Age spread in W3 Main: LBT/LUCI near-infrared spectroscopy of the
  massive stellar content
  We present near-infrared multi-object spectroscopy and JHKs imaging of the
massive stellar content of the Galactic star-forming region W3 Main, obtained
with LUCI at the Large Binocular Telescope. We confirm 15 OB stars in W3 Main
and derive spectral types between O5V and B4V from their absorption line
spectra. Three massive Young Stellar Objects are identified by their emission
line spectra and near-infrared excess. The color-color diagram of the detected
sources allows a detailed investigation of the slope of the near-infrared
extinction law towards W3 Main. Analysis of the Hertzsprung Russell diagram
suggests that the Nishiyama extinction law fits the stellar population of W3
Main best (E(J-H)/E(H-Ks) = 1.76 and R_(Ks) = 1.44). 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 derive a stellar mass of W3 Main of (4 +- 1)
10^3 Msun, by extrapolating from the number of OB stars using a Kroupa IMF and
correcting for our spectroscopic incompleteness. We have detected the
photospheres of OB stars from the more evolved diffuse HII region to the much
younger UCHII regions, suggesting that these stars have finished their
formation and cleared away their circumstellar disks very fast. Only in the
hyper-compact HII region (IRS5), the early type stars seem to be still
surrounded by circumstellar material.
