Properties of Molecular Outflows From Class 0 sources
Date Issued
2010
Date
2010
Author(s)
Wang, Liang-Yao
Abstract
The outflow phenomena are among the most interesting findings in the field of star formation. High-velocity molecular outflows emerging bipolarly from the youngest Class 0 protostellar objects are intensively studied over the past few decades. Increasingly higher resolution interferometric observations at millimeter and submillimeter wavelengths of molecular transition lines such as CO and SiO reveal properties of the outflows in this earliest evolutionary stage. Typical Class 0 molecular outflows like L1448 and HH211 appear dominated by an extremely high velocity components (EHV, velocities > 40 km s-1), and generally appear highly collimated with collimation factors >10 relative the low velocity and weakly collimated classical molecular outflow. Two distinct components are observed with CO emission, where a shell-like structure is seen in the low-velocity channel, and a more collimated jet-like component is present in the high-velocity one. On the other hand, low-velocity shell components are almost not detected in SiO emission, while the high-velocity SiO jet appear thinner than that of the CO.
Motivated by the observational results, we adopt the unified wind model of Shang et al. (2006), in which both shell and jet components are present, to study the properties of Class 0 molecular outflows. Critical densities above which level populations of the molecules are thermalized are calculated for CO and SiO, and are applied to the wind model as density criteria. Typical values of 1e6 cm^-3 and 3e4 cm^-3 for SiO and CO, respectively are adopted and the synthetic maps and position-velocity diagram capture the difference of CO and SiO emission. The result implies that the density effect could partly explain the different emission properties between CO and SiO.
Emission property of an object is closely related to its physical conditions. Since it is usually too difficult to model the emission of a source in detail, simplification methods are often required. Large Velocity Gradient (LVG) approximation is one of the methods to simplify the radiative transfer problem, and the results are also used by several authors to constrain the physical properties of molecular outflows. We follow the method adopted by Hirano et al. (2006) and attempt to place constraints on physical conditions of HH211 molecular jet. The constrain with SiO J=5-4 and SiO J=1-0 yields reasonable results, while that with SiO J=8-7 and SiO J=5-4 is inconclusive. Possible interpretations are discussed. The insufficient resolution and the comparison between data from different observations may cause uncertainties.
Finally, the emission pattern of Class 0 molecular outflow IRAS 04166+2706 is studied with the wind model. The puzzling slope feature observed in the position-velocity diagram is reproduced by adopting episodic ejection history of high and low velocity components in the simulations. The pattern is a natural consequence of the catching up process which occurs when high velocity material catches up and interact with material ejected with lower velocity in earlier episodes.
Subjects
jets and out ow
star formation
LVG
Critical density
Type
thesis
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