Planetary nebulae with close-binary nuclei are most probably ejected at the end of a common-envelope phase, which occurs when the primary star in a wide binary expands and engulfs its companion. Theoretical considerations indicate that the outcome of a common-envelope interaction depends on the evolutionary state of the primary star at the onset of this phase. If the primary is a first-ascent red giant, coalescence of the binary is likely unless the secondary star is fairly massive; if envelope ejection does occur, it will be preferentially in the orbital plane, leading to a high-density contrast (between the equatorial and polar directions) in the resulting planetary nebula. If the common-envelope interaction is delayed until the primary is an asymptotic giant branch star, then coalescence is less likely, and the density contrast in the resulting nebula is lower. The nebular morphology that will be observed is determined by the interaction of a fast wind from the hot primary star with the slowly moving material ejected earlier, and possibly by interaction with the interstellar medium. Thirteen planetary nebulae are known or suspected to have short-period binary nuclei. Ten of the central stars are periodic photometric variables, with orbital periods of 0.11 to 16 days; three are systems containing a rapidly rotating late-type star and a hot component revealed by IUE observations. Definitive orbital periods are not known for any of the latter group (the Abell 35-like systems), but are likely to be short. We present CCD or photographic imagery for nine of these planetary nebulae and provide references for imagery of the remaining four. The morphologies are then discussed in the context of common-envelope ejection. None of the planetaries with close-binary nuclei show the double or multiple shells that are common among planetary nebulae in general. This suggests that multiple shells are ejected during evolutionary phases of single stars that the primary star in a close binary cannot achieve because it encounters its companion first. There is a strong tendency for close-binary nuclei to be found in low surface brightness nebulae, but this could be an observational selection effect. Half of the planetary nebulae with binary nuclei show the butterfly (HtTr 4 and NGC 2346) or elliptical (A41, A65, LoTr 1, and probably DS 1, A63, and LoTr 5) morphologies that are expected for an extreme-to-moderate density contrast produced by binary ejection, followed by shaping by a fast wind. Sp 1, however, is a nearly perfectly round nebula, which we can understand only if it is a toroidal structure seen nearly pole-on. Several nebulae (A46, HFG 1, and A35) are peculiar (in the sense that they do not fit easily into the round-elliptical-butterfly scheme), and we suspect that their morphologies have been heavily modified by interaction with the interstellar medium. K1-2 is extremely peculiar, with evidence for jet ejection from the central binary. The fraction of planetary nebulae with nonspherical morphology (≳80%) is larger than the fraction known to contain close-binary nuclei (∼15%). This shows either that nonspherical nebulae can be ejected from single stars as well as from binaries, or that many central stars are undetected binaries.
All Science Journal Classification (ASJC) codes
- Astronomy and Astrophysics
- Space and Planetary Science