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Published February 20, 2003 | Submitted + Published
Journal Article Open

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


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.

Additional Information

© 2003 American Astronomical Society. Received 2002 July 9; Accepted 2002 October 29. We are very appreciative of the careful and insightful reading of the manuscript by Dermott Mullan (Bartol) and the anonymous referee. E. D. F. also greatly benefited from discussions with participants of stellar magnetism workshops in Santiago, Boulder, and Toulouse during 2001–2002. Patrick Broos (Penn State), Steven Pravdo (JPL), and Yohko Tsuboi (Penn State/Chuo) played critical roles in the Chandra ACIS Orion project. Sofia Randich (Arcetri) and William Herbst (Wesleyan) provided valuable unpublished results and comments. This work was principally supported by NASA contract NAS 8-38252 (Garmire, PI).

Attached Files

Published - Feigelson_2003_ApJ_584_911.pdf

Submitted - 0211049.pdf


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