Planetary Nebulae and Nitrogen Enrichment in the Galaxy

Abstract

All stars in the mass interval 1-4Mʘprobably evolve through a double shell source phase before losing their entire envelopes through stellar wind and the ejection of planetary nebula shells. The remnant carbon-oxygen core rapidly evolves to a blue nucleus, illuminates the surrounding nebula for a few tens of thousands of years and finally cools off as a white dwarf (Paczynski, 1970, Harm and Schwarzschild 1975). A large fraction of the mass contained in the main-sequence stars of mass 1-4Mʘthus returns to the interstellar medium. A common feature of the double shell source (DSS) evolution of stars is the occurrence of He shell flashes. Evolutionary studies by Schwarzschild and Harm (1967), Iben (1975) and others show that the flash-driven convection zone carries helium burning products towards the hydrogen-rich layers. The consequences of mixing between the outer convective envelope and the intershell region have received a great deal of attention in recent years particularly in connection with the interpretation of carbon stars. If a deep temporary convection zone exists extending from the surface of the star to a point near the helium burning shell and mixing is allowed to take place for sufficiently long time Sackmann, Smith, and Despain (1974) found that the flash-produced12C underwent further ON0 processing enriching the surface of the star in12N. Iben (1976) has also speculated on the possibility of surface enrichment of14N during the DSS evolution of intermediate-mass stars. The subsequent loss of this envelope as a planetary nebula shell can thus cause nitrogen enrichment of the interstellar gas. In the present work we have evaluated the extent of this enrichment and have also derived the gradient of nitrogen abundance in the disc of the Galaxy based on the simple model of galactic evolution due to Talbot and Arnett (1973, hereinafter TA).