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 Xray 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 production 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 -2emission; (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.

Coronal X-Ray Emission from an Intermediate-Age Brown Dwarf

“…The resulting spectrum, shown in Figure 2, is unusually soft, with peak emission at 0.7 keV and no photons above 1.5 keV. Bestfit parameters with 90% confidence levels are temperature keV, line-of-sight column density L Ӎ 1 # 10 X powerful solar flare, the typical temperatures rise to T Ӎ 1-2 keV, much hotter than seen in TWA 5B but typical of other PMS stars and BDs (e.g., Feigelson et al 2002). A light curve of the photon arrival times was examined and no significant variations were found during the 11 ks duration of the observation.…”

“…However, X-rays from several young BDs with ages Myr t ∼ 1 old were discovered with ROSAT in nearby star-forming regions (Neuhäuser & Comerón 1998;Neuhäuser et al 1999;Mokler & Stelzer 2002). After the launch of the Chandra X-Ray Observatory, the number of X-ray-detected young BDs has increased an order of magnitude Imanishi, Koyama, & Tsuboi 2001a;Imanishi, Tsujimoto, & Koyama 2001b;Preibisch & Zinnecker 2001;Feigelson et al 2002). In young stellar clusters around 1 Myr old, hard and variable Xrays are now detected from about one-fourth of the BD population near the "saturation level" of .…”

The Origin of T Tauri X‐Ray Emission: New Insights from the Chandra Orion Ultradeep Project

“…This problem of small and often biased samples has, however, recently been overcome with two Chandra studies of the Orion Nebula Cluster (ONC) (Feigelson et al 2002a;Flaccomio et al 2003b), both of which found no significant relation between X-ray activity and rotation. This strongly puts into question the solar-like dynamo activity scenario for TTSs.…”

“…The ONC has been observed with both imaging instruments on board Chandra. The results of two Advanced CCD Imaging Spectrometer observations with a combined exposure time of 23 hr were reported in Garmire et al (2000) and Feigelson et al (2002aFeigelson et al ( , 2002bFeigelson et al ( , 2003. A total of 1075 individual sources were detected, 91% of which could be identified with known stellar members of the cluster.…”

Star formation history of Canis Major R1

“…In particular, following up on pioneering ROSAT observations, intermediate to distant clouds have been covered by Chandra: the Orion nebula (Garmire et al 2000;Feigelson et al 2002Feigelson et al , 2003Flaccomio et al 2003), part of the Rosette nebula and molecular cloud, together with M17 (Townsley et al 2003;Wang et al 2008Wang et al , 2009); Mon R2 (Kohno et al 2002); RCW38 (Wolk et al 2002); M16 (Linsky et al 2007), and by XMMNewton: M8 (Rauw et al 2002); Carina (Albacete Colombo et al 2003); Vela OB2 (Jeffries et al 2009); Orion (López-Santiago & Caballero 2008); and others. Up to thousands of point X-ray sources per cloud are detected, almost all identified with young stars, down to the brown dwarf regime.…”