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Star formation occurs in cold Galactic interstellar clouds where the gas is found mostly in molecular form. Because the clouds are penetrated by cosmic rays, some small fraction of the gas particles will be ionized. Though the ions are badly outnumbered by the neutrals (by about a million-to-one) they nevertheless ensure strong coupling between the cloud's gas content and the ambient interstellar magnetic field. So magnetic fields can be expected to play important roles in the evolution of star forming clouds, perhaps controlling the rate at which stars form and even determining the masses of stars.
Indeed, many theories and models have been developed in which magnetism plays crucial roles in star formation. On the other hand, there are also theories and models in which the role of magnetic fields is relatively minor. In particular, some theorists advocate control of star formation by super-Alfvenic turbulent flows, in which case the magnetic field is too weak to have a decisive influence. Because Galactic fields are difficult to observe, especially in molecular clouds, it has not yet been possible to resolve this debate. However, the ongoing polarimetric upgrade of BLAST to BLAST-pol will allow unprecedented views of the magnetic fields of molecular clouds by taking advantage of the phenomenon of magnetic grain alignment.
Astronomers have known since the 1940s that the rapidly-spinning dust grains that are mixed in with the gas in interstellar space manage to acquire a net alignment with respect to the magnetic field. The physical mechanism causing this alignment has not been established, but it seems to be ubiquitous, and as a result the submillimeter thermal emission from these small grains is linearly polarized in a direction orthogonal to the sky-plane projection of the ambient magnetic field. Thus, submillimeter polarimetric observations like those planned with BLAST-pol serve to trace out the magnetic field lines of interstellar clouds.
Ground-based submillimeter polarimetric observations have given us insight into the general characteristics of molecular cloud magnetic fields, but with only a few exceptions (e.g., Fig. 1) these maps have been restricted to dense cloud cores. For scores of molecular clouds, BLAST-pol will map the large-scale magnetic fields with sufficient sensitivity to observe out to their edges and sufficient angular resolution to observe into their cores. We will learn whether these fields are ordered or disordered, whether the fields of cloud cores are linked to large-scale cloud fields, and whether the filamentary structure commonly seen in molecular clouds is related to magnetic effects. This new knowledge will provide important constraints for star formation theories.
Figure 1: Ground-based submillimeter polarimetry of NGC 6334, a massive molecular cloud at a distance of 1700 pc, from Novak, Dotson, and Li (2009). The vectors in the map at upper left were obtained using SPARO at South Pole, and the "blow- ups" at right and bottom are from Hertz/CSO on Mauna Kea. The orientation of each vector shows inferred ﬁeld direction, and the length of each vector is proportional to the degree of polarization. With BLAST-pol, we will map scores of such clouds, with sky coverage exceeding that of SPARO and angular resolution approaching that of Hertz.
For example, measurements of the degree of order of cloud-scale magnetic fields can constrain the field strength which is crucial to star formation theory. Figure 2 shows results of 3-d non-linear magnetohydrodynamic simulations of turbulent, self-gravitating molecular clouds. The cloud on the left has a strong magnetic field, while that on the right has a weak field. Observations of large-scale molecular cloud fields with BLAST-pol should allow us to conclusively rule out one of these models, and also to explore the effects of molecular cloud evolution and stellar feedback.
Figure 2: Simulated strong-field (left) and weak-field (right) molecular clouds, from MHD turbulence simulations by Ostriker, Stone, and Gammie (2001). The color image shows column density and magenta vectors indicate projected magnetic field lines as traced by polarimetric observations. For both images, the large-scale cloud field is assumed to lie in the plane of the sky. Because this is not generally true for real molecular clouds, distinguishing between these two models will require good statistics, which BLAST-pol will provide.