We search for low-mass pre-main sequence (PMS) stars, so-called T Tauri stars (TTS), among the large sample of unidentified ROSAT All-Sky Survey sources by means of ground-based optical follow-up observations. ROSAT source counterparts with Halpha emission and Lithium 6707 Å absorption together with a late spectral type (late F to M) can be regarded as young stars, as Lithium is burned in low-mass, i.e. fully convective, stars during the first few million years by proton-proton caption at the bottom of the convection zones.
For the newly identified young stars, we can then try to estimate their ages in two different ways:
1. Doing UBVRI photometry for estimating the bolometric luminosity and the effective temperature, so that we can place the stars into the Hertzspung-Russell diagram. Then, we can compare their location with theoretical isochrones and evolutionary tracks to estimate ages and masses. This has been done in the Chamaeleon star forming region (see Fig. 1).
2. We can obtain high-resolution spectra to estimate the exact Lithium abundance, so that we can compare these with Lithium abundances of zero-age main sequence (ZAMS) stars. Stars with more Lithium than ZAMS stars of the same spectral type are younger than ZAMS, hence T Tauri stars. This approach has been taken in an area south of the Taurus star forming region (see Fig. 2).
In both regions, we have found very young T Tauri stars even outside the clouds, where stars form. These young stars outside the clouds may have been ejected from their birth places by three-body encounters, so-called run-away T Tauri stars (e.g. the Chamaeleon clouds), or they may be members of the Gould Belt (e.g. south of Taurus).
R. Neuhäuser (1997) Science 276, 1363-1370
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