X‐Ray–emitting Young Stars in the Orion Nebula

The Orion Nebula Cluster and the molecular cloud in its vicinity have beenobserved with the ACIS-I detector on board the Chandra X-ray Observatory with23 hours exposure. We detect 1075 X-ray sources: 91% are spatially associatedwith known stellar members of the cluster, and 7% are newly identified deeplyembedded cloud members. This provides the largest X-ray study of a pre-mainsequence stellar population. We examine here the X-ray properties of Orionyoung stars as a function of mass. Results include: (a) the discovery of rapidvariability in the O9.5 31 M_o star \theta^2A Ori, and several early B stars,inconsistent with the standard model of X-ray production in small wind shocks;(b) support for the hypothesis that intermediate-mass mid-B through A typestars do not themselves produce significant X-ray emission; (c) confirmationthat low-mass G- through M-type T Tauri stars exhibit powerful flaring buttypically at luminosities considerably below the `saturation' level; (d)confirmation that the presence or absence of a circumstellar disk has nodiscernable effect on X-ray emission; (e) evidence that T Tauri plasmatemperatures are often very high with T >= 100 MK, even when luminosities aremodest and flaring is not evident; and (f) detection of the largest sample ofpre-main sequence very low mass objects showing high flaring levels and adecline in magnetic activity as they evolve into L- and T-type brown dwarfs.Comment: 82 pages, 16 figures, 6 tables. To appear in the Astrophysical Journal. For a version with high quality images and electronic tables, see ftp://ftp.astro.psu.edu/pub/edf/orion1