Large Scale Distribution of T Tauri Stars as Observed with the ROSAT X-ray Survey

Abstract

The X-ray satellite ROSAT has performed the first all-sky survey (RASS) in the soft (0.1 to 2.4 keV) X-ray domain detecting ~ 10⁵ sources. Optical follow-up observations of unidentified sources have also shown that a significant fraction of these X-ray sources are low-mass pre-main sequence stars, so-called T Tauri stars (TTSs). We report here on the discovery of hundreds of TTSs in several different SFRs Our target selection for ground-based optical follow-up observations of X-ray sources is based on the large sample of RASS sources. The advantage as compared to previous X-ray missions is the complete spatial coverage. The RASS flux limit is comparable with typical Einstein Observatory pointed observations. Most of the newly discovered TTSs are weak-line TTSs (wTTSs), one of their main characteristics is strong X-ray emission; the main characteristic of classical TTSs (cTTSs) is their infrared excess emission indicative of circumstellar nebulae and/or protoplanetary discs. Based on the spatially complete RASS, we can confirm that wTTSs outnumber cTTSs by a large factor. As new wTTSs are also found far from star forming cores, the question arises as to how and where they were formed. Two scenarios have been proposed: either they may have been formed locally in cloudlets which have depleted by now or in star forming regions where multiple protostellar systems lead to dynamical ejection events by close encounters. In the latter case, there should be mostly wTTSs far from SFRs, as effective disc stripping might take place during the encounter. In the former case, though, regions with many newly discovered wTTSs should also be populated by cTTSs. They can hardly be discovered with the flux-limited RASS. With deep ROSAT pointed observations, one can also detect previously known cTTSs and discovered hitherto unknown cTTSs, but only in a few selected fields. With an all-sky infrared survey, one should be able to discover many such cTTSs.