Infra-red sources in the H II region W3

High resolution mapping and photometric observations in the wavelength range 1.65–20 µ have led to the discovery of nine distinct infra-red objects in W₃. Four of them, including one coincident with W₃ (OH), are identified with compact radio H II condensations ∼0.1 pc in diameter. They show strong emission in the range 3–20 µ which is attributable to dust grains at a temperature of ∼ 150 K mixed with the ionized gas. On the other hand, the 2-µ emission from these objects is considerably less than is predicted from the radio measurements, indicating that the H II condensations are surrounded by obscuring dust with up to 50 mag of visual extinction. One of the condensations contains an unresolved source which is almost certainly a highly obscured O star that excites the component. Four other infra-red sources have no associated radio continuum source; one of these, an unresolved source with a luminosity ∼3 × 10⁴ L⊙ is coincident with a source of H₂O maser emission and may be a massive protostar.