The structure and dynamics of an environment forming high-mass stars
Moore, Toby John Terry
A detailed investigation is presented of the physical environment associated with high-mass star formation. This is carried out by means of an in-depth multi-wavelength study of one such region in W75N. An analysis is made of the nature of the major embedded luminosity sources and the cloud core from which they have recently formed,from the hot dust very close to each object to the cold dust and molecular gas surrounding them. The dynamical relationship between the large molecular outflow and the ambient cloud is studied with regard to limitations that might be placed on plausible flow generation mechanisms. The details of the reflection nebulae associated with the mass outflows are interpreted in terms of the scattering properties of dust.A recently-formed stellar cluster has been found, associated with the known signs of higli-mass star formation in W75N (compact HII regions, O H and H2O masers,CO outflow etc.). The main heating source and origin of the molecular outflow is identified and found to be very deeply embedded (Av > 90 mag). Imaging near-infrared polarimetry, millimetre-wave molecular line spectroscopy and submillimetre continuum observations at scales of 0.01 to 0.2 pc have shown no disk or torus structure in the dense obscuring material around this source. However, circumstantial evidence exists in the near-infrared colours for significant amounts of dusty material close to the major luminosity sources. Therefore very small-scale (few X100 A.U.) disk-like formations may exist and play an important dynamical role.The large molecular outflow in W75N is found to be unable to overcome the gravitational binding force of the large surrounding core unless the flow is initially highly collimated. The lobes of high-velocity gas possess a large degree of collimation,probably induced through confinement by the ambient material, but the outflow as a whole is irregular and possibly multipolar and appears to be intrinsically largely isotropic. The flow lobes are not wind-blown bubbles but are filled with high velocity molecular material. The driving mechanism is most likely to be a massive, semi-isotropic stellar wind but there could still be a cylindrically symmetrical, rotation-driven mechanism acting close to the star.Of the two reflection nebulae in the region the largest and brightest is associated with the blue-shifted molecular outflow lobe. Features in the nebula, similar to those in other such objects, are consistent with scattering from large grains which produce diffraction-affected and strongly forward-biased scattering patterns. It is shown that large grains should dominate the scattering in the near-infrared, regardless of the detailed grain size distribution. The ubiquitous suppression of backward-directed reflection lobes in bipolar sources may be caused by forward scattering and not by large obscuring disks. A smaller reflection nebula surrounds the less luminous source IRS-2,indicating a limited-scale outflow from this object also. Hydrogen recombination line ratios in IRS-2 are consistent with current models of massive, partly-ionised stellar winds.