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Astron. Astrophys. 321, 492-496 (1997)

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1. Background

T Tauri stars (TTSs) were discovered more than 50 years ago (Joy 1945) and were soon recognized as very young low-mass stars (Ambartsumian 1947). Their observational properties have been subject of extensive reviews (Herbig 1962, Haro 1983, Appenzeller & Mundt 1989, Bertout 1989). Presently, there is general agreement that TTSs can be divided in two groups; the classical TTSs (CTTSs), and the weak TTSs (WTTSs) 1, depending on the strength of their optical emission lines. The most commonly used criterion for distinguishing between these two categories is the equivalent width of H [FORMULA]   in emission. While some authors use W(H [FORMULA]  ) [FORMULA] 10 Å  for defining the WTTS (Appenzeller & Mundt 1989), others use W(H [FORMULA]  ) [FORMULA] 5 Å (Herbig & Bell 1988). The quantitative H [FORMULA]   boundary between CTTSs and WTTSs remains somewhat arbitrary.

The classification of TTSs into two subtypes became necessary only after the launch of the Einstein Observatory (EO). A number of X-ray sources associated with molecular clouds were discovered occupying the same region in the H-R diagram as the previously known TTSs (e.g. Walter et al. 1988). However, they did not show the spectroscopic characteristics which defined the T Tauri class (Herbig 1962). The coverage provided by the EO of the nearest star-forming regions was very incomplete. This problem was solved with the advent of the ROSAT All-Sky Survey (RASS), which is spatially unbiased and has similar sensitivity than typical EO pointings. It was expected that RASS would reveal new WTTSs towards molecular clouds, but it came as a surprise the finding of hundreths of WTT candidates very far from the clouds (Alcalá et al. 1995, 1996; Neuhauser et al. 1995; Wichmann et al. 1996). Whether these new stars are really WTTSs, or post TTSs (PTTSs), or even young main-sequence (YMS) stars, is still an open question and a matter of ongoing debate (e.g. Briceño et al. 1997, Feigelson 1996). This paper revisits the classification of pre-main sequence (PMS) low-mass stars, trying to be as quantitative as possible on the basis of PMS evolution physics, with the ultimate goal of clarifying the nature of the young stars discovered by X-ray satellites.

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998
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