2 Data sample and observations

2.1 The sample

In order to test the findings on previous samples of X-ray selected stars, we selected a new large sample of active nearby late type stars. To this end we used the data from the EUV all-sky survey obtained with the Wide Field Camera (WFC) on board the ROSAT satellite. The WFC survey is dominated by active late type stars, that constitute more than one third of the full sample (Pounds et al. 1993; Pye et al. 1995). Given that the absorption by the interstellar medium is very efficient at EUV wavelengths, the WFC survey is mainly made of stars in the solar neighbourhood and/or very active. Thus, it is an ideal database to start with.

From the 479 sources listed in the 2RE WFC catalogue (Pye et al. 1995) we selected 190 stars of spectral type between F and K and with magnitude mainly in the $5<\,V\,< 10$ range. We concentrated on this range of magnitudes for two reasons. On the one side stars brighter than 5$^{\rm th}$ magnitude are usually well studied and the relevant information on them can be found in the literature. On the other side stars weaker than 10$^{\rm th}$ magnitude are too faint to be studied with the telescopes we used, in particular with respect to the high resolution spectroscopic observations. Nevertheless, we left in our sample few brighter (one in the sub-sample presented here) and weaker (two in this sub-sample) stars. Moreover, we used the information found in the literature, in particular using the SIMBAD database, to exclude all stars that are already well known and for which high resolution spectroscopy is already available (e.g. many of the most known and active RS CVn binaries that are in the WFC catalogue). At the end we obtained a list of 104 stars for which, when we started this project, we could not find detailed information in the literature with respect to at least one of the spectral lines we were interested in, i.e. Li 6708Å, H$\alpha$ and Ca II H&K lines. Among the selected stars, HD71285 is present in Pounds et al. (1993) WFC-BSC catalogue but not in Pye et al. (1995) WFC-2RE catalogue. In fact the detection of HD71285, although real, is just below the acceptance criteria for the 2RE. Finally, we note that 13 stars (i.e. HD16699, HD35114, HD36869, HD124672, HD143937, HD156498, HD195434, SAO91772, SAO111210, SAO150508, SAO150676, SAO196024, SAO234124) are listed in the 2RE but not in the BSC catalogue, due to an improved detection algorithm (see Pye et al. 1995). Among the 104 stars of our list 51 were suitable for observations in the southern hemisphere and were indeed observed with the 1.4m ESO CAT telescope in various observing runs. Photometric observations of 45 stars were obtained with the 50cm ESO telescope, while for the remaining six stars of the sample we used the information obtained from either the HIPPARCOSoutput star catalogue or the literature.

2.2 Photometric observations

For all stars but six $UBV(RI)_{\rm c}$ multicolor photometry has been obtained and has been used to determine the physical parameters and the distance of the stars (see Sect. 3.4). In particular, the data presented in Table 1 were collected over the intervals 19 November - 03 December 1993 (N93), 24 November - 04 December 1994 (N94), 15-28 January 1995 (J95), 19 August - 03 September 1995 (A95) and 03-30 December 1996 (D96) at the European Southern Observatory (La Silla, Chile) by using the 50cm ESO telescope equipped with a single channel photon-counting photometer, a thermoelectrically cooled Hamamatzu R-943/02 photomultiplier and standard ESO filters matching the $UBV(RI)_{\rm c}$ system. Table 1 also includes data for the two short period eclipsing binaries HD9770 and HD195434, already published by Cutispoto et al. (1997a) and those of a preliminary study of some of the stars in our sample presented by Cutispoto et al. (1995). Finally, as our list includes a few well-known active stars, we could also use magnitude and colors obtained with the same instrument by our group and already available in the literature. The typical accuracy of our absolute photometry, details are given in Cutispoto (1998), is of the order of 0.01 magnitudes. For the six stars we were unable to observe, the V data are from the HIPPARCOSoutput star catalogue or from the literature; their visual magnitudes are marked with "*" in Table 1. Information on optical variability, that was detected for the first time in 15 stars of our sample, and is likely to be due to the presence of photospheric spots, is given in Table 2. There, together with the brightest magnitude ($V_{\rm m}$) and the light curve amplitude observed by us, we also give the brightest visual magnitude ($V_{\rm min}$) reported in the literature, in order to ascertain the presence of long-term variability. In the case of eclipsing binaries, the amplitude of the light curve refers to the out-of-eclipse data. A preliminary version of the photometric section of this paper was given by Cutispoto et al. (1998). The results presented here are based on a refined analysis of our photometric data.

  

Table 1: Star name, maximum luminosity and colors, epoch of observations, spectral type inferred from the colors, parallax measured by the HIPPARCOS satellite, photometric parallax

Epoch: (N93) = 19 Nov. - 03 Dec. 1993; (N94) = 24 Nov. - 04 Dec. 1994; (J95) = 15-28 Jan. 1995; (A95) = 19 Aug. - 03 Sep. 1995; (D96) = 03-30 Dec. 1996; (P1) = A&AS 89, 435; (P2) = A&AS 84, 397; (P3) = A&AS 115, 41; (P4) = A&AS 131, 321; (P5) = IBVS 4419.

  

Table 2: Star name, variable name, maximum luminosity reported in the literature ($V_{\rm min}$), maximum luminosity ($V_{\rm m}$) and amplitude of variation ($\Delta V$) observed by us, photometric or orbital period measured by us or from the literature (the latter are marked with *), $v\sin i$(kms^-1) observed by us, EW of the Ca I 6717.7 Å (EW Ca I) measured by us (mÅ)

Notes: (a) both components have $v\sin i$ >60; (b) the photometry refers to the whole system, the $v\sin i$ only to the primary component; (c) SAO 232842 is included; (d) the photometry refers to the whole system, the $v\sin i$ only to the secondary component; (e) used for the Ca I 6717.7 Å vs. B-V calibration.

2.3 Spectroscopic observations

High resolution spectroscopy has been obtained at ESO (La Silla, Chile) during two different runs, 20-27 January and 24-30 August 1995, using the Coudé Echelle Spectrometer (CES) fed by the 1.4m CAT Telescope. Coupling the CES with the Long Camera and a CCD detector, the set-up was fixed to a nominal resolving power of about 60 000 (H$\alpha$) and 65 000 (Li, Ca) for the January run and of about 110 000 (H$\alpha$) and 120 000 (Li, Ca) for the August run. In January we used a $1024\times 640$ pixel CCD, resulting in a scale 0.1 (red) and 0.06 (blue) Å/pixel. In August a $2048\times2048$ pixel CCD was employed, resulting in a scale of 0.07 (red) and 0.04 (blue) Å/pixel. Almost all integrations have a typical signal-to-noise ratio greater than 150-200 and were made primarily in three wavelength regions centered at 6705Å, 6560Å, and 3935Å (see Table 3). The data reduction was performed by using the IRAF package. After bias subtraction, the spectra were flat-fielded by using the spectrum of a quartz lamp. A Th-Ar lamp was used for wavelength calibration.