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PROSAC: A Submillimeter Array Survey of Low-Mass Protostars. I. Overview of Program: Envelopes, Disks, Outflows and Hot Cores

This paper presents a large spectral line and continuum survey of 8 deeply embedded, low-mass protostellar cores using the Submillimeter Array. Each source was observed in high excitation lines of some of the most common molecular species, CO, HCO+, CS, SO, H2CO, CH3OH and SiO. Line emission from 11 species originating from warm and dense gas have been imaged at high angular resolution (1-3"; typically 200-600 AU) together with continuum emission at 230 GHz (1.3 mm) and 345 GHz (0.8 mm). Compact continuum emission is observed for all sources which likely originates in marginally optically thick circumstellar disks, with typical lower limits to their masses of 0.1 M_sun (1-10% of the masses of their envelopes) and having a dust opacity law with beta approximately 1. Prominent outflows are present in CO 2-1 observations in all sources: the most diffuse outflows are found in the sources with the lowest ratios of disk-to-envelope mass, and it is suggested that these sources are in a phase where accretion of matter from the envelope has almost finished and the remainder of the envelope material is being dispersed by the outflows. Other characteristic dynamical signatures are found with inverse P Cygni profiles indicative of infalling motions seen in the 13CO 2-1 lines toward NGC1333-IRAS4A and -IRAS4B. Outflow-induced shocks are present on all scales in the protostellar environments and are most clearly traced by the emission of CH3OH in NGC1333-IRAS4A and -IRAS4B. These observations suggest that the emission of CH3OH and H2CO from these proposed "hot corinos" are related to the shocks caused by the protostellar outflows. Only one source, NGC1333-IRAS2A, has evidence for hot, compact CH3OH emission coincident with the embedded protostar.