Dissecting high-mass star-forming regions; tracing back their complex
  formation history
  We present near-infrared JHKs imaging as well as K-band multi-object
spectroscopy of the massive stellar content of W3 Main using LUCI at the LBT.
We confirm 13 OB stars by their absorption line spectra in W3 Main and spectral
types between O5V and B4V have been found. Three massive Young Stellar Objects
are identified by their emission line spectra and near-infrared excess. From
our spectrophotometric analysis of the massive stars and the nature of their
surrounding HII regions we derive the evolutionary sequence of W3 Main and we
find evidence of an age spread of at least 2-3 Myr. While the most massive star
(IRS2) is already evolved, indications for high-mass pre--main-sequence
evolution is found for another star (IRS N1), deeply embedded in an ultra
compact HII region, in line with the different evolutionary phases observed in
the corresponding HII regions. We have detected the photospheres of OB stars
from the more evolved diffuse HII region to the much younger UCHII regions,
suggesting that the OB stars have finished their formation and cleared away
their possible circumstellar disks very fast. Only in the hyper-compact HII
region (IRS5), the early type stars are still surrounded by circumstellar
material.
