SIMBAD references

Many planetary nebulae (PNs) are known to possess halos, the origins of which are ill determined. We suggest that several of the brighter halos, at least, are likely to arise through contraction of ionization fronts within the nebular shell, a situation that may arise where central star temperatures and luminosities decline at intermediate phases of PN evolution. We show that such halos may be long-lived and possess appreciable levels of emission for periods of ∼10³ yr or so. We also find that several well-known halo sources possess properties consistent with this hypothesis and that two further sources may have developed halos as a result of late helium flash.