Testing magnetic star formation theory

Richard M. Crutcher, Nicholas Hakobian, Thomas H. Troland

Research output: Contribution to journalArticlepeer-review


Zeeman observations of molecular clouds yield the line-of-sight component B LOS of the magnetic vector B, which makes it possible to test the two major extreme-case theories of what drives star formation - ambipolar diffusion or turbulence. However, only one of the three components of B is measurable, so tests have been statistical rather than direct, and they have not been definitive. We report here observations of the Zeeman effect in the 18 cm lines of OH in the envelope regions surrounding four molecular cloud cores toward which detections of B LOS have been achieved in the same lines, and evaluate the ratio of mass-to-magnetic flux, M/Φ, between the cloud core and envelope. This relative M/Φ measurement reduces uncertainties in previous studies, such as the angle between B and the line of sight and the value of [OH/H]. Our result is that for all four clouds, the ratios of the core to the envelope values of M/Φ are less than 1. Stated another way, the ratios of the core to the total cloud M/Φ are less than 1. The extreme case or idealized (no turbulence) ambipolar diffusion theory of core formation requires the ratio of the central to total M/Φ to be approximately equal to the inverse of the original subcritical M/Φ, or . The probability that all four of our clouds have is 3 × 10-7; our results are therefore significantly in contradiction with the hypothesis that these four cores were formed by ambipolar diffusion. Highly super-Alfvénic turbulent simulations yield a wide range of relative M/Φ, but favor a ratio , as we observe. Our experiment is limited to four clouds, and we can only directly test the predictions of the extreme-case "idealized" models of ambipolar-diffusion driven star formation, which have a regular magnetic field morphology. Nonetheless, our experimental results are not consistent with the "idealized" strong field, ambipolar diffusion theory of star formation. Comparisons of our results with more realistic models and simulations that include both ambipolar diffusion and turbulence may help to refine our understanding of the relative importance of magnetic fields and turbulence in the star formation process.

Original languageEnglish (US)
Pages (from-to)844-855
Number of pages12
JournalAstrophysical Journal
Issue number1
StatePublished - Feb 10 2009


  • ISM: clouds
  • ISM: evolution
  • ISM: magnetic fields
  • polarization
  • stars: formation

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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