L - band ( 3 . 5 μ m ) IR - excess in massive star formation II . RCW 57 / NGC 3576

We present colour-colour and colour-magnitude diagrams for RCW 57 (NGC 3576) based on L-band data at 3.5 μm taken with SPIREX (South Pole Infrared Explorer), and 2MASS JHKs data at 1.25-2.2 μm. A total of 251 sources were detected. More than 50% of the 209 sources included in the diagrams have an infrared excess. The IR-excess is interpreted as coming from circumstellar disks and hence gives the cluster disk fraction. Comparison with other JHKsL surveys, including results on 30 Doradus from a previous paper, support a very high initial disk fraction (>80%) even for massive stars, although there is an indication of a possible faster evolution of circumstellar disks around high mass stars. 33 sources only found in the L-band indicate the presence of heavily embedded, massive Class I protostars. We also report the detection of diffuse PAHs emission throughout the