The apparent overdensity of open clusters in the Canis Major overdensity

Abstract

The exciting debate on the existence and nature of the relatively recent discovery of the so-called Canis Major (CMa) overdensity is re-examined here based on the positions, reddenings, ages and metallicities of Galactic open clusters (OCs). The data used to carry out the current study were taken from the recently updated version of the Dias et al.'s 1776 OC catalogue. We found that only Tombaugh 2 is physically located within the main body of CMa. Even if we take into account the whole sample of catalogued OCs with unknown distances, it is statistically expected that only one additional OC could be found within the CMa region. Since the CMa overdensity appears to be quite transparent to dust, only a few OCs in that zone could have been missed. Both metallicity and age distributions of OCs located in the Galactic outer disc, including Tombaugh 2 and those projected on to CMa, are in good agreement with the paradigm of a main dispersion of age and metallicity values. There are only marginal indications for a radial abundance variation. We conclude that CMa does not contain a significant population of OCs which share its properties. This result does not favour the dwarf galaxy scenario. © 2008 The Authors. Journal compilation © 2008 RAS.