2003A&A...404..255N

In the framework of a multi-wavelengths study of NGC 3603 we have performed near and mid infrared imaging to investigate the nature and the evolutionary status of the highly reddened sources NGC3 603 IRS9 A-C. These sources are located on the OB cluster facing side of the massive molecular cloud core NGC 3603 MM2 and were apparently only recently revealed from most of their natal environment by strong stellar winds and energetic radiation originating from the nearby high mass cluster stars. On the basis of the steeply rising spectral energy distributions and the large near and mid infrared excess emission we conclude that IRS9 A-C represent (the brightest members of) a sparse association of high mass protostars, solely embedded in the gravitationally bound material of their circumstellar envelopes but largely blown free from gas+dust of the pristine molecular cloud core. Fitting blackbody functions to the overall spectral energy distribution at near and mid infrared wavelengths we identify two dust components at temperatures of 250K and 1150K for IRS9A and 250K and 700-750K for IRS9B/9C. From the mid infrared fluxes we deduce lower limits for the gas+dust masses of the circumstellar envelopes: >0.1M_☉ for IRS9A and >3x10^–3M_☉ for IRS9B/9C. Estimates of the total luminosities are on the order of 10⁵L_☉ and 10³L_☉ for IRS9A and IRS9B/9C, respectively. Finally, our interpretation of IRS9A-C being protostars is supported by recent ATCA data. The measured 3cm and 6cm radio fluxes of IRS9A can neither be explained by non-thermal emission nor by thermal dust emission. From the spectral index α_radio∼0.3 we conclude that the observed radio emission is thermal bremsstrahlung and most likely excited externally by ionizing photons originating from the nearby OB cluster.