Magnetic fields and star formation
Mapping large scale magnetic fields – testing star formation theories
The relationship between young stars and their relative concentrations within molecular clouds is a strong indication that stars form in the dense cores found in their interiors (Beichman et al, 1986). While the transition from these dense cloud cores or, as they are commonly referred to, pre-stellar cores to protostar is controlled by gravity, the physics governing the processes involved in the formation and evolution of these giant molecular clouds and their associated sub-structures e.g. cores and filaments, are far from understood and are the subject of much debate. Estimates of the giant molecular cloud lifetimes, which typically contain masses (~ 20-80 solar masses), range from one free- fall collapse time τ ff ( ~ 1,000,000 years) – Fukui & Kawamura (2010) estimated that giant molecular clouds in the Large Magellanic Cloud have lifetimes of 20-30 million years. If these clouds have lifetimes much longer than their free fall times, they must be supported from gravitational collapse: the question is by what? The interstellar medium is known to be both turbulent and magnetized, so it comes as no surprise that these two media are fundamental quantities in star formation theories. Early studies of star formation proposed theories in which the magnetic field controls the formation and evolution of the molecular clouds, pre-stellar cores, and the gravitational collapse of these pre- stellar cores into protostars, which eventually develop into main sequence (Tassis & Mouschovias 2004). In these models, the magnet field is strongly coupled ( ‘f lux freezing ’ ) with the ions and electrons present in the media. Through a process known as ‘ ambipolar diffusion ’ , the neutral gas and dust gravitationally contract towards the core centres as they are not directly influenced by the magnetic field, thereby increasing the mass in the cloud cores, while the magnetic field resists compression and remains behind in the cloud envelope. Consequently, clouds cores that are initially stable against gravitational collapse (subcritical) will, driven by ambipolar diffusion, become unstable (supercritical), collapse and form stars. The fact that the ambipolar diffusion timescale (> 10 7 yr) for the formation of supercritical clouds is far longer than their free fall lifetimes provides an explanation for the observed low star formation rates – due to the magnetic field supporting subcritical clouds against gravitational collapse. This is a strong argument for the ambipolar diffusion driven models, because if ambipolar diffusion regulates the star formation rate, the timescales are approximately correct. However, this model has been challenged by the ‘w eak field ’ star formation theory which proposes that the magnetic fields are too weak and that supersonic, solar wind turbulence gas flows are the dominant agents controlling the formation of molecular clouds and pre-stellar cores (Padoan et al. 2004). In this theory dense cores are generated in shocks (intersections of supersonic turbulent flows) by supersonic turbulences, some of which can become self-gravitating and collapse to form stars and, if they are not gravitationally bound, disperse back into the interstellar medium. The star formation inefficiency is understood in terms of the low fractions of mass, of sufficiently dense gas, within the clouds to become gravitationally bound.
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