▪ Abstract We review the state of observational and theoretical studies of the shaping of planetary nebulae (PNe) and protoplanetary nebulae (pPNe). In the past decade, high-resolution studies of PNe have revealed a bewildering array of morphologies with elaborate symmetries. Recent imaging studies of pPNe exhibit an even richer array of shapes. The variety of shapes, sometimes multiaxial symmetries, carefully arranged systems of low-ionization knots and jets, and the often Hubble-flow kinematics of PNe and pPNe indicate that there remains much to understand about the last stages of stellar evolution. In many cases, the basic symmetries and shapes of these objects develop on extremely short timescales, seemingly at the end of AGB evolution when the mode of mass loss abruptly and radically changes. No single explanation fits all of the observations. The shaping process may be related to external torques of a close or merging binary companion or the emergence of magnetic fields embedded in dense outflowing stellar winds. We suspect that a number of shaping processes may operate with different strengths and at different stages of the evolution of any individual object.

We report new results from high spatial resolution Wide Field Planetary Camera 2 imaging studies of "FLIERs" and other microstructures in the planetary nebulae NGC 3242, 6826, 7009, and 7662. Most FLIERs have head-tail morphologies, with the tails pointing outward from the nucleus. Ionization gradients that decrease with distance from the nebular center are ubiquitous. These are consistent with an ionization front in neutral knots of density ≈10⁴ cm^-3. Can neutral knots account for the properties of FLIERs? We compare two broad classes of possible explanations for FLIERs with the new images: high-speed bullets ramming through the shells of planetary nebulae, and photoevaporated gas swept by winds into head-tail shapes. Both classes of models fail basic consistency tests. Hence an entirely new conceptual paradigm is needed to account for the phenomenology of FLIERs.

We present a simple, robust mechanism by which an isolated star can produce an equatorial disk. The mechanism requires that the star have a simple dipole magnetic field on the surface and an isotropic wind acceleration mechanism. The wind couples to the field, stretching it until the field lines become mostly radial and oppositely directed above and below the magnetic equator, as occurs in the solar wind. The interaction between the wind plasma and magnetic field near the star produces a steady outflow in which magnetic forces direct plasma toward the equator, constructing a disk. In the context of a slow (10 km s^-1) outflow (10^-5 M_☉ yr^-1) from an asymptotic giant branch star, MHD simulations demonstrate that a dense equatorial disk will be produced for dipole field strengths of only a few Gauss on the surface of the star. A disk formed by this model can be dynamically important for the shaping of planetary nebulae.

A sample of 27 planetary nebulae (PNs) in the Small Magellanic Cloud (SMC) have been observed with the Hubble Space Telescope Imaging Spectrograph (HST/STIS) to determine their morphology, size, and the spatial variation of the ratios of bright emission lines. The morphologies of SMC PNs are similar to those of LMC and Galactic PNs. However, only a third of the resolved SMC PNs are asymmetric, compared to half of those in the LMC. The low-metallicity environment of the SMC seems to discourage the onset of bipolarity in PNs. We measured the line intensity, average surface brightness, and photometric radius of each nebula in Hα, Hβ, [O III] λλ4959 and 5007, [N II] λλ6548 and 6584, [S II] λλ6716 and 6731, He I λ6678, and [O I] λλ6300 and 6363. We show that the surface brightness-to-radius relationship is the same as in LMC PNs, indicating its possible use as a distance scale indicator for Galactic PNs. We determine the electron densities and the ionized masses of the nebulae where the [S II] lines were measured accurately, and we find that the SMC PNs are denser than the LMC PNs by a factor of 1.5. The average ionized mass of the SMC PNs is 0.3 M_☉. We also found that the median [O III]/Hβ intensity ratio in the SMC is about half that of the corresponding LMC median. We use CLOUDY to model the dependence of the [O III]/Hβ ratio on the oxygen abundance. Our models encompass very well the average observed physical quantities. We suggest that the SMC PNs are principally cooled by the carbon lines, making it hard to study their excitation based on the optical lines at our disposal.

New Hubble Space Telescope images and Cerro Tololo Inter-American Observatory echelle spectra of the nebula around the symbiotic Mira Henize 2-104 are presented. The nebula consists of two nested hourglass-shaped structures and a collimated polar jet. Spatiokinematical modeling shows that all three outflows were ejected during approximately the same epoch, namely, about 5700 years ago, with the adopted distance of 4.4 kpc, in a mass-loss event unrelated to the present outburst. We present a tentative model in which the outer pair of lobes and jets are produced by a high-velocity outflow from the white dwarf companion to the Mira, while the inner, slowly expanding lobes are produced by the Mira wind itself.