Proper motions of faint ROSAT WTT stars in the Chamaeleon region

Abstract

We present proper motions of 59 stars of the ROSAT All-Sky Survey (RASS) located in direction of the Chamaeleon star forming region (SFR) in the magnitude range B=5.1 - 17 mag. Proper motions of the fainter stars were newly derived utilizing survey Schmidt plates from the GSC II plate archive and from a set of special plates taken with the ESO Schmidt telescope. The vector point diagram (VPD) indicates that the certified WTT stars cluster away from the region occupied by the brighter pre-main-sequence stars (PMS) in Cha I. The distance to this new association is estimated at ~ 100 pc, sensibly smaller than the 150 pc generally assumed for the SFR. This yields an upper limit of 2 km s(-1) for the velocity dispersion of this new kinematic group. The de-reddened CM diagram of the group members suggest the WTT stars are still PMS objects, but older (3-30 Myr) and less massive than previous determinations. These revised age estimates, the newly derived group peculiar velocity, and current distance estimates to the Cha I/II/III complex would favour in-situ formation against that predicted by high velocity cloud models. Finally, based on a redetermination of the peculiar motions of stars and gas, we speculate that the whole SFR originated from the local Orion spur as a result of more classical mechanisms like interactions with the spiral arms. Based on observations collected at the European Southern Observatory (Chile) and on data from the Hipparcos astrometry satellite. Table~1 is available only in electronic form at the CDS via anonymous ftp at 130.79.128.5.