Abstract. We present first results obtained by a survey of the Lupus star forming region in search of new T Tauri stars. This study has been performed on the basis of deep pointed ROSAT observations in the Lupus dark clouds as well as data from the ROSAT All-Sky-Survey in the surrounding, less obscured regions. Our survey covers an area of about 230 square degrees, located between 15h 6 m and 16 h 24 m in right ascension and between −47• and −32• in declination. Identification of ROSAT All-Sky-Survey sources in this area by means of optical spectroscopy revealed 89 T Tauri stars, 86 of them "weak-line" T Tauri stars (WTTS) not known from previous studies of this region. Our pointed ROSAT observations led to the identification of 47 more T Tauri stars, giving a total of 136 new T Tauri stars. The large area of our study, as compared with previous works, allows us to study the spatial distribution of WTTS in this star forming region on a large scale. We find the new WTTS to be distributed over the whole area of our survey, indicating that their spatial distribution might extend well beyond our study area. Contrary to the Lupus T Tauri stars known prior to this study, the WTTS discovered by the ROSAT All-SkySurvey are not clustered in the regions of highest extinction, i.e. the dark clouds.

Abstract. -We present results of the spectroscopic and photometric follow-up observations of the ROSAT allsky survey in the direction of the Orion cloud complex. The main goal of these observations is the search for X-ray emitting pre-main sequence stars. 820 X-ray sources were detected with high confidence in about 450 square degrees. The mean density of X-ray sources in this region is a factor of about two higher than that of the whole RASS. 5% of the RASS sources in this region are identified with previously known and likely pre-main sequence stars. We have investigated spectroscopically 181 new RASS sources widely distributed over the entire cloud complex. On the basis of the presence of strong Li I λ6707 absorption, spectral type later than F0 and chromospheric emission, 112 new weak-line T Tauri stars could be found. We present coordinates, X-ray count-rates and finding charts of the new PMS. Optical UBV (RI)KC, near-infrared JHKLM and uvby-β photometry for the new WTTS is also provided. In addition 24 dKe-dMe stars were also found on the basis of the RASS data.

Aims. We report on our analysis of the high resolution spectra (R ≈ 86 000) of a sample of 42 late-type active stars (with measured log R HK spanning from ≈−3 to ≈−5) acquired with the Italian 3.6 m Telescopio Nazionale Galileo (TNG) using the SARG spectrometer in the 4960−10 110 Å range. The high quality of the spectra and the good activity-level coverage allow us to measure two different chromospheric indicators that can be derived from the Ca ii infrared triplet (Ca ii IRT) lines: the residual equivalent width (EQW) and the chromospheric indicator R IRT . The aim of this work is determine and test the best way of deriving activity-level information and errors from the Ca ii IRT lines, in preparation of the GAIA Cornerstone mission by ESA, by which the Ca ii IRT spectral range will be spectroscopically observed for millions of stars. Methods. The R IRT index is calculated for each observed star as the difference between the calculated NLTE photospheric central intensity and the observed one. The residual EQW, ∆W IRT , is calculated as the area of the positive profile obtained as the difference between the calculated NLTE photospheric and the observed profiles. We correlate log R HK with R IRT and the ∆W IRT .Results. This analysis indicates that Ca ii IRT lines are good chromospheric diagnostics. We find that both ∆W IRT and the R IRT quantities can be used as chromospheric indicators, although the former exhibits a tighter correlation with the log R HK index. Furthermore, we find that the total chromospheric excess EQW in the Ca ii IRT is almost linearly correlated with the excess in the Ca ii H & K doublet, as estimated through the log R HK index.

Abstract. This work is part of a larger project on the study of activity in stars of spectral type similar to, or later than the Sun, from PMS to ZAMS, based on the analysis of the high resolution Ca  InfraRed Triplet (Ca  IRT: λ = 8498, 8542, 8662 Å) observed profiles. Here, a preliminary study on the diagnostic power of these calcium lines has been performed by means of NLTE calculations of the line profiles with an approximate treatment of UV line-blanketing, for a grid of photospheric models with T eff = 4200, 5200, 6200 K, log g = 4.0, 4.5, 5.0 and [A/H] = 0.0, −1.0, −2.0. We used these calculations to estimate the sensitivity of the profiles to changes in stellar parameters and the effect of departures from LTE. As found by other authors, the Ca  triplet NLTE EQWs (equivalent widths) are quite sensitive to photospheric parameters, in particular to T eff and [A/H]. On the other hand, we find that the dependence of the Ca  triplet lines central depression (CD = 1-central relative flux) on log g and T eff , and to a lesser extent to [A/H], is very weak. The departure from LTE is negligible when we consider EQWs, unless very metal-poor atmospheres are considered, while CDs can be affected by NLTE by more than 20%. This analysis indicates that in the use of these lines as activity indicators (where the details of the line profile in the core are important), a NLTE treatment is required. Furthermore, we show that a new chromospheric indicator, which we denote R IRT , can be derived from measurements of Ca  IRT line central depressions, provided that rotational broadening is taken into proper account. In order to facilitate the use of the Ca  IRT lines as activity diagnostics, we give interpolation formulae for estimating line CDs within the range of stellar parameters of our NLTE calculations.