MORPHOLOGICAL EVOLUTION OF BIPOLAR OUTFLOWS FROM YOUNG STELLAR OBJECTS

摘要

We present Spitzer IRAC images that indicate the presence of cavities cut into the dense outer envelope surrounding very young pre-main-sequence stars. These young stellar objects (YSOs) characterized by an outflow represent the earliest stages of star formation. Mid-infrared photons thermally created by the central protostar/disk are scattered by dust particles within the outflow cavity itself into the line of sight. We observed this scattered light from 27 nearby, cavity-resolved YSOs and quantified the shape of the outflow cavities. Using the grid models of Robitaille et al., we matched model spectral energy distributions (SEDs) to the observed SEDs of the 27 cataloged YSOs using photometry from IRAC, MIPS, and IRAS. This allows for the estimation of geometric and physical properties such as inclination angle, cavity density, and accretion rate. By using the relative parameter estimates determined by the models, we are able to deduce an evolutionary picture for outflows. Our work supports the concept that cavities widen with time, beginning as a thin jetlike outflow that widens to reveal the central protostar and disk until the protostellar envelope is completely dispersed by outflow and accretion.