Is the Plasma Within Bubbles and Superbubbles Hot or Cold?

I review what is known about the temperature of the plasma within stellar wind bubbles and superbubbles. Classical theory suggests that it should be hot, with characteristic temperatures of order a million degrees. This temperature should be set by the balance between heating by the internal termination shocks of the central stellar winds and supernovae, which expand at thousands of km/s, and cooling by conductive evaporation of cold gas off the shell walls. However, if the hot interior gas becomes dense enough due to evaporation or ablation off of interior clouds, it will cool in less than a dynamical time, leading to a cold interior. The observational evidence appears mixed. On the one hand, X-ray emission has been observed from both stellar wind bubbles and superbubbles. On the other hand, no stellar wind bubble or superbubble has yet been observed emitting at the rate predicted by the classical theory: they are either too faint or too bright, by up to an order of magnitude. Alternate explanations have been proposed for the observed emission, including off-center supernova remnants hitting the shell walls of superbubbles, and residual emission from highly-ionized gas out of coronal equilibrium. Furthermore, the structures of post-main sequence stellar wind bubbles, expanding into what are presumably old stellar wind bubbles, appear in at least some cases to show that the bubble interior is cold, not hot. (The classical example of this is NGC 6888.) What is the actual state of bubble and superbubble interiors?Comment: 7 pages, 1 figures, to be published in Astrophysical Plasmas: Codes, Models and Observations, RMxAA Conf Ser, 2000. Requires rmaa.cl

Turbulent Velocity Structure in Molecular Clouds

We compare velocity structure in the Polaris Flare molecular cloud at scales ranging from 0.015 pc to 20 pc to simulations of supersonic hydrodynamic and MHD turbulence computed with the ZEUS MHD code. We use several different statistical methods to compare models and observations. The Delta-variance wavelet transform is most sensitive to characteristic scales and scaling laws, but is limited by a lack of intensity weighting. The scanning-beam size-linewidth relation is more robust with respect to noisy data. Obtaining the global velocity scaling behaviour requires that large-scale trends in the maps not be removed but treated as part of the turbulent cascade. We compare the true velocity PDF in our models to velocity centroids and average line profiles in optically thin lines, and find that the line profiles reflect the true PDF better unless the map size is comparable to the total line-of-sight thickness of the cloud. Comparison of line profiles to velocity centroid PDFs can thus be used to measure the line-of-sight depth of a cloud. The observed density and velocity structure is consistent with supersonic turbulence with a driving scale at or above the size of the molecular cloud and dissipative processes below 0.05 pc. Ambipolar diffusion could explain the dissipation. The velocity PDFs exclude small-scale driving such as that from stellar outflows as a dominant process in the observed region. In the models, large-scale driving is the only process that produces deviations from a Gaussian PDF shape consistent with observations. Strong magnetic fields impose a clear anisotropy on the velocity field, reducing the velocity variance in directions perpendicular to the field. (abridged)Comment: 21 pages, 24 figures, accepted by A&A, with some modifications, including change of claimed direct detection of dissipation scale to an upper limi