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Much attention has been placed on the dust distribution in protostellarenvelopes, but there are still many unanswered questions regarding thestructure of the gas. We aim to start identifying the factors that determinethe chemical structure of protostellar regions, by studying and comparinglow-mass embedded systems in key molecular tracers. The cold and warm chemicalstructures of two embedded Class 0 systems, IRAS16293 and VLA1623 arecharacterized through interferometric observations. DCO+, N2H+ and N2D+ areused to trace the spatial distribution and physics of the cold regions of theenvelope, while c-C3H2 and C2H from models of the chemistry are expected totrace the warm (UV-irradiated) regions. Both sources show a number of strikingsimilarities and differences. DCO+ consistently traces the cold material at thedisk-envelope interface, where gas and dust temperatures are lowered due todisk shadowing. N2H+ and N2D+, also tracing cold gas, show low abundancestowards VLA1623, but for IRAS16293, the distribution of N2D+ is consistent withthe same chemical models that reproduce DCO+. c-C3H2 and C2H show differentspatial distributions for the two systems. For IRAS16293, c-C3H2 traces theoutflow cavity wall, while C2H is found in the envelope material but not theoutflow cavity wall. In contrast, toward VLA1623 both molecules trace theoutflow cavity wall. Finally, hot core molecules are abundantly observed towardIRAS16293 but not toward VLA1623. We identify temperature as one of the keyfactors in determining the chemical structure of protostars as seen in gaseousmolecules. More luminous protostars, such as IRAS16293, will have chemicalcomplexity out to larger distances than colder protostars, such as VLA1623.Additionally, disks in the embedded phase have a crucial role in controllingboth the gas and dust temperature of the envelope, and consequently thechemical structure.

Tracing the cold and warm physico-chemical structure of deeply embedded protostars: IRAS 16293-2422 versus VLA 1623-2417