The X-ray telescope ROSAT and its detectors are described in detail by
Trümper (1983) and Pfeffermann et al. (1988).
The Position Sensitive Proportional Counter (PSPC) on board ROSAT performed
an All-Sky Survey scanning the sky in great circles with a 
diameter field of view. Observations in our selected area have been
performed in August and September 1990 and in February 1991. Vignetting
corrected RASS exposure times vary roughly with 
where 
denotes ecliptic latitude. In our studied area, the exposure varies between
and 
seconds. During the RASS, all areas in the
sky have been observed in 
second long scans, separated by
minutes and spread over almost two days. The ROSAT PSPC has
255 instrumental energy channels sensitive from 0.1 keV to 2.4 keV
(broad band). The spectral resolution of the ROSAT PSPC instrument (
at 0.93 keV) permits a reasonable spectral analysis in three energy
bands:
In the flux-limited RASS, most of the previously known wTTS but only
few cTTS have been detected (Neuhäuser et al. 1995b). It has
been shown (Neuhäuser et al. 1995a,b) that wTTS and cTTS can
be discriminated by their X-ray spectral hardness ratios. If 
,
, and 
denote count rates for the ROSAT energy
channels soft, hard 1, and hard 2, respectively, we define two hardness
ratios as follows:
Figure 1: Taurus-Auriga: In our studied area (enclosed by a box) we find
new PMS stars (filled symbols), possible new PMS stars (stars), new
dKe/dMe stars (plusses) and other non-PMS stars (dots)
We have extracted from the merged photon event, exposure, and attitude
files of the RASS data those data which pertain to our studied area
south of the Taurus-Auriga dark clouds. This area is shown in
Fig. 1 (click here); it includes a region just south of the Taurus-Auriga dark
cloud complex (i.e. with 
) and an attached strip
perpendicular to the galactic plane. Neuhäuser et al.\
(1995c) refer to the same area, while the region north of 
has been searched for new TTS by Wichmann et al.\
(1996). Source detection and local background determination were
done separately in five different ROSAT energy bands: broad, soft, hard 1,
hard 2, and hard (0.5 to 2.1 keV). Detected sources were merged and tested
again with a maximum likelihood technique. Details on RASS data reduction
and source detection algorithm can be found in Neuhäuser et al.\
(1995b). These authors showed that all sources with a maximum
likelihood of existence of at least 7.4 can be accepted as real sources;
such a low likelihood value is justified as we search for unknown TTS which
may be X-ray faint.