A study of the Chamaeleon star-forming region from the ROSAT All-Sky Survey

We analyse the nature of the optical counterparts of theROSATall-sky survey (RASS)X-ray sources identifiedwith new weak-line T Tauri (WTTS) stars in the Chamaeleon star forming region (SFR). The newWTTS are distributed throughout the whole SFR, while the classical T Tauri stars (CTTS) are found only in the cloud cores. Adopting a distance of 150pc we derive the stellar parameters and place the new WTTS in the HR diagram. By comparison with theoretical pre-main sequence (PMS) evolutionary tracks, we find masses in the range of 0.2-2.5 M and ages from a few 105yr to 5×107yr. Many of the youngest WTTS are located far away from the main Chamaeleon dark clouds. By comparing the properties of the new WTTS with those of the previously known Chamaeleon members, 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 lowermass PMS stars; ii) for stellar masses between 2.5 and 0.5 M , the combined mass distribution of the PMS stars is consistent with the initial mass function (IMF) for field stars, but declines rapidly for masses between 0.5 and 0.1M , where the strongest selection effects are expected; iii) a weak trend for increasing age with increasing angular distance from the cloud cores is observed but we cannot establish an age segregation since very youngWTTSare also found far away from themolecular clouds; iv) the age distributions of the new WTTS and the Cha I population are nearly identical, while that of the Cha II population is shifted towards younger stars indicating that Cha II is probably in an earlier evolutionary phase as compared with Cha I and the new WTTS; v) no decrease of the number density of WTTS is observed with increasing distance to the clouds; vi) the level Send offprint requests to: J.M. Alcalá ? Based on observations collected at the European Southern Observatory at La Silla, Chile ?? Tables 1 and 2 are available electronically at the CDS via anonymous ftp 130.79.128.5 or http://cdsweb.u-strasbg.fr/Abstract.html of X-ray emission of the new WTTS is higher than that of the previously known Chamaeleon members, and the fraction of energy released 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 clusters. Finally, we discuss possible mechanisms which may give rise to the observed spatial distribution of the PMS stars in Chamaeleon.