X-ray Properties of Low-Mass Pre-Main Sequence Stars in the Orion Trapezium Cluster

Kavli Affiliate: David P. Huenemoerder

| First 5 Authors: Norbert S. Schulz, David P. Huenemoerder, Moritz Guenther, Paola Testa, Claude R. Canizares

| Summary:

The Chandra High Energy Transmission Gratings (HETG) Orion Legacy Project
(HOLP) is the first comprehensive set of observations of a very young massive
stellar cluster which provides high resolution X-ray spectra of very young
stars over a wide mass range (0.7 – 2.3 Msun). In this paper, we focus on the
six brightest X-ray sources with T Tauri stellar counterparts which are
well-characterized at optical and infra-red wavelengths. All stars show column
densities which are substantially smaller than expected from optical extinction
indicating that the sources are located on the near side of the cluster with
respect to the observer as well as that these stars are embedded in more dusty
environments. Stellar X-ray luminosities are well above $10^{31}$ erg/s, in
some cases exceeding $10^{32}$ erg/s for a substantial amount of time. The
stars during these observations show no flares but are persistently bright. The
spectra can be well fit with two temperature plasma components of 10 MK and 40
MK, of which the latter dominates the flux by a ratio 6:1 on average. The total
EMs range between 3 – 8$times10^{54}$ cm$^{-3}$ and are comparable to active
coronal sources. Limits on the forbidden to inter-combination line ratios in
the He-Like K-shell lines show that we observe a predominantely optically thin
plasma with electron densities below $10^{12}$ cm$^{-3}$. Observed abundances
compare well with active coronal sources underlying the coronal nature of these
sources. The surface flux in this sample of 0.6 to 2.3 Msun classical T Tauri
stars shows that coronal activity and possibly coronal loop size increase
significantly between ages 0.1 to 10 Myrs.

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