Understanding Accretion Outbursts in Massive Protostars through Maser
  Imaging
  The bright maser emission produced by several molecular species at centimeter
to long millimeter wavelengths provides an essential tool for understanding the
process of massive star formation. Unimpeded by the high dust optical depths
that affect shorter wavelength observations, the high brightness temperature of
these emission lines offers a way to resolve accretion and outflow motions down
to scales below $\sim$1 au in deeply embedded Galactic star-forming regions at
kiloparsec distances. The recent identification of extraordinary accretion
outbursts in two high-mass protostars, both of which were heralded by maser
flares, has rapidly impacted the traditional view of massive protostellar
evolution, leading to new hydrodynamic simulations that can produce such
episodic outbursts. In order to understand how these massive protostars evolve
in response to such events, larger, more sensitive ground-based centimeter
wavelength interferometers are needed that can simultaneously image multiple
maser species in the molecular gas along with faint continuum from the central
ionized gas. Fiducial observations of a large sample of massive protostars will
be essential in order to pinpoint the progenitors of future accretion
outbursts, and to quantify the outburst-induced changes in their protostellar
photospheres and outflow and accretion structures. Knowledge gained from these
studies will have broader impact on the general topic of accretion onto massive
objects.
