Equipartition of Energy for Turbulent Astrophysical Fluids: Accounting for the Unseen Energy in Molecular Clouds
Abstract
Molecular clouds are observed to be partially supported by turbulent pressure. The kinetic energy of the turbulence is directly measurable, but the potential energy, which consists of magnetic, thermal, and gravitational potential energy, is largly unseen. We have extended previous results on equipartition between kinetic and potential energy to show that it is likely to be a very good approximation in molecular clouds. We have used two separate approaches to demonstrate this result: For smallamplitude perturbations of a static equilibrium, we have used the energy principle analysis of Bernstein et al. (1958); this derivation applies to perturbations of arbitary wavelength. To treat perturbations of a nonstatic equilibrium, we have used the Lagrangian analysis of Dewar (1970); this analysis applies only to shortwavelength perturbations. Both analysis assume conservation of energy. Wave damping has only a small effect on equipartition if the wave frequency is small compared to the neutralion collision frequency; for the particular case we considered, radiative losses have no effect on equipartition. These results are then incorporated in a simple way into analyses of cloud equilibrium and global stability. We discuss the effect of Alfvenic turbulence on the Jeans mass and show that it has little effect on the magnetic critical mass.
 Publication:

The Astrophysical Journal
 Pub Date:
 February 1995
 DOI:
 10.1086/175216
 Bibcode:
 1995ApJ...439..779Z
 Keywords:

 Energy Budgets;
 Hydrodynamics;
 Interstellar Matter;
 Molecular Clouds;
 Plasma Turbulence;
 Equilibrium Equations;
 Gravitational Effects;
 Kinetic Energy;
 Magnetic Effects;
 Mathematical Models;
 Potential Energy;
 Virial Theorem;
 Astrophysics;
 HYDRODYNAMICS;
 ISM: CLOUDS;
 ISM: MOLECULES;
 TURBULENCE