Previous
abstract 
Next
abstract
\def\arcsec {\hbox{$^{\prime\prime}$}} \def\HII {\hbox{H~II}} \def\CEIO {\hbox{${\rm C}^{18}{\rm O}$}} %C18O \def\CTHFOS {\hbox{${\rm C}^{34}{\rm S}$}} % C34S
Many different models have been presented to explain the different observed morphology of compact, ultra-compact and hyper-compact \HII\ regions. The only definite answer that can be given today is that the young stars that excite these \HII\ regions are born within the densest and most compact clumps of molecular clouds. We present sub-arcsecond resolution images using multifrequency, multiconfiguration observations of the Very Large Array of the W3 \HII/molecular cloud complex. These high resolution images reveal a whole cluster of hyper-compact \HII\ regions with angular sizes $<$ 1\arcsec, embedded within the molecular cloud. One of these has been associated with IRS~5, a likely candidate for a ``proto-star''. Others seem to be associated with strong sub-mm peaks, that had previously been thought to be ``radio-quiet''. Ultra-compact \HII\ regions with angular diameters $<$ 10\arcsec\ are also found embedded within the molecular cloud, although they seem to group around its edges. For most of these regions, the infrared sources that are most likely to excite them have been identified. The compact and diffuse \HII\ regions are found at the very edge of the molecular cloud. These regions are the most expanded and probably cleared away the remaining molecular gas. Towards the most diffuse \HII\ regions, over an arcminute away from the densest molecular core region, the exciting star can be observed in the visible now. Although the debate about the evolution and morphology of \HII\ regions is still ongoing, W3 is a unique region, that shows \HII\ regions of each type classified. Studies of this \HII/molecular cloud complex can help us understand the process of star-formation in its very early stage.
