EPoS Contribution
EPoS Contribution
The Role of Magnetic Fields in Star Formation

Chris McKee
UCB, Berkeley, US
Historically, it has been conjectured that ambipolar diffusion is essential for allowing gravitational forces to exceed magnetic forces in molecular clouds, thereby enabling star formation. However, Zeeman observations of magnetic field strengths in molecular cloud cores have failed to find evidence of cores that are magnetically dominated. Magnetic field in ideal magnetohydrodynamic (MHD) simulations of a region in a turbulent molecular cloud are in good agreement with the Zeeman observations of the line-of-sight magnetic field in molecular clouds. We show that the observed line-of-sight field in molecular clumps scales approximately as n2/3 as inferred by observations for the total field; in addition, the total field in our simulation approximately obeys this scaling. The simulations show how tangling of the field lines reduces the field measured by the Zeeman effect. The observed mass-to-flux ratio is density weighted and underestimates the actual mass-to-flux ratio; we infer that the median mass-to-flux ratio of the cores in the observed sample is three times the critical value, confirming that ambipolar diffusion is relatively unimportant in these cores. Comparing simulations with an initially weak field (initial Alfven Mach number MA=10) to one with an initially moderate field (MA=1), we find that only the initially moderate field is in good agreement with the observations of magnetic field structure. Finally, we find some evidence for turbulent reconnection in our ideal MHD simulation: ideal MHD in the presence of turbulence is not ideal.
P.S. Li, UCB
R.I. Klein, LLNL/UCB
Key publication

Suggested Session: Magnetic Fields