From the Convergence of Filaments to Disk-Outflow Accretion:
  Massive-Star Formation in W33A
  Interferometric observations of the W33A massive star-formation region,
performed with the Submillimeter Array (SMA) and the Very Large Array (VLA) at
resolutions from 5 arcsec (0.1 pc) to 0.5 arcsec (0.01 pc) are presented. Our
three main findings are: (1) parsec-scale, filamentary structures of cold
molecular gas are detected. Two filaments at different velocities intersect in
the zone where the star formation is occurring. This is consistent with
triggering of the star-formation activity by the convergence of such filaments,
as predicted by numerical simulations of star formation initiated by converging
flows. (2) The two dusty cores (MM1 and MM2) at the intersection of the
filaments are found to be at different evolutionary stages, and each of them is
resolved into multiple condensations. MM1 and MM2 have markedly different
temperatures, continuum spectral indices, molecular-line spectra, and masses of
both stars and gas. (3) The dynamics of the "hot-core" MM1 indicates the
presence of a rotating disk in its center (MM1-Main) around a faint free-free
source. The stellar mass is estimated to be approximately 10 Msun. A massive
molecular outflow is observed along the rotation axis of the disk.
