Submillimetre continuum images of the NGC 2024 star-forming ridge

We present 450- and 800-mu m images, made with the James Clerk Maxwell Tele scope, of the NGC 2024 molecular ridge. The seven previously known compact cores, FIR1-7, have been detected, and FIR5 has been resolved into a compac t object and an associated extended source to the east. The estimated masse s of the dense cores vary between 1.6 and 5.1 M. per 14-arcsec beam, assumi ng a dust temperature of 30 K and a dust opacity of kappa 800 mu m = 0.002 m(2) kg(-1). A spectral index map made from the 450- and 800-mu m images sh ows spatial variations, with the spectral index, alpha (F-nu proportional t o nu(alpha)), bring systematically lower towards the dense cores. We interp ret this as evidence for a lower value of the frequency dependence of the d ust opacity, beta, towards the denser cores relative to the surrounding mol ecular material. This may indicate that grain growth is occurring in the co res, prior to planetesimal formation. By comparing the high-resolution 450- mu m image with interferometer maps of the integrated CS(2-1) emission, the previously reported discrepancy between dust continuum emission and molecu lar line emission is found to be very localized. Depletion and temperature variations are discussed as possible explanations.