Using data from the ROSAT All Sky Survey (RASS), several recent studies (cf. Wichmann et al. 1996 , Alcalá et al. 1996a , Sterzik et al. 1995 , Neuhäuser et al. 1995b , Krautter et al. 1996 ) have addressed the problem of the large-scale spatial distribution of pre-main sequence (PMS) stars in star forming regions (SFRs). The exciting result of these studies was that in all the SFRs investigated, the population of X-ray selected weak-line T Tauri stars (WTTS) extends far beyond the regions of high obscuration, where the optically selected T Tauri stars (TTS) - mainly classical T Tauri stars (CTTS) - are located.
As pointed out by Herbig (1978 ), there should exist a numerous population of TTS that have evolved from the population of CTTS towards the main sequence (post-T Tauri-stars - PTTS) and therefore should show properties already quite similar to those of zero-age main sequence (ZAMS) dwarfs. Only few stars that might belong to this population have been identified so far, since many of the known WTTS are more or less coeval with the CTTS. Since PTTS are by necessity young, they are expected to be active, and hence X-ray surveys provide a powerful tool to search for PTTS candidates.
A comprehensive review of the previous work in Lupus is given by Krautter (1991 ). Some 69 possible PMS objects have been identified by Schwartz's H emission survey (Schwartz 1977 ). In a recent study Krautter et al. (1996 ) discovered some 130 new WTTS in the Lupus SFR on the basis of ROSAT X-ray data. We have studied these WTTS photometrically in order to determine their evolutionary status and their relation to the hitherto known population of PMS stars in the Lupus SFR. We are specifically interested in whether these stars represent the long searched-for PTTS or not. We also investigate the history of star formation in the Lupus SFR. Of special interest is the peculiar mass function of the hitherto known Lupus TTS population. We investigate the possibility that the initial mass function in Lupus is time-dependent, i.e. showing a trend towards lower masses with time. Furthermore, we study the relationship between the X-ray and non-X-ray characteristics of the Lupus TTS, and discuss the various correlations proposed in the literature.
© European Southern Observatory (ESO) 1997
Online publication: July 3, 1998