Astronomy and Astrophysics – Astrophysics
Scientific paper
Mar 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995apj...442..186m&link_type=abstract
The Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 442, no. 1, p. 186-196
Astronomy and Astrophysics
Astrophysics
61
Interstellar Magnetic Fields, Ionization, Magnetic Flux, Magnetohydrodynamic Waves, Molecular Clouds, Turbulence, Abundance, Ambipolar Diffusion, Density Distribution, Photoionization, Reynolds Number, Velocity Distribution
Scientific paper
Observations of diffuse, dark, and giant molecular clouds and their cores are analyzed to determine properties of their turbulent motions. Estimates of characteristic cloud internal density, external extinction, and external radiation field intensity are used to deduce the electron fraction chie due to both photoionization and cosmic rays. This ionization fraction exceeds that due to cosmic rays alone, by factors approximately 5 for dark cloud cores to approximately 4000 for giant molecular clouds with embedded OB stars. Estimates of characteristic cloud size, density, velocity dispersion, ionization fraction, and magnetic field strength then indicate that four diagnostic numbers exceed unity by a significant factor: the Reynolds number, the magnetic Reynolds number, the Hartmann number, and the 'wave coupling number,' or ratio of cloud size to minimum hydromagnetic wavelength. These results indicate that virtually all observed interstellar clouds have strong coupling between the magnetic field and the neutral gas, through ion-neutral collisions, even if the field is weaker than its equipartition value. This coupling allows energetically significant magnetohydrodynamic (MHD) waves to propagate above cutoff, so that MHD waves, chaotic motions, and clumpy density structure are probably more pervasive in interstellar clouds than would be expected from cosmic-ray ionization alone. This strong coupling implies that the timescale for ambipolar diffusion is at least approximately 107 yr for low-mass cores, and is at least approximately 108 yr for the gas around cores. These timescales may be too long for all of the mass in a low-mass core to condense via ambipolar diffusion. The observed velocity dispersion is strongly correlated with the estimated electron fraction, according to the power law nu approximately chie p, with p approximately = 0.3. This trend, and those already known among velocity dispersion, size, and density, suggest that increasing extinction may influence the structure of cloud density and velocity dispersion by driving a cycle of decreasing ionization, decreasing MHD wave activity, decreasing velocity dispersion, and increasing density.
Khersonsky Valery K.
Myers Phil C.
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