Mid-infrared interferometric observations of the high-mass protostellar candidate NGC 3603 IRS 9A

Abstract

We investigate the question of how high-mass stars form by combining mid-infrared observations from the worlds largest ground- and space-based telescopes and interferometers. The target of this study is IRS 9A, a promising candidate for the rare class of very young, high-mass protostars. In the first part of this work we present the immediate results and implications of the individual observations, and in the second part we try to devise a model for IRS 9A and its circumstellar structure that can account for these observations. We also make use of a publicly available grid of spectral energy distributions which has been calculated for a large number of protostellar objects. We find that neither geometrical models of the brightness distribution nor simple one-dimensional models for the density structure can explain IRS 9A's appearance in the mid-infrared. However, using radiative transfer models that comprise circumstellar disks and envelopes, we are able to simultaneously reproduce all our observational data. Moreover, the comparison with the grid of protostellar objects independently confirms IRS 9A to be a high-mass protostar. Hence our study provides further support to the idea that high-mass stars form in a similar manner to their low- and intermediate-mass counterparts.