Probing the early evolution of dust grains through detailed YSO models

Understanding the evolution of young stellar objects (YSOs) from a protostar to a main-sequence star is key to understanding the origins and evolution of planetary systems. High-resolution polarimetric observations from the Hubble Space Telescope Near Infrared Camera and Multi-Object Spectrometer (NICMOS) of objects known to span the earliest stellar evolutionary phases are combined with (1) 3-D radiative transfer codes, and (2) a grid of dust grain models, to gain insight into the initial phases of dust grain growth and evolution away from an interstellar medium (ISM) distribution. Fractional polarization is a strong function of wavelength; therefore by developing detailed models of polarimetric images in the infrared, we can begin to sensitively constrain not only the geometry and optical depth of the scattering medium, but also the grain size distribution. We are studying four YSOs known to span the earliest stellar evolutionary phases (IRAS04302+2247 (Class I), IRAS04016+2610 (Class I), CoKu Tau/1 (Class I/II) and DG Tau B (Class I/II)) as their circumstellar disks transition from containing primarily small ISM dust grains to larger size grains indicative of protoplanetary disks. Results show that an ISM dust grain distribution under-predicts the NICMOS polarization observations. Therefore, the dust grain composition surrounding these YSOs no longer behaves like ISM grains at 2 mu m. We present data and model comparisons of YSO fractional polarization.