Introduction

Although circumstellar discs play an important role in many of the phenomena associated with star formation there is little direct evidence for them at optical wavelengths. In polarization studies of reflection nebulae surrounding young stars and protostars we have noticed a deviation in the expected polarization pattern that appear to indicate the presence of circumstellar discs. We call this feature the ‘polarization disc’.

Examples and properties of polarization discs

Figure 1 shows a polarization map of the reflection nebulosity illuminated by the star HL Tau. At large distances from the star the polarization pattern has the expected centrosymmetric form but in inner regions the pattern deviates to form an anomalous band running across the illuminating star which itself is linearly polarized. We identify this inner pattern, the so called polarization disc, with a circumstellar disc of dusty material. Table 1 gives a comprehensive list of objects possessing such discs and indicates any additional peculiarities in the polarization data.

The properties of polarization discs are summarized below. Obviously not all of these properties are found in every object but they seem to represent various facets of the same phenomenon.

1. (1) The polarization disc consists of an anomalous band of polarization centred on the apical region of reflection nebulae.

2. (2) This band is normally present regardless of the visibility of the central source.

3. […]