The magnetic decoupling stage of star formation

S. J. Desch, Telemachos Ch Mouschovias

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89 Scopus citations


We present the results of the first numerical calculations to model magnetic decoupling in a collapsing molecular cloud core. Magnetic decoupling is the stage during which the motion of the neutrals ceases to significantly affect the magnetic field strength and the magnetic field ceases to significantly affect this motion. We have analyzed the resistivity of a weakly ionized, magnetic gas, and we have separated the contributions of ohmic dissipation and ambipolar diffusion. The chemical model used to determine the abundances of ionized particles accounts for, among other things, a distribution of grain radii. The evolution of an axisymmetric, magnetic molecular cloud core is followed from central densities of 300 to 2 x 1012 cm-3. Typically, magnetic decoupling begins at a central density of 3 x 1010 cm-3 and is complete by a density of 2 x 1012 cm-3. We find that the mechanism responsible for magnetic decoupling is ambipolar diffusion, not ohmic dissipation, and that decoupling precedes the formation of a central stellar object. When the central density is a few times 1012 cm-3, a nearly uniform magnetic field of Bdec ≈ 0.1 G extends over a region ≈ 20 AU in radius that contains a mass ≈ 0.01 M. This value of Bdec is consistent with measurements of remanent magnetization in meteorites. Magnetic decoupling at this stage is not accompanied by hydromagnetic shocks. We estimate that the "magnetic flux problem" of star formation is resolved by ambipolar diffusion before the magnetic field is refrozen in the matter because of thermal ionization.

Original languageEnglish (US)
Pages (from-to)314-333
Number of pages20
JournalAstrophysical Journal
Issue number1 PART 1
StatePublished - Mar 20 2001
Externally publishedYes


  • ISM: magnetic fields
  • MHD
  • Plasmas
  • Solar system: formation
  • Stars: formation

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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