Thomas Preibisch, Yong -Cheol Kim, Fabio Favata, Eric D. Feigelson, Ettore Flaccomio, Konstantin Getman, Giusi Micela, Salvatore Sciortino, Keivan Stassun, Beate Stelzer, Hans Zinnecker

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

Astrophysical Journal Supplements, COUP Special Issue, 160, 401-422 (2005)

The Chandra Orion Ultradeep Project (COUP) provides the most comprehensive dataset ever acquired on the X-ray emission of pre-main sequence stars. In this paper, we study the nearly 600 X-ray sources that can be reliably identified with optically well characterized T Tauri stars (TTS) in the Orion Nebula Cluster. With a detection limit of $L_{\rm X,min} \sim 10^{27.3}$~erg/sec for lightly absorbed sources, we detect X-ray emission from more than 97\% of the optically visible late-type (spectral types F to M) cluster stars. This proofs that there is no ``X-ray quiet'' population of late-type stars with suppressed magnetic activity. We use this exceptional optical, infrared, and X-ray data set to study the dependencies of the X-ray properties on other stellar parameters. All TTS with known rotation periods lie in the saturated or super-saturated regime of the relation between activity and Rossby numbers seen for main-sequence (MS) stars, but the TTS show a much larger scatter in X-ray activity than seen for the MS stars. Strong near-linear relations between X-ray luminosities, bolometric luminosities and mass are present. We also find that the fractional X-ray luminosity $L_{\rm X}/L_{\rm bol}$ rises slowly with mass over the $0.1 - 2\,M_\odot$ range. The plasma temperatures determined from the X-ray spectra of the TTS are much hotter than in MS stars, but seem to follow a general solar-stellar correlation between plasma temperature and activity level. The scatter about the relations between X-ray activity and stellar parameters is larger than the expected effects of X-ray variability, uncertainties in the variables, and unresolved binaries. This large scatter seems to be related to the influence of accretion on the X-ray emission. While the X-ray activity of the non-accreting TTS is consistent with that of rapidly rotating MS stars, the accreting stars are less X-ray active (by a factor of $\sim 2-3$ on average) and produce much less well defined correlations than the non-accretors. We discuss possible reasons for the suppression of X-ray emission by accretion and the implications of our findings on long-standing questions related to the origin of the X-ray emission from young stars, considering in particular the location of the X-ray emitting structures and inferences for pre-main-sequence magnetic dynamos.

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