The Damping Rates of Embedded Oscillating Starless Cores

In a previous paper we demonstrated that nonradial hydrodynamic oscillations of a thermally supported (BonnorEbert) sphere embedded in a low-density, high-temperature medium persist for many periods. The predicted column density variations and molecular spectral line profiles are similar to those observed in the Bok globule B68, suggesting that the motions in some starless cores may be oscillating perturbations on a thermally supported equilibrium structure. Such oscillations can produce molecular line maps which mimic rotation, collapse, or expansion and, thus, couldmake determining the dynamical state from such observations alone difficult. However, while B68 is embedded in a very hot, low-densitymedium, many starless cores are not, having interior/exterior density contrasts closer to unity. In this paper we investigate the oscillation damping rate as a function of the exterior density. For concreteness we use the same interior model employed by Broderick et al., with varying models for the exterior gas. We also develop a simple analytical formalism, based on the linear perturbation analysis of the oscillations, which predicts the contribution to the damping rates due to the excitation of soundwaves in the external medium.We find that the damping rate of oscillations on globules in dense molecular environments is always many periods, corresponding to hundreds of thousands of years and persisting over the inferred lifetimes of the globules. Subject headingg s: hydrodynamics — ISM: clouds — ISM: globules — stars: formation