On the helium and nitrogen enrichment of the interstellar medium by planetary nebulae.

To study the contribution from stars in the 1 < M/M⊙ < 5 range to the enrichment of the interstellar medium a discussion of the various types of planetary nebulae (PN) is made. From observations of PN in the Galaxy and in the Magellanic Clouds it is found that PN of Type I comprise about 20% of the total number of PN. Type II PN delineate well defined abundance gradients across the galactic disk. Type I PN enrich significantly the interstellar medium with freshly made helium while Type II PN present the He/H abundance ratios with which they were formed. Evidence is presented that suggests that Type I PN are more massive than Type II PN and that the dividing line between them corresponds to objects of 2.4 M⊙.