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

We use the data of the Chandra Orion Ultradeep Project (COUP) to study thenearly 600 X-ray sources that can be reliably identified with optically wellcharacterized T Tauri stars (TTS) in the Orion Nebula Cluster. We detect X-rayemission from more than 97% of the optically visible late-type (spectral typesF to M) cluster stars. This proofs that there is no ``X-ray quiet'' populationof late-type stars with suppressed magnetic activity. All TTS with knownrotation periods lie in the saturated or super-saturated regime of the relationbetween activity and Rossby numbers seen for main-sequence (MS) stars, but theTTS show a much larger scatter in X-ray activity than seen for the MS stars.Strong near-linear relations between X-ray luminosities, bolometricluminosities and mass are present. We also find that the fractional X-rayluminosity rises slowly with mass over the 0.1 - 2 M_sun range. The plasmatemperatures determined from the X-ray spectra of the TTS are much hotter thanin MS stars, but seem to follow a general solar-stellar correlation betweenplasma temperature and activity level. The large scatter about the relationsbetween X-ray activity and stellar parameters seems to be related to theinfluence of accretion on the X-ray emission. While the X-ray activity of thenon-accreting TTS is consistent with that of rapidly rotating MS stars, theaccreting stars are less X-ray active (by a factor of ~2-3 on average) andproduce much less well defined correlations than the non-accretors. We discusspossible reasons for the suppression of X-ray emission by accretion and theimplications of our findings on long-standing questions related to the originof the X-ray emission from young stars.Comment: accepted for ApJS, COUP Special Issu