2002MNRAS.336..621L

The combination of mid-infrared data from the MSX satellite mission and ground-based near-infrared photometry is used to characterize the properties of the mid-infrared population of the Galactic plane. The colours of the youngest sources still heavily embedded within their natal molecular clouds are in general different from those of evolved stars shrouded within their own dust shells. Our main motivation is to use MSX for an unbiased search for a large (∼1000) sample of massive young stellar objects (MYSOs). A simple analysis shows that the MSX point-source catalogue should contain most of the MYSOs within our Galaxy. We develop colour-selection criteria using combined near- and mid-infrared data for MYSOs, which produces a list of 3071 objects, excluding the Galactic Centre region. The programme of follow-up observations already under way to separate the MYSOs from compact HII regions and other remaining objects is briefly described. We also show that these data can be used, just as IRAS data have been previously, to provide a separation between evolved stars with carbon-rich and oxygen-rich dust. These data may also be used to search for evidence of dust around normal main-sequence stars, such as low-mass pre-main-sequence stars or the Vega-excess class of objects where debris discs are presumed to remain from the planet formation process. We discuss the accuracy and completeness of the MSX point-source catalogue, and show that the errors present tend to be of a kind that is not significant for the main stellar populations we discuss in this paper.