Abstract The observational evidence for the presence of discs around protostars and young stars consists of spectral and polarimetric data from which the existence of circum-stellar discs are indirectly inferred and data in which discs are directly imaged. A review of both the direct and indirect evidence for discs is presented as well as summary of the properties of these discs and their relationship to bipolar outflows and stellar jets.

Introduction

This review of discs associated with protostars and young stars stars is limited to two types of young stellar objects (YSOs): infrared sources (objects that emit only at infrared wavelengths) and optically visible T Tauri and FU Orionis stars. Current star formation models suggest that discs should commonly be associated with protostars and young stars. In fact, the flattened nature of our Solar system provides strong circumstantial evidence that discs have played a role in the formation of at least one star and planetary system.

Interest in circum-stellar discs has been heightened by the recent discovery of bipolar molecular outflows and stellar jets associated with many YSOs. An attractive model for the collimation and generation of energetic outflows assumes that accretion of material onto a young star through a viscous disc ultimately powers this energetic phenomenon. Unfortunately, direct imaging of these discs has proven difficult and only a few circumstellar discs have been unambiguously detected. Instead the efforts to detect circumstellar discs have frequently uncovered evidence for much larger structures, often called “interstellar discs”, surrounding YSOs.