1994A&A...284..559O

We present high signal-to-noise, high angular resolution, submillimetre continuum and C¹⁸O (J=2->1) maps of the core of the W 3 molecular cloud. Continuum maps at 450 and 800µm reflect the distribution of the dust, but maps at 1100, 1300 and 2000µm are contaminated by free-free radiation from the HII radio sources: W 3(A),(B),(C) and (D). We adopt a suitable model for this emission which we then subtract from the continuum maps to reveal the true distribution of the dust. The continuum emission of the embedded infrared sources is distinguished from that of the molecular cloud, enabling us to model the quiescent cloud and the sources separately. An isothermal fit to the data gives an average dust emissivity β-index of ∼1 for the quiescent cloud, indicating that the grains are composed of amorphous carbon or silicate. No large-scale variation in β-index is found, with a maximum value of β=1.4 at the position of IRS5. We compare the morphology and physical properties as determined by the line and continuum maps. A striking difference is found around IRS4, which we attribute to temperature and optical depth effects. The submillimetre source IRS4 south is found to be cooler than the other sources and optically thick at 450µm. The maps are split into 3 regions for which we calculate the mass independently from C¹⁸O and 800µm data. Derived masses agree closely for an assumed average temperature of 35K and we derive a total mass for the core of 1700M_☉. We emphasize that the common assumption of a uniform temperature across a region such as W 3 is frequently invalid and can lead to an overestimate of the mass. The relative merits of line and continuum observations as mass tracers are discussed. C¹⁸O is a better tracer of mass, whilst continuum emission better identifies embedded sources.