On the Nature of Collinder 121: Insights from the Low-Mass Pre-Main Sequence

Abstract

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 sight-line. de Zeeuw et al (1999) 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 sight-line, 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 the original description of Cr 121 (Collinder, 1931), and we argue that this distant compact cluster should retain its original designation. The moving group detected by de Zeeuw et al (1999) resembles a foreground association and we agree with Eggen (1981) that this should be called CMa OB2. This study demonstrates that although the de Zeeuw et al (1999) census of OB associations is an invaluable resource for studying local star formation, it must be interpreted in the context other data when considering structure over distances of the same order as the limits of the Hipparcos parallaxes.

arXiv comment8 pages, 6 figures, MNRAS accepted. Catalogues from this paper and others produced by the cluster collaboration are available from http://www.astro.ex.ac.uk/people/timn/Catalogues/description.html

References in corpus (9)

• A Hipparcos census of the nearby OB associations
• New catalogue of optically visible open clusters and candidates
• The SuperCOSMOS Sky Survey. Paper I: Introduction and Description
• The SuperCOSMOS Sky Survey. Paper III: Astrometry
• Photometry and membership for low mass stars in the young open cluster NGC 2516
• Optimal photometry for colour-magnitude diagrams and its application to NGC 2547
• The discovery of a low mass, pre-main-sequence stellar association around gamma Velorum
• XMM-Newton and VLA Observations of the Variable Wolf-Rayet Star EZ CMa: Evidence for a Close Companion?
• The discovery of low-mass pre main sequence stars in Cepheus OB3b

Cited by in corpus (14)

• 3D maps of the local ISM from inversion of individual color excess measurements
• The stellar association around Gamma Velorum and its relationship with Vela OB2
• Pre-main-sequence isochrones -- II. Revising star and planet formation timescales
• Empirical isochrones and relative ages for young stars, and the radiative-convective gap
• Pre-main-sequence isochrones -- I. The Pleiades benchmark
• The Keele-Exeter young cluster survey: I. Low mass pre-main sequence stars in NGC 2169
• Can variability account for apparent age spreads in OB association colour-magnitude diagrams?
• Do accretion discs regulate the rotation of young stars?
• Pre-main-sequence isochrones -- III. The Cluster Collaboration isochrone server
• Contamination and exclusion in the sigma Orionis young group
• A lithium depletion boundary age of 22 Myr for NGC 1960
• Characterising the i-band variability of YSOs over six orders of magnitude in timescale
• The Structure and the Distance of Collinder 121 from Hipparcos and Photometry: Resolving the Discrepancy
• Stellar clustering and the kinematics of stars around Collinder 121 using Gaia DR3