New evidence for magnetic dynamo processes in young stars

Abstract

A recent observation of the Orion Nebula Cluster withthe Chandra X-ray Observatory detected 1075 sources, providing auniquely large and well-defined sample to study the dependence ofmagnetic activity on bulk properties for pre-main sequence (PMS) starsdescending the Hayashi tracks. The principal result is that X-rayemission does not show the activity-rotation relation which is thedominant correlate of magnetic activity indicators in main sequencestars. Instead, other effects are seen in PMS stars: X-rayluminosities are correlated with stellar bolometric luminosity, massand size for M <2-3 M_⊙, but the emission drops precipitouslyin some or all PMS stars with M >2-3 M_⊙. The absence of an X-ray/rotation correlation in PMS stars, andparticularly the high X-ray values seen in some very slowly rotatingstars, is a clear indication that the mechanisms of magnetic fieldgeneration differ from those operating in main sequence stars. Themost promising possibility is a turbulent dynamo distributed throughoutthe convective zone, but other models such as a "supersaturated"α - Ω dynamo or relic core fields are not immediatelyexcluded. The evidence does not support X-ray production inlarge-scale star-disk magnetic fields. This work is described indetail by Feigelson et al. (2003).