Axisymmetric outflows from single and binary stars

Summary

Abstract

Mechanisms that can produce axisymmetric planetary nebulae are reviewed. It is suggested that the “interacting winds” model, in the presence of a “density contrast” between the equatorial and polar directions, can produce most of the observed morphologies.

Mechanisms that can produce a density contrast are examined and it is suggested that binary stellar companions and even brown dwarfs and massive planets may play an important role in the generation of such a contrast, either via common envelope evolution or by spinning up the envelope of the AGB star. It is shown that the statistics of planetary nebulae of different morphological types are consistent with models which rely on the presence of binary companions.

Introduction

Axisymmetric outflows are associated with many nebulae (e.g. He 2-36, BI Cru, My Cn 18, IC 4406, K 3-72, Corradi & Schwarz 1993a, b, c; OH 17.7–2.0, La Bertre 1986; R Aquarii, Burgarella & Paresce 1992) and with Be stars. I will concentrate in the present review mainly on planetary nebulae (PNe).

An examination of the catalogue of narrow band images of Schwarz, Corradi and Melnick (1992) and other images in the literature reveals a few interesting morphologies. In some cases, almost perfect rings are observed (e.g. ScWe 3, ScWe 2, Schwarz, Corradi & Melnick 1992; Hen 1357, Bobrowsky 1993).