature of the envelope, and consequently the chemical structure.