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

Abstract

A recent observation of the Orion Nebula Cluster with the ACIS instrument on board the Chandra X-Ray Observatory detected 1075 sources, 525 of which are pre-main-sequence (PMS) stars with measured bulk properties such as 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 Lt 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_☉, an order of magnitude below the main-sequence saturation level; (2) the X-ray emission drops rapidly below this level in some stars with 2 < M < 3 M_☉; (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 decline of X-ray emission with increasing 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 α-Ω 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 in T Tauri stars.

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