Infrared observations of NGC 3603^*

III. The enigmatic, highly reddened sources of IRS 9

European Southern Observatory, Casilla 19001, Santiago 19, Chile Institut für Theoretische Physik und Astrophysik, Univ. Würzburg, Am Hubland, 97074 Würzburg, Germany

Corresponding author: dnuernbe@eso.org

Received: 30 September 2002
Accepted: 18 March 2003

Abstract

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 NGC 3603 IRS 9A–C. These sources are located on the OB cluster facing side of the massive molecular cloud core NGC 3603 MM 2 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 IRS 9A–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 250 K and 1150 K for IRS 9A and 250 K and 700–750 K for IRS 9B / 9C. From the mid infrared fluxes we deduce lower limits for the gas + dust masses of the circumstellar envelopes: 0.1  for IRS 9A and   for IRS 9B / 9C. Estimates of the total luminosities are on the order of 10⁵  and 10³  for IRS 9A and IRS 9B / 9C, respectively. Finally, our interpretation of IRS 9A–C being protostars is supported by recent ATCA data. The measured 3 cm and 6 cm radio fluxes of IRS 9A can neither be explained by non-thermal emission nor by thermal dust emission. From the spectral index  ~ 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.