Infrared observations of NGC 3603^*

II. A 11.9 μm and 18 μm survey

¹ European Southern Observatory, Casilla 19001, Santiago 19, Chile
² Institut für Theoretische Physik und Astrophysik, Univ. Würzburg, Am Hubland, 97074 Würzburg, Germany
³ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany

Corresponding author: D. E. A. Nürnberger, dnuernbe@eso.org

Received: 2 July 2002
Accepted: 23 December 2002

Abstract

We present results of the first sub-arcsec resolution mid infrared survey of the southern hemisphere giant H ii region NGC 3603. We have observed selected fields in the vicinity of the OB cluster at wavelengths of 11.9 μm and 18 μm using TIMMI 2 mounted on the ESO 3.6 m telescope. These fields comprise areas with dense molecular cores, embedded near infrared sources as well as several OH, H₂O and CH₃OH maser sources, which give indications of ongoing star formation processes. We report the detection of 36 mid infrared point sources and additionally provide flux measurements for 42 knots of diffuse emission. In the area surveyed the protostar IRS 9A is found to be the most luminous source at both 11.9 μm and 18 μm. Located in its immediate vicinity two more sources (IRS 9B and IRS 9C) also exhibit significant 11.9 μm and 18 μm emission, thus providing further indications for IRS 9 being an association of protostars in its own right. Several other 11.9 μm point sources are related to near infrared sources with strong K-band excess emission and/or to maser sources, which classifies them as young sources, too. In contrast, the second strongest 11.9 μm source, IRS 4, appears to be in a more evolved stage. Towards the center of the OB cluster we observe mid infrared emission arising from the three Wolf-Rayet stars WR 43abc, providing evidence for dust production and/or the presence of plasma in their circum stellar envelopes. Spread all over the cluster, we detect a number of sources with mid infrared fluxes close to the sensitivity limit (~0.01 Jy) of our 11.9 μm data, which apparently have very red colours. We suggest that these sources are circum stellar disks which are externally heated by the nearby massive stars. Towards the south and west of the OB cluster, large amounts of diffuse emission are found closely correlated with ionized material. We identify at least 7 shocks and ionization fronts, reflecting the enormous impact of the fast stellar winds and ionizing photons, originating from the massive cluster stars, on the adjacent gas and dust. This is impressively emphasized by the shocked and ionized material associated with the heads of the two prominent pillars. Both pillars are easily seen in our 11.9 μm and 18 μm data: the western one rather prominent in emission, the eastern one more pronounced in absorption against a strong diffuse mid infrared background. Among those sources, for which our data do not reveal any point-like mid infrared counterpart, are IRS 1 as well as the three “proplyds”. However, at least for “proplyd” 3 we detect extended, rim-like 11.9 μm emission. Therefore, we consider it likely that NGC 3603's “proplyds” simply represent scaled-down versions of the neighbouring pillars, i.e. remnant density enhancements of the pristine molecular cloud which to date were able to resist the ionizing and photoevaporating radiation from the nearby OB stars.