A recent observation of the Orion Nebula Cluster with the ACIS instrument on board the Chandra X-ray Observatory detected 1075 sources ( Feigelson et al .
2002 ) , 525 of which are pre-main sequence ( PMS ) stars with measured bulk properties ( bolometric luminosities , masses , ages and disk indicators ) .
Nearly half of these stars have photometrically measured rotational periods .
This provides a uniquely large and well-defined sample to study the dependence of magnetic activity on bulk properties for stars 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 with bolometric luminosity with average ratio \log L _ { t } / L _ { bol } = -3.8 for stars with masses 0.7 < M < 2 M _ { \odot } , an order of magnitude below the main sequence saturation level ; ( 2 ) the X-ray emission drops rapidly below this level in some or all stars with 2 < M < 3 M _ { \odot } ; ( 3 ) the presence or absence of infrared circumstellar disks has no apparent relation to X-ray levels ; and ( 4 ) X-ray luminosities exhibit a slight rise as rotational periods increase from 0.4 to 20 days .
This last finding stands in dramatic contrast to the strong anticorrelation between X-rays and period seen in main sequence stars .

The absence of a strong X-ray/rotation relationship in PMS stars , and particularly the high X-ray values seen in some very slowly rotating stars , is a clear indication that the mechanisms of magnetic field generation differ from those operating in main sequence stars .
The most promising possibility is a turbulent dynamo distributed throughout the deep convection zone , but other models such as \alpha - \Omega dynamo with ‘ supersaturation ’ or relic core fields are not immediately excluded .
The drop in magnetic activity in intermediate-mass stars may reflect the presence of a significant radiative core .
The evidence does not support X-ray production in large-scale star-disk magnetic fields .