Circumstellar Environments around the Youngest Protostars: Interferometric Observations and Modeling by Hsin-fang Chiang Dissertation

This thesis presents observations and modeling of nearby embedded sources in the earliest stage of protostellar evolution, i.e., Class 0 young stellar objects, using interferometric data of the Berkeley-Illinois-Maryland Association array and the Combined Array for Research in Millimeter-wave Astronomy. Protostars form through gravitational collapse inside their natal envelopes, and these circumstellar envelopes contain valuable information about the physical processes of star formation. In this thesis, both molecular lines and dust continuum are utilized to investigate the nature of the collapsing envelopes. In particular, I focus on the isolated edge-on low-mass protostar L1157. While a large-scale (∼20,000 AU) flattened envelope is detected in both the N2H + line and the 8 μm extinction perpendicular to the outflow orientation, the dust continuum shows spherical structures at scales between ∼102 and ∼103 AU. The N2H observations not only reveal the outer envelope that is too dim to be detected in dust continuum, but they also unveil the kinematic structures of the flattened envelope. The dust continuum is compared with theoretical collapse models using radiative transfer calculation and Bayesian inference. The modeling techniques, as well as the associated uncertainties, are detailed. The results show that a power-law envelope model with a density index p ∼ 2 provides a better fit to the observations than the simple Shu model or the commonly-used Terebey-Shu-Cassen model. Furthermore, I discuss the implications of the modeling results on the dust grain properties and the constraints they place on the youngest circumstellar disk embedded inside the envelope.