High resolution magnetic field measurements in high-mass star-forming regions

A number of different formation scenarios have been proposed to explain the formation of stars with masses larger than about 8 M⊙. These include formation through the merger of less massive stars (Coalescence model) or through the accretion of unbound gas from the molecular cloud (Competitive accretion model). In the third scenario, Core accretion model, massive stars form through gravitational collapse, which involves disc-assisted accretion to overcome radiation pressure. This scenario is similar to the favored picture of low-mass star formation, in which magnetic fields are thought to slow the collapse, to transfer the angular momentum and to power the bipolar outflows. Moreover, during the formation of low-mass stars the gravitational collapse of molecular clouds proceeds preferentially along the magnetic field lines, giving rise to large rotating disc or torus structures orthogonal to the magnetic field. Consequently, the molecular bipolar outflows, which originate from the protostar, are driven parallel to the magnetic field. Although the magnetic fields play such an important and crucial role in the formation of low-mass stars, its role in the formation of high-mass stars is still under debate. The debate is due to the several unanswered questions about the role of the magnetic fields in high-mass star formation. The difficulties in answering these questions is due to their fast evolution (of order of hundreds thousands years). As a result of this, the massive star-forming regions are rare and typically found at fairly large distance. Moreover the massive star-forming regions consist of a large number of protostars that make the identification of individual massive protostar very complex. Consequently it is very difficult to observe and measure the magnetic fields during the protostellar phase of high-mass stars. Current observations of magnetic fields in massive star-forming regions are often limited to low density regions and/or envelopes at scales of several thousands astronomical units. Linear polarization observations of dust also only provide information on the magnetic field in the plane of the sky, so these observations have been yet unable to probe the strength and the full structure of the magnetic field close to the protostars