Oral Presentation

Polarization and Magnetic Field in Star formation

Ya-Wen Tang, Patrick M. Koch and Paul T. P. Ho

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 deg.
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.