SCUBA observations of the Horsehead nebula – what did the horse swallow?

We present observations taken with the Submillimetre Common-User Bolometer Array (SCUBA) on the James Clerk Maxwell Telescope (JCMT) of the Horsehead Nebula in Orion (B33), at wavelengths of 450 and 850 μm. We see bright emission from that part of the cloud associated with the photon-dominated region (PDR) at the ‘top’ of the horse’s head, which we label B33-SMM1. We characterise the physical parameters of the extended dust responsible for this emission, and find that B33-SMM1 contains a more dense core than was previously suspected, with a mass of ∼2 M⊙ in a region of 0.31 × 0.13 pc, and a peak volume density of ∼6 × 10 cm. We compare the SCUBA data with data from the Infrared Space Observatory (ISO) and find that the emission at 6.75-μm is offset towards the west, indicating that the mid-infrared emission is tracing the PDR while the submillimetre emission comes from the molecular cloud core behind the PDR. We calculate the virial balance of this core and find that it is not gravitationally bound but is being confined by the external pressure from the HII region IC434, and that it will either be destroyed by the ionising radiation, or else may undergo triggered star formation. Furthermore we find evidence for a lozenge-shaped clump in the ‘throat’ of the horse, which is not seen in emission at shorter wavelengths. We label this source B33SMM2 and find that it is brighter at submillimetre wavelengths than B33-SMM1. We calculate the physical parameters of SMM2 and find it has a mass of ∼4 M⊙ in a region 0.15 × 0.07 pc, with a peak volume density of ∼2 × 10 cm and peak column density of ∼9 × 10 cm. SMM2 is seen in absorption in the 6.75-μm ISO data, from which we obtain an independent estimate of the column density in excellent agreement with that calculated from the submillimetre emission. We calculate the stability of this core against collapse and find that it is in approximate gravitational virial equilibrium. This is consistent with it being a pre-existing core in B33, possibly pre-stellar in nature, but that it may also eventually undergo collapse under the effects of the HII region.