The Nature of X-Rays from Young Stellar Objects in the Orion Nebula Cluster -- A Chandra HETGS Legacy Project

The Orion Nebula Cluster (ONC) is the closest site of very young (~ 1 Myrs) massive star formation. The ONC hosts more than 1600 young and X-ray bright stars with masses ranging from ~ 0.1 to 35 M_. The Chandra HETGS Orion Legacy Project observed the ONC with the Chandra high energy transmission grating spectrometer (HETGS) for $2.1\,$Ms. We describe the spectral extraction and cleaning processes necessary to separate overlapping spectra. We obtained 36 high resolution spectra which includes a high brilliance X-ray spectrum of θ¹ Ori C with over 100 highly significant X-ray lines. The lines show Doppler broadening between 300 and 400\;km\;s⁻¹. Higher spectral diffraction orders allow us to resolve line components of high Z He-like triplets in θ¹ Ori C with unprecedented spectral resolution. Long term light curves spanning ~20 years show all stars to be highly variable, including the massive stars. Spectral fitting with thermal coronal emission line models reveals that most sources show column densities of up to a few times 10²²\,cm⁻² and high coronal temperatures of 10 to 90 MK. We observe a bifurcation of the high temperature component where some stars show a high component of 40 MK, while others show above 60 MK indicating heavy flaring activity. Some lines are resolved with Doppler broadening above our threshold of ~200\;km\;s⁻¹, up to 500\;km\;s⁻¹. This data set represents the largest collection of HETGS high resolution X-ray spectra from young pre-MS stars in a single star-forming region to date.