SIMBAD references

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 (YSOs). Six nearby YSOs, 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 interferometer observations in combination with large beam observations, we construct a detailed portrait of the structure, excitation, and abundances in each of the target sources, and explore patterns in the development of low-intermediate mass star forming regions.

We observe an overall pattern of gas phase depletions in each of the sources, consistent with scenarios of early star formation being characterized by a `freezing-out' of molecules onto grain surfaces. Later, as the region is heated up by radiation and energetic activity, these materials are liberated and most gas phase abundances return to quiescent values. The relative severity of the observed depletions in the surrounding condensation as compared to denser, more compact scales may be used as a crude estimate of the relative ages of embedded 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 rough estimate of the onset time and the nature of the activity. We do not see a strong correlation between source luminosity and the magnitude of enhancements in these shock/energetic tracers on small scales; thus, even low-luminosity sources are capable of radically altering the composition of their local environments.

Overall, the effects of molecular depletions and shock chemistry can be used to understand the puzzling variations seen in the emission distributions toward many star forming regions, particularly in molecules thought to be tracing similar excitation conditions.

Observations of millimeter continuum emission reveal massive amounts of material distributed on scales of ≤ 5000 AU; the exact nature of these structures is unclear but their properties are inconsistent with either circumstellar disks or free-falling envelopes. Centimeter continuum emission toward young stellar objects is often found to be variable and non-thermal in nature; more study is required but such observations may provide insights into mass loss phenomena and the magnetic characteristics of young stellar objects.