606 MASS EJECTION FROM OLD STARS - AN EXAMPLE OF A POSSIBLE MECHANISM M.P. FitzGerald Department of Physics, University of Waterloo, Ontario, Canada S. Kwok and C. R. Pruton CRESS, York University, Toronto, Ontario, Canada The basic ideas in this paper are described in recently published papers by Ahern et al. (1977) and Kwok (1977). There are two main points to be made in addition, which remove the differences between these two papers. 1) If smaller dust grains are used, say of 0.1 ]i instead of the 1 ^ grains of Kwok, then the dust is more efficient in absorbing the uv radiation from the star. While we have not carried out detailed calculations yet, we believe that such a medium will produce a cut-off to the HII region in a p^r density distribution, so that Kwok's previously ejected nebula is not necessary, provided the star ejected material moderately uniformly for a period of say 10,000 years. For a total mass ^ 0.3 M & , it appears this model will fit the detailed grain calculations and the radio outer cut off well. As time goes on the inner radius expands, along with the whole nebula which becomes more ionized. Eventually we can expect, after say a further 10,000 years, to see a fully fledged, ionized, planetary, whereas at present we see all the dust, but only a small fraction (^0.02 Mg) of the gas, that is the ionized component. 2) From the work of Cahn and Wyatt (1976) and also Wood and Cahn (1977) we see that the birth rate and death rate of planetaries, the death rate of stars with 1M 0 <M< 5K the production rate of white dwarfs, and the death rate of Miras are all of the order of -3 -3 -1 3 X 10 kpc yr . Hence we expect to see about 1 planetary formed every 10 years within "x> 5 kpc of the sun. V1016 Cygni and HM Sge are suitable candidates and understanding the physics for them could be a major clue to understanding mass ejection. Our conclusion is that the V1016 Cygni phenomena is representative of the way a planetary nebula is formed. We may summarize it as follows: 1. Mass ejection at i* 3 X 10 M yr for ^ 10 years of a dusty envelope from an LPV. This gives a density distribution of of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0252921100069736 Downloaded from https://www.cambridge.org/core. IP address: 207.90.37.89, on 12 Jun 2019 at 11:22:07, subject to the Cambridge Core terms