Probing the Early Evolution of Dust Grains Through Detailed YSO Models

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 (Class I - Class I/II). We use both well-developed 3-D radiative transfer codes and variable dust grain models to sensitively constrain not only the geometry and optical depth of the scattering medium, but also the grain size distribution. We simultaneously fit multi-wavelength (submicron to millimeter) spectral energy distributions (SEDs) for our objects to further constrain the model results. Scattered light from neither ISM nor spherical small grains provides enough polarization to reproduce the NICMOS observations. Non-spherical aligned grains may produce a larger polarization depending on the orientation of the magnetic field. We present data and model YSO polarization, image morphology, and SEDs for varying dust grain models.