Magnetic fields in molecular clouds: Regulators of star formation

Abstract

Magnetic fields cause relatively violent behavior in diverse astrophysical systems. Yet they also mitigate would-be violent behavior. Although they destabilize the interstellar medium on scales ≈1 kpc and lead to the formation of clouds, they also provide effective support against the self-gravity of interstellar clouds. On the one hand they allow gravitational contraction of clouds and star formation to take place by redistributing angular momentum (magnetic braking). Yet, on the other hand, they transform the would-be violent gravitational collapse of interstellar clouds to an almost quasistatic, but still rapid, contraction of their cores which often endures to densities >108 cm−3; this is achieved by a well controlled redistribution of mass in the central flux tubes of clouds (ambipolar diffusion). We show that magnetic fields also unavoidably introduce a natural length scale in cloud interiors. The typical mass inside this length scale is ≈ 1 M⊙ which, deprived of effective magnetic support due to ambipolar diffusion, contracts relatively rapidly, while the surrounding envelope is much better supported by magnetic forces. Hence, magnetic fields may also play a crucial role in the determination of the initial (stellar) mass function as they tend to inhibit accretion of the envelope onto a protostellar core. These quantitative results and the star formation scenario that they suggest are qualitatively different from current ideas concerning star formation in nonmagnetic clouds or in cores which have lost their magnetic support. The latter ideas postulate the absence of any characteristic length (or protostellar mass); we prove its existence and calculate its precise magnitude.