Introduction
Proto-planetary Nebulae
INTRODUCTION:
The protoplanetary nebula (PPN, plural PPNe) phase of stellar evolution is a short but important one. Sandwiched between the mass-losing Asymptotic Giant Branch (AGB) star, and the fully evolved planetary nebula, the PPN provides useful information about both stages. The spherically symmetric, pulsating mass loss from the AGB is followed by a large outburst with complex symmetries, during which ~0.1 MSun of atomic or molecular gas is lost. During the PPNe stage, this complex outburst interacts with the spherical remnants of the pulsations. This interaction is responsible for the wide array of observed planetary nebula morphologies. Studies of PPNe help us to understand these evolved morphologies. In a similar way, many of the properties of a PPN are fossils of the mass-losing phase of the AGB star which produced it. Observations of PPNe further constraint the problems of wind-driving and time variability which are still incompletely solved. Although their existence was long theorized, teh discovery of PPNe is relatively recent, since they are in general highly obscured sources. The IRAS survey of 1983 has been used to identify a large number of PPNe. These objects hav ebeen further studied in molecular lines, as well as in the near-IR and the optical. Attention has been primarily focused in the IR and mm wavelengths, since teh dusty envelope tends to be bright in the IR, and since the molecular evolution of these objects is fundamental to problems of planetary nebula composition and ISM enhancement.
Recent observations of both post-AGB stars (Palen 1999) and fully evolved planetaries (Balick and Hajian 1999) indicate that the influence of magnetic fields in PPNe may be far from trivial in some objects. In addition to enhancing maser action via the Cook mechanism, these magnetic fields may be responsible for channeling the outflow along shell walls, and into the bipolar symmetries so commonly observed in evolved planetaries. Creating a magnetic field at these distances from the central star has always been a bit of a puzzle. Given that the dust grains seem to be carrying charge, it is possible to generate local magnetic fields due to the motions of these charged particles (currents). In addition, the magnetic fields may be primordial, and carried out from the central star by the outflowing mass, in a similar manner to that in which they were collected from the ISM during the star formation phase (?). Until recently, the magnetic fields were hoped to have only a small effect on the character of the objects, and thus have been conspicuously absent from dynamical treatments of the envelope evolution. When combined with recent observations, the continued failure of hydrodynamic models to adequately and robustly explain all morphological characteristics leads us to the conclusion that magnetic fields must be present, and must be more important than previously considered. No complete treatment of the magneto-hydrodynamical problem has ever been conducted. The fundamental science is difficult and time-consuming, but also unavoidable.