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Astron. Astrophys. 325, 613-622 (1997)

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1. Introduction

The T-association Taurus-Auriga (Tau-Aur) is a large star forming region with several hundreds of pre-main sequence stars (PMS) known. Its distance of only about 140 pc (Elias 1978, Kenyon et al. 1994) makes it an ideal object for studying the properties of the cloud resulting from ongoing star formation, e.g. its stellar content, age, distribution and motion of the stars and so on.

Before the launch of ROSAT about 150 T Tauri stars which are associated with Taurus-Auriga have been known, most of them classical T Tauri stars (CTTS). The ROSAT All-Sky Survey (RASS) and some deep ROSAT pointings have revealed so far 4 new CTTS and 82 weak-line T Tauri stars (WTTS) in the central region of Taurus-Auriga, between [FORMULA] and [FORMULA] in right ascension and [FORMULA] and [FORMULA] in declination (Wichmann et al. 1996, Strom & Strom 1994, Carkner et al. 1996). Both CTTS and WTTS show the characteristic strong Li I [FORMULA] 6708 Å absorption line, but in contrast to CTTS, WTTS are stronger X-ray-emitters (Neuhäuser et al. 1995a) and so easier to detect by ROSAT. WTTS however lack the strong H [FORMULA] emission, Ca II emission and IR-excess of the CTTS, most of which were discovered by objective prism surveys.

In addition to these new TTS discovered in the central region of Taurus-Auriga pre-main sequence stars have also been found in a large area south of the Taurus-Auriga dark clouds (Neuhäuser et al. 1995b, Magazzù et al. 1997, Neuhäuser et al. 1997).

Proper motions of all these stars are required to kinematically investigate their membership to the association and to address the question whether it is possible that the stars outside the commonly adopted boundaries of Taurus-Auriga (based on CO surveys (Ungerechts & Thaddeus 1987) ) have formed near the centre of the region or rather near their present locations. Until now proper motions have been known only for about 70 mostly classical T Tauri stars in selected areas inside the large region (Jones & Herbig 1979, Walter et al. 1987, Hartmann et al. 1991, Gomez et al. 1992).

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© European Southern Observatory (ESO) 1997

Online publication: April 28, 1998