The structure of dust discs around G35.2N, NGC 2071 and LkHα234

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Abstract
Thermal continuum emission from the dust around three young luminous stars has been mapped using the James Clerk Maxwell Telescope. In all three cases, the isophotes are elongated perpendicular to known bipolar flows, and are centred on the exciting star. Interpreting this as evidence of symmetric discs or toroids, we derive the density structure of the dust surrounding the three stars. In two cases (G35.2N and NGC 2071) model fits to the structure observed along the disc plane show density decreasing as r−β in the region 0.01–1.0 pc (β = 0.5 and 1.0, respectively). In addition both of these objects have evidence of excess emission extended perpendicular to the plane at shorter wavelengths, indicating the presence of hot dust in the outflow region. If this is heated by the mechanical energy of the stellar wind, the results would imply a wind mechanical luminosity 5–40 per cent of the bolometric luminosity – considerably larger than that estimated from the molecular observations. The third object studied – LkHα234 – has a dust ring of radius ≈ 0.15 pc, with a mass 10 times that of the material inside it. This confirms that LkHα234 is a more evolved object, as most of the surrounding disc material has been forced out. By comparing the results with published ammonia data, the dust morphologies indicate that ammonia is depleted within 0.1 pc of all three luminous stars.

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