Variability in the Assembly of Protostellar Systems
  Understanding the collapse of clouds and the formation of protoplanetary
disks is essential to understanding the formation of stars and planets. Infall
and accretion, the mass-aggregation processes that occur at envelope and disk
scales, drive the dynamical evolution of protostars. While the observations of
protostars at different stages constrain their evolutionary tracks, the impact
of variability due to accretion bursts on dynamical and chemical evolution of
the source is largely unknown. The lasting effects on protostellar envelopes
and disks are tracked through multi-wavelength and time domain observational
campaigns, requiring deep X-ray, infrared, and radio imaging and spectroscopy,
at a sufficient level of spatial detail to distinguish contributions from the
various substructures (i.e., envelope from disk from star from outflow).
Protostellar models derived from these campaigns will illuminate the initial
chemical state of protoplanetary disks during the epoch of giant planet
formation. Insight from individual star formation in the Milky Way is also
necessary to understand star formation rates in extragalactic sources. This
cannot be achieved with ground-based observatories and is not covered by
currently approved instrumentation. Requirements: High (v < 10 km/s for survey;
v < 1 km/s for followup) spectral resolution capabilities with relatively rapid
response times in the IR (3-500 um), X-ray (0.1-10 keV), and radio (cm) are
critical to follow the course of accretion and outflow during an outburst.
Complementary, AU-scale radio observations are needed to probe the disk
accretion zone, and 10 AU-scale to probe chemical and kinematic structures of
the disk-forming regions, and track changes in the dust, ice, and gas within
protostellar envelopes.
