A study of the dynamical signatures of star formation

Abstract

A multi-pronged study aimed at disentangling the kinematical signatures of the earliest stages of star formation is presented. Radiative transfer calculations of millimeter and submillimeter molecular line emission from fully three-dimensional models of protostars are reported. These models are compared with detailed submillimeter molecular line observations of dynamical motions towards seven Class 0 protostellar objects. The radiative transfer calculations are performed for two classes of protostellar collapse solutions: (1) "self-consistent", nonspherical, hydrodynamic, collapsing, rotating protostellar systems (Boss 1993); (2) parameterized, semi-analytic, rotating collapse solutions of Terebey, Shu and Cassen (1984) The morphology of the gas and dust emission is found to be a strong function of collapse time and angular resolution. From model centroid velocity maps, a distinctive new infall signature called the "blue-bulge" infall signature is derived. The blue-bulge infall signature can be observed in the centroid velocity maps of protostellar objects when infall dominates over rotation. This infall signature can be detected under a wide variety of source conditions, and should be easily observable using single-dish submillimeter telescopes. At high angular resolutions, models with moderate to high rotational rates exhibit the "polar blue-bulge" - a centroid velocity signature of underlying Keplerian rotation in an embedded cloud core. Submillimeter transitions of HCO+ and CS are found to be better than millimeter transitions in detecting infall, especially at early collapse times. Using new submillimeter observations in CS and HCO+ towards IRAS 16293-2422, the first detection of the "blue-bulge" signature towards a protostellar object is presented. The mass accretion rate through the infall region appears consistent with an inside-out collapse model for the source. Using new submillimeter HCO+ and CO observations, a detailed study was performed of six other nearby Class 0 objects. The blue-bulge signature of infall is detected in five sources. Among these, SMM4 and B335 are known infall candidates. VLA 1623, L483 and L1262 are new sources for which evidence for infall is derived in this work. SM1N, which does not exhibit a blue-bulge appears to be a pre-protostellar object. A low luminosity bipolar outflow was detected toward SM1N, suggesting that it may be in an extremely early stage of collapse. Of the six sources, only three, SMM4, B335 and L1262 exhibited the classic blue asymmetric line profile signature of infall, suggesting that the blue-bulge signature is more robust in detecting infall than traditional line profile techniques. Evolutionary trends are seen between observationally obtainable source parameters and model derived timescales for the Class 0 sources presented in this work. Such a study when extended to a larger sample of YSOs will help in the understanding of the evolution of YSOs from the embedded protostellar stage to revealed pre-main-sequence objects.