1997A&A...318..931T

The HII/molecular cloud complex W3 has been imaged at 4.9GHz, 14.9GHz and 22.5GHz in the radio continuum and the H66α radio recombination line with subarcsecond angular resolution using combined multiple VLA configurations. Several hypercompact continuum sources with diameters ≤1000AU have been detected toward IRS 4 and IRS 5, reminiscent of the ``Orion Radio Zoo''. They have been imaged with a maximum angular resolution of 0.1". From the flux densities at 1.3cm, 2cm and 6cm we determine their spectral indices. The compact and ultracompact HII regions with diameters <20,000AU exhibit spectral indices α in the range -0.1 to <1.5. The gradients in spectral indices across these regions correspond to asymmetries in their radio continuum intensities as well as gradients in the densities of the surrounding molecular gas. This indicates gradients in the electron density. From a direct comparison of the continuum emission of the ionized hydrogen gas with the emission of the dense molecular gas and dust continuum, we refine the analyses of the interaction of the radio components with the molecular gas. From our H66α recombination line data we compare the radial velocities of the HII regions with those found for the molecular gas toward W3. We find linewidth and velocity gradients in the ionized gas which are indicative of expansion and turbulent flows caused by pressure gradients in the ambient neutral gas. We propose that the observed morphologies of compact and ultracompact HII regions are determined by turbulent expansion of the ionized gas into highly anisotropic and clumpy molecular gas. Thus, we believe there can be no definite prediction for the morphologies observed in HII regions with current kinematic models without considering these inhomogeneities. Furthermore, we propose that the spatial and kinematic relation of the compact, ultracompact and hypercompact radio continuum regions toward W3 is indicative of sequentially triggered star formation caused by the pressure of the expanding HII regions and the subsequent compression of the molecular gas.