Spatially Resolved Study of Chemical Abundances in Planetary Nebulae

Martin A. Guerrero
1997 Publications of the Astronomical Society of the Pacific  
We have obtained spatially resolved intermediatedispersion spectroscopy observations for a sample of extended planetary nebulae (PNe) representative of different morphological classes. The study has included multiple shell PNe (M 1-46, NGC 2438, NGC 5882, and NGC 6720), bipolar PNe (K 4-55, M 1-75, NGC 2440, and NGC 2818 and hydrogen-poor PNe (A 30 and A 58). The information obtained has allowed an analysis of the chemical composition gradients in the ionized gas. In particular, we have
more » ... ar, we have specially studied the He/H, N/O, and Ne/O ratios. A comparative study of the chemical abundances and physical properties of the halos and main nebulae of the multiple shell PNe has been carried out. The chemical abundances obtained are in agreement with the theoretical predictions that low-and intermediate-mass star models propose for progenitor stars initially as massive as 1-2.5 Af 0 . In addition, spatially resolved high-dispersion spectroscopy data were obtained for M 1-46 and NGC 6720, thus allowing an analysis of their kinematics and threedimensional structures, and a detailed knowledge of the lossmass history. NGC 6720, despite its ring-like appearance, is an expanding shell which is being destroyed by some kind of instabilities. The kinematics of M 1-46 cannot be explained as a uniform shell expanding at a fixed velocity. Taking the ^TInUrpulM (y 1 *) Fig. 1 -Comparison between the intershell time A and the theoretical time between thermal pulses A The diagonal follows the ratio 1:1. Each point has been labeled with the name of the corresponding PN. available data in the literature for other multiple shell PNe, we find that the kinematical ages calculated for each shell are in agreement with the theoretical predictions for the interpulse time lag (Fig. 1) . It should be noted, however, that the time intershells shows a clear trend to have lower values than the predicted time interpulse. The chemical abundances of the innermost regions of A30 and A58 have been studied in detail in this work, finding that between 75% and 95% of the initial hydrogen have been converted into helium. The central knots of A30 are segregated in two different groups according to their chemical composition. The central region of A58 clearly shows the results of shock excitation. The bipolar PNe show high chemical enrichment of helium and N/O ratio, thus indicating that the progenitor stars were massive intermediate-mass stars (4-8 M©). A noticeable fact is the presence of strong-emission ring-shaped central regions where the high N/O ratios found require a high efficiency of the convective processes which dredge-up processed material on the AGB. There is a clear segregation between the chemical abundances exhibited by bipolar PNe and the central regions of multiple shell PNe (Fig. 2) . He/H Fig. 2 -N/O ratio vs. He/H according to theoretical predictions of enrichment for PNe of Renzini and Voli (1981, A&A, 94, 175) after the second dredge-up process. Empty circles correspond to the values found for the bipolar PNe, and filled circles for the main nebula of the MSPNe. A clear segregation is present between the two morphological groups. 71
doi:10.1086/133862 fatcat:mxmq7t2difaw3o37xjktnvlhwu