The formation of stars is a complex process which is poorly understood at present, although recently important progress has been made both on a theoretical and observational ground. It is clear that star formation must proceed through contraction of a large molecular clump into a dense optically thick proto-stellar core: the obvious consequence is that conservation of angular momentum must force the material to spin up and flatten. Thus, formation of disks around newly formed stars is a very sensible expectation. Indeed, the recent development of instruments like the Hubble Space Telescope (HST) and the millimeter interferometers has allowed detection of several disks around low-mass young stellar objects (YSOs), such as the Keplerian disk in GG Tau (Guilloteau et al. 1999) and that seen with the HST in HH 30 (Burrows et al. 1996). The situation is quite different for high-mass YSOs. In this case, the evidence for disks is scarce, although a priori one would expect these to be more massive than those in low-mass YSOs and hence easier to detect. Various effects may complicate this simple-minded picture: for instance, magnetic field is likely to play an important role coupling the inner part of the collapsing cloud to its outer layers, thus making angular momentum conservation difficult to apply to any single “portion” of the cloud; depending on the ratio between disk and stellar mass, the disk may be unstable and hence short-lived; the effects of the stellar wind and radiation have to be taken into account; finally, the mass and size of the disk depend on the accretion process, which is not well understood. All these caveats probably explain why disks around massive YSOs are difficult to detect.
Hubble Space Telescope Imaging of the Disks and Jets of Taurus Young Stellar Objects