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

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Quantitative spectroscopic criteria for the classification of pre-main sequence low-mass stars

E.L. Martín

Instituto de Astrofísica de Canarias, E-38200 La Laguna, Tenerife, Spain

Received 1 October 1996 / Accepted 7 November 1996


The discovery of hundreths of X-ray emitting stars possibly associated with pre-main sequence (PMS) low-mass stars far from molecular clouds, makes it necessary to adopt quantitative spectroscopic criteria for classifying them. T Tauri stars have young ages ([FORMULA]  10 Myr) and low masses (M [FORMULA] 2 [FORMULA]  ). As a consequence, it is shown that they must verify two spectroscopic conditions: (1) spectral type in the range K0-M6, and either (2a) strong emission lines and UV-optical-NIR continuum excesses, or (2b) weak-emission lines and a photospheric Li I [FORMULA] 670.8  absorption feature with a "minimum" equivalent width which depends on its spectral type. Classical T Tauris meet criteria (1) and (2a), while weak T Tauris (WTTS) meet criteria (1) and (2b).

T Tauri stars occupy a different region in the [FORMULA]   - [FORMULA] diagramme than the low-mass members of young open clusters. Post T Tauri stars (PTTSs) later than about K2 can be clearly identified in the same diagramme because they fill an empty region (PTT-gap), intermediate between the T Tauris and the young cluster stars. The application of the spectroscopic criteria defined in this work to the PMS stars claimed to have been discovered in recent X-ray surveys of molecular clouds is hampered by the lack of high-resolution optical spectra for most of them. On the basis of the sparse and modest-resolution data that is available, the preliminary results indicate that the majority of these stars ([FORMULA] 60%) are not WTTSs. Only [FORMULA] 25% of the non-WTTS X-ray discovered stars are clearly PTTSs according to this study. The PMS status of the remaining stars is dubious. It seems unlikely that the PTTSs indentified in X-ray surveys outnumber the T Tauri stars. Far away from molecular clouds, the number of WTTSs and PTTSs appear to decrease significantly.

Key words: stars: pre-main sequence; fundamental parameters – X-rays: stars

Send offprint requests to: E.L. Martín, e-mail: ege@iac.es

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998