Magnetically Aligned Dust Grains in Young Stellar Objects

Young stellar objects (YSOs) evolve from being dominated by a circumstellar envelope, which overtime collapses onto a circumstellar disk and disperses via jets and outflows, while the disk material accretes onto the forming star. Planets form in the circumstellar disks from dust grains that begin with an interstellar medium (ISM) grain size distribution, and grow from sizes typically less than 1um to planets with radii greater than 6000 km. Light from the forming central star scatters off of the small dust grains in the disk, envelope and outflow regions resulting in polarization at near-infrared wavelengths. Studying the polarized light in these regions provides insight into the size and distribution of the dust grain population, which changes as the YSO evolves; thus facilitating our understanding of both stellar and planetary formation. We model high-resolution Hubble Space Telescope Near Infrared Camera and Multi-Object Spectrometer (NICMOS) imaging and polarimetry for a group of four (IRAS04302+2247, IRAS04016+2610, CoKu Tau/1, DG Tau B) Taurus-Auriga YSOs known to span the earliest stellar evolutionary phases. We use both well-developed 3-D radiative transfer codes and magnetically aligned non-spherical dust grain models to predict imaging and polarimetry. We simultaneously fit multi-wavelength (submicron to millimeter) spectral energy distributions (SEDs) for our YSOs to further constrain the model results. We present data and model imaging, polarimetry and SED comparisons using magnetically aligned non-spherical dust grains in varying magnetic field geometries. Dichroic scattering and extinction by magnetically aligned non-spherical grains may lead to significant polarization in circumstellar environments as compared to scattering from spherical grains, which were the focus of our previous research effort. Radiative transfer models of our four YSOs indicate that spherical dust grains do not produce as much polarization as is observed by NICMOS. Therefore, we seek to better understand the significance of magnetically aligned gains in the polarization production, as we anticipate these non-spherical grains may be a better representation of circumsteller environments.