X‐Ray–emitting Young Stars in the Orion Nebula

Abstract

The Orion Nebula Cluster and the molecular cloud in its vicinity have been observed with the ACIS-I detector on board the Chandra X-ray Observatory with 23 hours exposure in two observations. We detect 1075 X-ray sources, most with sub-arcsecond positional accuracy. Ninety-one percent of the sources are spatially associated with known stellar members of the cluster, and an additional 7% are newly identified deeply embedded cloud members. This provides the largest X-ray study of a pre-main sequence stellar population and covers the initial mass function from brown dwarfs up to a 45 M ⊙ O star. Source luminosities span 5 orders of magnitude from log L x ≃ 28.0 to 33.3 erg s −1 in the 0.5 − 8 keV band, plasma energies range from 0.2 to >10 keV, and absorption ranges from log N H < 20.0 to ∼ 23.5 cm −2 . Comprehensive tables providing X-ray and stellar characteristics are provided electronically. We examine here the X-ray properties of Orion young stars as a function of mass; other studies of astrophysical interest will appear in companion papers. Results include: (a) the discovery of rapid variability in the O9.5 31 M ⊙ star θ 2 A Ori, and several early B stars, inconsistent with the standard model of X-ray pro-duction in small shocks distributed throughout the radiatively accelerated wind; (b) support for the hypothesis that intermediate-mass mid-B through A type stars do not themselves produce significant X-ray emission; (c) confirmation that low-mass G-through M-type T Tauri stars exhibit powerful flaring but typically at luminosities considerably below the 'saturation' level; (d) confirmation that the presence or absence of a circumstellar disk has no discernable effect on X-ray – 2 – emission; (e) evidence that T Tauri plasma temperatures are often very high with T ≥ 100 MK, even when luminosities are modest and flaring is not evident; and (f) detection of the largest sample of pre-main sequence very low mass objects showing flaring levels similar to those seen in more massive T Tauri stars and a decline in magnetic activity as they evolve into L-and T-type brown dwarfs.