Magnetic fields and star formation
Polarization science
On its journey, starlight interacts with interstellar dust. Part of the light becomes absorbed by the dust grains. Because the grains are asymmetric and elongated, they preferentially absorb light along their long axis. This causes the light to become polarized along the short axis of the grain.
In 1949, the polarization of background star light was independently discovered by Hall and Hiltner. Their findings quickly led to the assertion that this phenomenon resulted from the alignment of the interstellar dust grains with respect to the local magnetic fields. Further, to produce the observed polarization, the interstellar dust grains must be asymmetrical (elongated) and possess a preferential alignment of their short axes with one another, and parallel to the local magnetic field. Observations reveal that the sub-millimetre emission from cold dense cores within molecular clouds are indeed polarized by a few percent.
Grain alignment theories
Despite the significant progress in the years since the pioneering work of Davis & Greenstein (1951), the mechanisms responsible for grain alignment are not yet fully understood. Though recent advancements have made it possible to predict the degrees of polarization, and thus compare theory and numerical models with observation, a comprehensive theory still remains elusive. This situation has far-reaching consequences for star formation theories and interstellar processes, in which the magnetic field is a fundamental parameter, as the polarization resulting from dust emission provides a means to map the magnetic fields and indirectly estimate their strength. There are three broad classes of grain alignment theories: mechanical alignment, paramagnetic relaxation alignment and radiative alignment (Lazarian 2003). Although, it is generally accepted that the dominant processes responsible for alignment rely on the grain rotation interacting with the ambient magnetic field as in paramagnetic and radiative alignment. All paramagnetic relaxation mechanisms are based upon the premise that the thermally spinning grains acquire an induced magnetization and are aligned with their angular momentum parallel to the magnetic field due to the dissipation of their internal magnetization energy. This dissipation decreases the component of the spinning grains ’ rotational velocity perpendicular to the ambient magnetic and, as a result, the spinning grains eventually rotate with their velocity parallel to the ambient field. In recent years radiative alignment torque (RAT) theory has come to the fore as the most promising mechanism to explain dust alignment. Originally proposed by Dolginov & Mytrophanov (1976), the theory considers grains with a helicity (rotation about a well-defined axis in irregular grains) that present different cross sections for left-handed and right-handed photons. Grains are then spun up by the torques imbued by the un-polarized light as it scatters of the grains – thereby building up the grains’ net angular momentum. The induced magnetization causes the grain to precess around the ambient magnetic field and as with paramagnetic alignment, the grain is aligned with its angular momentum aligned parallel to the magnetic field. Unlike paramagnetic alignment, no paramagnetic relaxation is involved.
145
Made with FlippingBook - PDF hosting