1995A&A...297..418S

We establish a criterion for selecting low-mass, pre-main sequence star candidates from X-ray sources discovered in the ROSAT All-Sky Survey. X-ray properties and non-spectroscopic data (hardness ratios and X-ray to optical flux ratio) of 187 optically identified X-ray sources in the Orion star forming region are used as a training set for a non-parametric discrimination analysis. We show that high selection reliabilities of weak-line T-Tauri stars (wTTS) can be obtained with this method. We utilize the selection procedure to predict the large scale spatial distribution of TTS candidates in a 710deg² field around the Orion SFR. Five significant surface density enhancements are identified, four of them are well matched with OB subgroup associations (OB1a, λ-Ori, OB1b, OB1c). A dispersion time of 2-10Myr can be derived from their spatial extent, consistent with the ages of the stellar component in these regions. We suspect a young stellar cluster in the vicinity of NGC 1788, where a high concentration of TTS candidates resides. The largest fraction of the predicted wTTS population is distributed widely over an area many times greater than that of the molecular gas. If these sources really are wTTS, they must be either much older than usually assumed or have a high velocity dispersion.