  Observational signatures of infalling envelopes and outflowing material in
early stages of protostellar evolution, and at small radii from the protostar,
are essential to progress in the understanding of the mass-accretion process in
star formation. In this letter, we report a detailed study of the accretion and
outflow structure around a protostar in the well-known high-mass star-forming
region AFGL 5142. We focus on the mm source MM-1, which exhibits hot-core
chemistry, radio continuum emission, and strong water and methanol masers.
Remarkably, our Very Long Baseline Interferometry (VLBI) observations of
molecular masers over six years provided us with the 3-D velocity field of
circumstellar molecular gas with a resolution of 0.001-0.005 arcseconds and at
radii <0.23 arcseconds (or 400 AU) from the protostar. In particular, our
measurements of methanol maser emission enabled, for the first time, a direct
measurement of infall of a molecular envelope (radius of 300 AU and velocity of
5 km/s) onto an intermediate- to high-mass protostar. We estimate an infall
rate of 0.0006 n_8 Msun/year, where n_8 is the ambient volume density in units
of 10^8 cm-3 (required for maser excitation). In addition, our measurements of
water maser (and radio continuum) emission identify a collimated bipolar
molecular outflow (and ionized jet) from MM-1. The evidence of simultaneous
accretion and outflow at small spatial scales, makes AFGL 5142 an extremely
compelling target for high-angular resolution studies of high-mass star
formation.
