INTERSTELLAR MOLECULAR DISKS AROUND YOUNG STARS

We present a review of observational evidence for the existence of int erstellar and circumstellar molecular discs and compare the evidence w ith theoretical expectations for such objects. Molecular line, submill imetre dust continuum and infrared observations are used to describe t he structures of the molecular cores surrounding candidate sources of a wide range of luminosity (100-10(5) L.). The effects of molecular de pletion onto and evaporation off dust grains complicate the interpreta tion. The emission from the cores is elongated and frequently reveals a strong density concentration towards the centre (density varying app roximately as r-1.5 to r-2). However, fragmentation within an envelope can also occur. The structures appear as clumpy discs or bars which s ometimes but by no means always show evidence of rotation. Molecular m aterial 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 outf lows within these distances. In the more luminous objects the closest material appears to be expanding. Recent 3D numerical simulations offe r a good chance of explaining the observed features particularly if th ey can be generalised to include a magnetic field.