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A recent Chandra/ACIS observation of the Orion Nebula Cluster detected 1075sources (Feigelson et al. 2002), providing a uniquely large and well-definedsample to study the dependence of magnetic activity on bulk properties forstars descending the Hayashi tracks. The following results are obtained: (1)X-ray luminosities L_t in the 0.5-8 keV band are strongly correlated withbolometric luminosity with <log L_t/L_bol> = -3.8 for stars with masses 0.7<M<2Mo, an order of magnitude below the main sequence saturation level; (2) theX-ray emission drops rapidly below this level in some or all stars with 2<M<3Mo; (3) the presence or absence of infrared circumstellar disks has no apparentrelation to X-ray levels; and (4) X-ray luminosities exhibit a slight rise asrotational periods increase from 0.4 to 20 days. This last finding stands indramatic contrast to the strong anticorrelation between X-rays and period seenin main sequence stars.  The absence of a strong X-ray/rotation relationship in PMS stars, andparticularly the high X-ray values seen in some very slowly rotating stars, isa clear indication that the mechanisms of magnetic field generation differ fromthose operating in main sequence stars. The most promising possibility is aturbulent dynamo distributed throughout the deep convection zone, but othermodels such as alpha-Omega dynamo with `supersaturation' or relic core fieldsare not immediately excluded. The drop in magnetic activity inintermediate-mass stars may reflect the presence of a significant radiativecore. The evidence does not support X-ray production in large-scale star-diskmagnetic fields.Comment: 51 pages, 8 figures. To appear in the Astrophysical Journa

X‐Rays in the Orion Nebula Cluster: Constraints on the Origins of Magnetic Activity in Pre–Main‐Sequence Stars