Massive star formation in the GMC G345.5+1.0: spatial distribution of the dust emission

Context. Massive condensations in giant molecular clouds (GMCs) are linked to the formation of high mass stars, which are the principal source of heavy elements and UV radiation, playing an important role in the evolution of galaxies. Aims. We attemp to make a complete census of massive-star formation within all of GMC G345.5+1.0. This cloud is located one degree above the Galactic plane and at 1.8 kpc from the Sun, thus there is little superposition of dust along the line-of-sight, minimizing confusion effects in identifying individual clumps. Methods. We observed the 1.2 mm continuum emission across the whole GMC using the Swedish-ESO Submillimetre Telescope (SEST) Imaging Bolometer Array (SIMBA) mounted on the SEST. Observations have a spatial resolution of 0.2 pc and cover 1. ◦ 8 × 2. ◦ 2 in the sky with a noise of 20 mJy beam −1 . Results. We identify 201 clumps with diameters between 0.2 and 0.6 pc, masses between 3.0 and 1.3 × 10 3 M� , and densities between 5 × 10 3 and 4 × 10 5 cm −3 . The total mass of the clumps is 1.2 × 10 4 M� , thus the efficiency in forming these clumps, estimated as the ratio of the total clump mass to the total GMC mass, is ∼0.02. The clump mass distribution for masses between 10 and 10 3 M� is well-fitted by a power law dN/dM ∝ M −α , with a spectral mass index α of 1.7 ± 0.1. Given their mass distribution, clumps do not appear to be the direct progenitors of single stars. Comparing the 1.2 mm continuum emission with infrared images taken by the Midcourse Space Experiment (MSX) and by the Spitzer satellite, we find that at least ∼20% of the clumps are forming stars, and at most ∼80% are starless. Six massive-star forming regions (MSFRs) embedded in clumps and associated with IRAS point sources have mean densities of ∼10 5 cm −3 , luminosities >10 3 L� , and spectral energy distributions that can be modeled with two dust components at different mean temperatures of 28 ± 5 and 200 ± 10 K.