(1) Astronomisches Institut der Unicersität Würzburg, Germany
(2) Thüringer Landessternwarte, Tautenburg, Germany
(3) Astronomisches Recheninsititut. Heidelberg, Germany
(4) Astrophysikalisches Institut Potsdan, Germany
Nearly 150 T Tauri stars have recently been found in the Scorpius
Centraurus (ScoCen) OB association by follow-up observations of ROSAT
and Einstein X-ray sources. However, before drawing any conclusions
from this sampls, e.g. about the mass function, it is very important to
know how complete the X-ray selected sample of T Tauri stars is.
publsihed in "Abstract Series of the Astronomical Geselschaft"
No. 13, 1997, p. 23
Strong evidence for the existence of a population of "X-ray quiet"
T Tauri stars has recently been found for the Orion nebula region: Wolk
(1996) could find new T Tauri stars not only amoung X-ray selected
stars but also amoung non-X-ray selected stars with similar positions
in the color-magnitude-diagram. This surprising results implies that
the X-ray selected sample might be considerably incomplete.
In order to search for X-ray quiet T Tauri stars in ScoCen, we have
perfomed a Lithium survey using the multiobject spectrograph FLAIR at
the UK Schmidt Telescope. We have taken optical spectra for 115 stars
that were not detected as X-ray sources in the ROSAT All Sky Sky
Survey but have proper motions indicating membership in ScoCen. The
width of the Lithium 6708 A line, which is known to be a good
indicator for stellar youth, has been used to discern bewteen T Tauri
stars and older stars.
Our result is that none of these 115 stars seems to be a T Tauri star.
Most of the stars show no detectable or very weak Lithium lines
(W(Li)<0.1 A), about 15% show Lithium lines with similar strength as
found for Plejades. This are presumably young (zero-age?) mani-sequence
stars or perhaps post T Tauri stars which might be related to the
Gould Belt system.
Thus, we find no evidence for the existence of a siginficant
popultaion of X-ray quiet T Tauri stars in ScoCen and conclude that
the X-ray selected sample of T Tauri stars seems to be essentially
complete down to Lbol=0.5Lo. This implies that basically all ScoCen
T Tauri stars with 0.5