SEARCH STRATEGY AND OBSERVATIONS

The high galactic latitude (b = -59^o) active star ER Eri}, despite not being an IRAS source, was observed during the PDS and classified as a WTT (Gregorio-Hetem et al. 1992, Torres 1998). Distinctly from our earlier search for the TWA region, where candidates were selected by their infrared emission, around ER Eri we used RASS sources. We began with an area of 10^o x 10^o} centered on ER Eri:

and later we enlarged it to about 20^o x 25^o:

in order to find the limits of the possible association. We also chose three control areas, two of them of 10^o x 10^o where no isolated PMS star was known to exist a priori . The first control area was chosen in the southern galactic hemisphere:

The second area was in the northern galactic hemisphere:

And the third one was in the direction of the known TWA, measuring 13^o x 14^o:

As possible optical counterparts, we selected stars brighter than about13 mag within the error boxes of the RASS, using the GSC and Tycho catalogues. We found 19, 46, 23, 33 and 23 X-ray sources with observable counterparts, respectively in the inner and outer areas around ER Eri, in the northern, in the southern and in the TWA regions. In the outer area around ER Eri we discarded, for observation, 10 bright stars, 9 being of the Bright Star Catalogue. These stars are, in general, well known and, in fact, none is a good candidate for their spectral type and space velocities. The only dwarf later than F is zeta¹ Ret, that has been studied recently by Peloso et al. (2000). These ten stars are included at the end of Table 1 and we take them into account when doing the statistics in this paper. We observed then 36 X-ray sources in this outer region. Figure 3 summarizes graphically our strategy around ER Eri. In September 1997 and January 1998 we observed the candidates, taking low resolution spectra using the B&C spectrograph installed at the Cassegrain focus of the 1.52 m ESO telescope at La Silla, Chile. The configuration used (grating of 1200 l/mm and the CCD #39) gives a final dispersion of 0.98 Å/pixel. In 1999, using the same telescope, we obtained high resolution spectra for the selected candidates, with the FEROS echelle spectrograph (Kaufer et al. 1999), which gives a total coverage of 4000 Å, and a resolving power of 50000. To complete the extended area around ER Eri we took also some spectra using the coudé spectrograph (grating of 600 l/mm; resolution of 9000; spectral coverage of 450 Å, centered at 6500 Å, using the CCD #101) of the 1.60 m telescope of the Observatório do Pico dos Dias (OPD), Brazil. UBV(RI)_C photometry for most of the selected stars (except in the southern control region) was obtained using FOTRAP (Jablonski et al. 1994) at the 0.60 m telescope of the OPD.