The role of magnetic fields in star formation
James Owen
Introduction
Though the physics of their interactions are far from understood, cosmic magnetic fields are ubiquitous, pervasive and are considered integral to the properties and evolution of almost everything in the universe. This is particularly true for the processes that drive star formation, which, to this day, remain one of the challenges for modern astrophysics. Stars form in cool dense regions of dust and gas in the interstellar medium, known as molecular clouds. Molecular clouds are composed of molecular hydrogen (~90%), helium (~10%) and traces of other molecules, with temperatures of 10 to 30 k which glow at far infra-red and sub-millimetre wavelengths. Dust particles of carbon or silicate materials represent approximately one per cent of the cloud’s mass. They are typically 100-10 6 times the mass of the Sun (M sun ) and can reach up to tens of parsecs (3.26 light years ~ 19.2 trillion miles) in size. The largest clouds (10 3 - 10 6 M sun ), which are referred to as giant molecular clouds (GMCs), contain clumps of denser regions known as ‘ prestellar ’ cores. These cores are thought to be at the first stages of stellar evolution. Cosmic rays passing through molecular clouds ionize a small percentage (~1/10 6 ) of the gas particles and these ionized particles strongly couple the cloud’s gas content with the ambient magnetic field. Given this strong coupling, it is not inconceivable that the magnetic field may play a significant role in the evolution of molecular clouds, perhaps even regulating the formation rates and masses of stars from which they are born. Indeed, the magnetic field is a fundamental quantity in many star formation theories and models (e.g., Allen et al 2003), while for others, it is considered to be too weak to play an influential role (e.g., Padoan et al. 2004). Galactic magnetic fields must be ‘ illuminated ’ in order to detect their presence. Fortunately, this illumination is provided by the phenomenon of magnetic grain alignment: the plane of the sky component of the magnetic field may be directly measured by tracing the linearly polarized emission from dust grains aligned with respect to the cloud’s ambient magnetic field from which the field strength may be calculated – at far infra-red and sub-millimetre wavelengths. Thus, the polarization reveals the magnetic field ’ s strength and direction across the entire molecular cloud. In addition, polarization studies at these wavelengths yield information about the magnetic field and grain structure that cannot be obtained at other wavelengths. Sub-millimetre polarimetry astronomy is now considered to be an essential tool for investigating the very cold matter associated with the earliest stages of star formation and is an active area of pioneering research, tracing the magnetic field across whole molecular clouds.
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