A STUDY OF THE CHAMELEON STAR-FORMING REGION FROM THE ROSAT ALL-SKY SURVEY .2. THE PRE-MAIN-SEQUENCE POPULATION

We analyse the nature of the optical counterparts of the ROSAT all-sky survey (RASS) X-ray sources identified with new weak-line T Tauri (WT TS) stars in the Chamaeleon star forming region (SFR). The new WTTS ar e distributed throughout the whole SFR, while the classical T Tauri st ars (CTTS) are found only in the cloud cores. Adopting a distance of 1 50pc we derive the stellar parameters and place the new WTTS in the HR diagram. By comparison with theoretical pre-main sequence (PMS) evolu tionary tracks, we find masses in the range of 0.2-2.5 M. and ages fro m a few 10(5)yr to 5 x 10(7)yr. Many of the youngest WTTS are located far away from the main Chamaeleon dark clouds. By comparing the proper ties of the new WTTS with those of the previously known Chamaeleon mem bers, we obtain the following results: i) the new WTTS are, on average , the more massive and luminous PMS stars in Chamaeleon, while the Cha II population contains the lower-mass PMS stars; ii) for stellar mass es between 2.5 and 0.5 M., the combined mass distribution of the PMS s tars is consistent with the initial mass function (IMF) for field star s, but declines rapidly for masses between 0.5 and 0.1 M., where the s trongest selection effects are expected; iii) a weak trend for increas ing age with increasing angular distance from the cloud cores is obser ved but we cannot establish an age segregation since very young WTTS a re also found far away from the molecular clouds; IV) the age distribu tions of the new WTTS and the Cha I population are nearly identical, w hile that of the Cha II population is shifted towards younger stars in dicating that Cha II is probably in an earlier evolutionary phase as c ompared with Cha I and the new WTTS: v) no decrease of the number dens ity of WTTS is observed with increasing distance to the clouds; vi) th e level of X-ray emission of the new WTTS is higher than that of the p reviously known Chamaeleon members, and the fraction of energy release d as X-ray emission, is higher in the new WTTS than in the Cha I TTS. The latter is similar to the X-ray emission level found in open cluste rs, Finally, we discuss possible mechanisms which may give rise to the observed spatial distribution of the PMS stars in Chamaeleon.