On the nature of Collinder 121: insights from the low-mass pre-main sequence

We present a VI photometric catalogue towards the open cluster Cr 121. XMM-Newton and ROSAT data are used to discover a low-mass pre-main sequence (PMS) along this sightline. de Zeeuw et al. have identified Cr 121 as a moving group, using Hipparcos data, at a distance of 592 pc; we reject the scenario that these low-mass PMS stars are associated with that association. By considering the higher mass main sequence stellar membership of the groups along this sightline, the density of low-mass PMS stars and their age spread we argue that the low-mass PMS stars are associated with a young, compact cluster at a distance of 1050 pc. This is consistent with Collinder's original description of Cr 121, and we argue that this distant compact cluster should retain its original designation. The moving group detected by de Zeeuw et al. resembles a foreground association and we agree with Eggen that this should be called CMa OB2. This study demonstrates that although the de Zeeuw et al. census of OB associations is an invaluable resource for studying local star formation, it must be interpreted in the context of other data when considering structure over distances of the same order as the limits of the Hipparcos parallaxes.