Turbulence in Star Formation: Tracing the Velocity Fields of Dense Cores

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The theory of star formation that has developed over the past several decades supposes that dense cores are quiescent and isolated from energetic events. However, observations of star-forming regions show that cores develop in active environments. Thus, although the "standard" theory is quantitatively rigorous, it can only explain a fraction of real star-forming events.

The point of departure for this work is the hypothesis that turbulence is a fundamental component of the star formation process. In a turbulent star formation theory, the effects of random gas motions extend from molecular cloud scales down to scales of thousands or hundreds of AU. Dense cores form rapidly at the collision interfaces of turbulent flows and evolve according to the specific physical conditions at those interfaces. Star formation is dynamic and interactive, rather than quasi-static and isolated.

This work presents evidence for turbulent motions in dense cores. The evidence comes from observations of cores in the Perseus cloud made with the BIMA interferometer and the FCRAO 14 m antenna. The cores were mapped in C18O J=1-0 emission with resolutions of ~44, 10, 5, and 3 arc-seconds. The higher angular resolutions correspond to physical scales within the characteristic core radius (~0.1 pc) identified in previous studies. In general, the range of velocities traced by the C18O, as well as the complexity of the fields, increases with resolution. No core resembles a quiescent condensation undergoing simple systematic rotation.

The cores are analyzed by applying a gridding technique developed by Ostriker, Stone, & Gammie (2001) to quantify the properties of model clouds. Spectra taken through the datacubes over a wide range of sizes are used to construct correlations between line widths and spatial scale, which show a broad range of line widths even at the smallest measurable scales. The narrowest lines must have a turbulent component at least as great as the thermal component, and for nearly all lines, the turbulent component makes the dominant contribution. A statistical analysis of the variations in line properties as a function of spatial separation across a core shows that the means and variances of the central velocity and line width difference distributions exhibit properties characteristic of a hierarchy of turbulent gas motions (Miesch & Bally 1994). The high resolution BIMA data reveal that these turbulent motions persist on sub-core scales.