arXiv subjects
Publications and source records attributed to Taoling Xie.
We further discuss the suggestion that the chemistry in a photon-dominated region is coupled to that in the UV-shielded region behind it by turbulent t ransport processes. In addition to transport time-scales, we discuss why MHD waves/turbulence will likely cause transport instead of prohibiting it.
Under the basic assumption that the observed turbulent motions in molecular clouds are Alfvenic waves or turbulence, we emphasize that the Doppler broadening of molecular line profiles directly measures the velocity amplitudes of the waves instead of the Alfven velocity. Assuming an equipartition between the kinetic energy and the Alfvenic magnetic energy, we further propose the hypothesis that observed standard scaling laws in molecular clouds imply a roughly scale-independent fluctuating magnetic field, which might be understood as a result of strong wave-wave interactions and subsequent energy cascade. We predict that $σ_{v}\propto ρ^{-0.5}$ is a more basic and robust relation in that it may approximately hold in any regions where the spatial energy density distribution is primarily determined by wave-wave interactions, including gravitationally unbound regions. We also discuss the fact that a scale-independent $σ_{B}^{2}$ appears to contradict existing 1-D and 2-D computer simulations of MHD turbulence in molecular clouds.
It has been proposed recently that the small size and long lifetime of ultra-compact HII regions (UCHIIs) could be due to pressure confinement if the thermal pressure of the ambient gas is higher than previous estimates. We point out that confinement by thermal pressure alone implies emission measures in excess of observed values. We show that turbulent pressure, inferred from observed non-thermal velocities, is sufficient to confine UC HIIs and explain their longevity. We predict an anti-correlation between the size of UCHIIs and the velocity dispersion of the ambient neutral gas, and show that it is consistent with existing observations.