Magnetic fields and star formation
The analytical model of grain alignment by radiative torques, proposed by Lazarian and Hoang (2007), showed that the grain alignment with the magnetic field, results from the averaging of the radiative torques and not from paramagnetic relaxation. This model has been instrumental in the advancement of the theory and specific theoretical predictions are relatively consistent with the molecular cloud emission polarimetry and its associated polarization spectrum. Based upon current observational data, radiative alignment theory still remains the most promising theory for explaining interstellar polarization. However, it must be noted that despite its many successes, there are still major problems with the theory, such as the non-alignment of large carbonaceous grains – why do silicates align and carbonaceous grains do not? A reminder that the physical properties of grains and their interaction with the magnetic field are as important as the field itself for grain alignment theories.
Measuring the magnetic field strength
The magnetic field strength in molecular clouds is a critical parameter for star formation studies, but they are very difficult to measure (see Crutcher et al. 2004). Two techniques used extensively to measure the magnetic field strength in molecular clouds are: Zeeman splitting, and more recently dust polarization. The Zeeman effect directly measures the magnetic field strength in molecular clouds. It involves the splitting of two circularly opposing polarized components of the radiation. The component of the magnetic field parallel to the line of sight is proportional to the separation of the opposing polarized components. Observations of Zeeman splitting at radio wavelengths, acting on circularly polarized radiation emitted by neutral atoms (HI) or thermally exited molecules (OH, SiO, H 2 O, CN), provide the strength and direction of the line sight component of the magnetic field. Unfortunately, Zeeman measurements are difficult and sparse for the densest molecular cores, due in part to the low densities of H 2 molecules. While linearly polarized dust emission traces the morphology of the magnetic field projected onto the plane of sky , it does not directly measure the strength . The dust polarization technique indirectly provides an estimate for the magnetic field strength. Their technique is based upon the understanding that small scale random turbulent or MHD-wave motions will cause irregularities in the magnetic field. The stronger regular magnetic fields will naturally offer greater resistance to the random turbulent motions, resulting in a scatter in polarization position angles with respect to those of the regular field. Thus, the strength of the regular magnetic field, in the plane of sky, may be deduced from the magnitude of these irregularities. Crutcher et al. (2004) compared field strength estimates of linearly polarized dust emission of prestellar cores with their respective Zeeman measurements and found them to be in essential agreement. Given the relative lack of Zeeman measurement for the densest molecular clouds, this technique makes an enormous contribution to studies involving the role of magnetic fields in star formation.
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