Wave Propagation in Molecular Clouds

Details Wave Propagation in Molecular Clouds Wave Propagation in Molecular Clouds

Jul 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996apj...465..775b&link_type=abstract

Astrophysical Journal v.465, p.775

Astronomy and Astrophysics

Astronomy

50

Ism: Clouds, Ism: Kinematics And Dynamics, Magnetohydrodynamics: Mhd, Shock Waves, Stars: Formation, Waves

Scientific paper

We study the linear propagation of waves in a partially ionized, magnetized, self-gravitating plasma. Three dispersion studies are presented. The first study is carried out in the strong coupling approximation, without self-gravity. The second is done for the strong coupling approximation, with self-gravity. The third is done in the two-fluid approximation, in which the strong coupling approximation is dropped. The eigenvectors for ideal, isothermal MHD are also constructed and examined. They provide important insights into the results that we obtain from the stability analysis.
We find that for parameters typical of molecular clouds, where the Alfvén speed exceeds the sound speed, the slow waves propagate without significant damping on short wavelengths, while the fast and Alfvén waves undergo rapid damping. Moreover, as the angle between the direction of propagation of the slow waves and the magnetic field is reduced, the damping of the slow waves is reduced. When this angle is zero, the waves propagate undamped. By examining the structure of the eigenvectors for ideal, isothermal MHD we show these propagation characteristics of the waves at short wavelengths to be physically justified. At long wavelengths, the slow waves bifurcate into a pair of modes, one of which is purely growing and the other that is purely damped. Thus, the gravitational collapse is carried entirely by the slow family of waves. The fast and Alfvén waves propagate without significant damping at long wavelengths. From the two-fluid dispersion analysis we find the length scale where the single-fluid approximation breaks down and examine wave propagation for wavelengths shortward of that wavelength.
These results allow us to see why observed line profiles in molecular clouds indicate turbulent motions that are supersonic but sub-Alfvénic. We build up an analog of the Reynolds number for partially ionized plasmas. We show it to be physically well justified. We realize that weak turbulence in a molecular cloud cannot be Alfvénic on length scales of interest. Further insights into the nature of turbulence in partially ionized, magnetized, self-gravitating plasmas are also gained from this work. We build up a dimensionless number called the "turbulent pressure support to gravitational collapse number." When this number drops below unity the turbulent pressure support to the self-gravitating gas is diminished. We identify gravitional collapse as an alternative source for sustaining the turbulence when energy input from massive star formation is not available. This provides an explanation for the fact that the Maddalena-Thaddeus molecular cloud has significant line widths, even though it shows little or no evidence for massive star formation.

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