Interpreting Jets from Young Stars

Abstract

The formation, propagation and termination of jets from young stellar objects (YSOs) is investigated. Although there is no consensus on their origin, YSO jets may initially form as the innermost, and most highly ionized part, of a centrifugally driven wind from an accretion disk. Such a disk seems mandatory if a jet is to form at all. While it is virtually certain that the emission from YSO jets comes from oblique shocks, it is shown that all models proposed so far to explain how these shocks arise (including the steady jet hypothesis, internal working surfaces and hydrodynamical instabilities) do not satisfactorily explain the observations. Finally we examine how jets terminate; the termination zone is perhaps the best understood part of the optical flow. While the gross features can be understood in terms of a simple Mach disk plus bow shock model, the origin of the knots seen in the region of the bow is not clear. Possible explanations for the latter include thermal and Rayleigh instabilities, inhomogeneities in the ambient medium and unsteadiness of the jet beam.

The true philosophers, Syrus, were, I think, quite right to distinguish the theoretical part of philosophy from the practical.

The Preface to Ptolemy’s Almagest