Oral Presentation

Grain Alignment in the Ophiuchus Molecular Cloud

Author(s): Kate Pattle (National Tsing Hua University)

Presenter: Kate Pattle (National Tsing Hua University)

If we wish to use submillimeter continuum polarization measurements as a tracer of magnetic fields in molecular clouds, it is essential that we understand the physical conditions in which we can safely do so. We thus require an understanding of how the alignment of dust grains with magnetic field varies with increasing column and volume gas density. Dust polarization fraction is typically observed to decrease toward dense regions, suggestive of grain misalignment and/or variation of the magnetic field on scales smaller than the beam. We present polarization observations of three dense clumps in the L1688 region of the Ophiuchus molecular cloud taken as part of the JCMT BISTRO (B-Fields in Star-forming RegiOns) Large Program, with unprecedented sensitivity and spatial extent at ~14-arcsec resolution. The high sensitivity of our measurements allows us to investigate the evolution of polarization fraction toward high column densities without the selection effects introduced into previous observations by low signal-to-noise. We find that while depolarization is observed in all three regions, efficient grain alignment persists to significantly higher column densities in Oph A, which is located between two massive stars and so has a significantly elevated flux of high-energy photons, than in the otherwise comparable Oph B and C regions. We discuss our results in the context of the radiative alignment torque theory of grain alignment.