Polarization and Magnetic Field in Star formation
Dust grains are known to align with their shorter axes parallel to the field lines in most circumstances. The plane-of-sky projected B field integrated along the line of sight can be traced by rotating the detected polarization of the thermal dust emission by 90^o.
Stars form in giant molecular clouds under the threads of turbulence and large-scale magnetic (B) fields. Theoretically, the significance of the B field influences how structures are formed, such as the density contrast within structures, the star formation rate, and the suppressed fragmentation. However, the B fields in star-forming clouds are not well-constrained observationally, because they are difficult to detect.
In this talk I will present our current studies of the magnetic (B) field in the star forming cores and in the envelopes of molecular clouds. In order to trace the B field, the dust continuum at wavelengths of 870 micron and its linearly polarized emission were observed with the Submillimeter Array (SMA). The B morphologies are resolved with an angular resolution up to 0.3". Dense structures with a number density 10^5 to 10^7 cm^-3 are traced. The B morphologies of sources at different evolutionary stages, from the collapsing cores to the ultra-compact HII regions, will be presented.
In a sequence of increasingly higher resolution observations, it becomes manifest how the field morphologies change from the envelope surface layer to the inner core and disk. The B field morphologies vary from uniform in the cloud scale (a few pc) to cometary and hourglass-like in the star forming core scales (10s milli-pc), suggesting the interplay of the B field with other forces at various scales.