SIMBAD references

Large values of the Hα/Hβ line ratio are preferentially found in planetary nebulae (PNe) with large abundance discrepancy factors (ADFs) for O+2. This connection can be understood if there is a cold component in these objects. An equation relating the ADF with Hα/Hβ was derived from a simple two-component model. This equation involves the density, temperature and mass of the hot and cold components, as well as their relative abundance of O+2. Sensible solutions were found in 12 of the 15 PNe considered in this paper. Solutions where the mean H+ temperature is closer to the Balmer jump temperature and/or the cold component temperature is closer to temperatures found from O+2 recombination line ratios, almost always imply an O+2 abundance that is a few times larger in the cold component, a cold-to-hot H+ mass ratio between 0.15 and 2.01 and a cold component density close to 100/ cm3.