Observational implications of precessing protostellar discs and jets

We consider the dynamics of a protostellar disc in a binary system where th e disc is misaligned with the orbital plane of the binary, with the aim of determining the observational consequences for such systems. The disc wobbl es with a period approximately equal to half the orbital period of the bina ry and precesses on a longer time-scale. We determine the characteristic ti me-scale for realignment of the disc with the orbital plane as a result of dissipation. If the dissipation is determined by a simple isotropic viscosi ty then we find, in line with previous studies, that the alignment time-sca le is of the order of the viscous evolution time-scale. However, for typica l protostellar disc parameters, if the disc tilt exceeds the opening angle of the disc, then tidally induced shearing within the disc is transonic. In general, hydrodynamic instabilities associated with the internally driven shear result in extra dissipation that is expected to drastically reduce th e alignment time-scale. For large disc tilts the alignment time-scale is th en comparable with the precession time-scale, while for smaller tilt angles delta, the alignment time-scale varies as (sin delta)(-1). We discuss the consequences of the wobbling, precession and rapid realignment for observat ions of protostellar jets and the implications for binary star formation me chanisms.