Because of the great number of dense cores in these molecular clouds, and the number of spectral lines that can be mapped, this is a program which will keep a large mm array busy for decades. A large new mm array will revolutionize studies of star formation. It will allow us to better trace the chemistry of star-forming regions, to detect and map protoplanetary disks, and to study the kinematics and dynamics of bipolar flows from young stellar objects. Molecular line profiles can indicate the infall, rotation, and outflows accompanying the birth of stars. Intensity ratios of lines from different levels of the same molecule can tell us the density and temperature in the circumstellar gas.
Molecular abundances are diagnostics of the evolutionary stage of a young stellar object.
Once the young star has formed, radiation heats the gas and evaporates the icy grain mantles. Shocks in the outflows can also return the mantles' refractory elements like silicon to the gas phase. It will be exciting to extend current research to the 0.1 scale and to weak lines in protoplanetary disks around young massive stars like MWC 349 as well as to stars of lower mass, and to study the chemistry of protosolar nebulae. Recent detections of the thermal emission of dust in the disks around HL Tau and L1151 IRS 5 show beams are needed to resolve the protostellar accretion disks, and obviously is needed to derive functions like temperature vs. radius. A large mm array would give us the ability to study these objects anywhere in the Galaxy with better linear resolution than we now have for objects within 500pc of the Sun.