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We present results of a study aimed at identifying the physical and chemical processes which shape the appearance, chemistry and evolution of gas associated with and participating in the formation of young stellar objects. Six nearby young stellar objects (Orion-S\&IRAS 05338-0624 (Orion, d$\sim$500 pc),S68FIRS1\&S68N (Serpens, d$\sim$310 pc), and NGC 1333 IRAS 4A\&IRAS 4B (Perseus, d$\sim$350 pc)), spanning a range in luminosity from 20 to several thousand solar luminosities, have been examined in species selected to highlight the structure and chemical activity on scales of a few hundred to a few thousand AUs. Using BIMA observations in combination with large beam observations, we construct a detailed portrait of the structure, excitation, and abundance patterns in each of the target sources, and explore patterns in the development of low-intermediate mass star forming regions.
We identify several key species (e.g. SO, SO$_2$, SiO, CH$_3$OH) which serve as dyes of differing levels of energetic activity and whose abundance ratios provide a crude estimate of the onset time of the activity. Broad abundance comparisons between sources with various levels of chemical processing yield patterns of molecular abundances which may be used to characterize the relative youth of the region. We find strong evidence for the depletion of common tracer molecules (such as HCN and CS), consistent with the earliest stages of stellar evolution being marked by a freezing out of molecules onto grain surfaces. Later as the object interacts with its surrounding, this material is released and gas phase enhancements occur, as seen in more processed sources such as Orion IRc~2.
