Interstellar molecular disks around young stars

We present a review of observational evidence for the existence of interstellar and circumstellar molecular discs and compare the evidence with theoretical expectations for such objects. Molecular line, submillimetre dust continuum and infrared observations are used to describe the structures of the molecular cores surrounding candidate sources of a wide range of luminosity (100-10(5) L.). The effects of molecular depletion onto and evaporation off dust grains complicate the interpretation. The emission from the cores is elongated and frequently reveals a strong density concentration towards the centre (density varying approximately as r-1.5 to r-2). However, fragmentation within an envelope can also occur. The structures appear as clumpy discs or bars which sometimes but by no means always show evidence of rotation. Molecular material is observed very close to the central stars (within 10(3) AU). A simple extrapolation of the deduced densities towards the centre as r-2 does not appear sufficient to confine the observed molecular outflows within these distances. In the more luminous objects the closest material appears to be expanding. Recent 3D numerical simulations offer a good chance of explaining the observed features particularly if they can be generalised to include a magnetic field.