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Heavy Element Abundances in Ionized Nebulae

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Heavy Element Abundances in Ionized Nebulae H e a v y E l e m e n t A b u n d a n c e s in I o n ized N e b u l a e loannis Tsamis A thesis submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy at the University of London University College London University of London UCL 2002 ProQuest Number: U641824 All rights reserved INFORMATION TO ALL USERS The quality of this reproduction is dependent upon the quality of the copy submitted. In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion. uest. ProQuest U641824 Published by ProQuest LLC(2015). Copyright of the Dissertation is held by the Author. All rights reserved. This work is protected against unauthorized copying under Title 17, United States Code. Microform Edition © ProQuest LLC. ProQuest LLC 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106-1346 A(^iepov Tm, TiTovç joi>SL(; pov Teuj'pyto nai Qeone pTrrp^ y ta TO (£LU ceAAa KaL t o e v (^Siv. Etti ap<; g t o v aôep^po pov Mavuj \p pe evxs Ç yicn pia K,aXp aTaSioSpopi a ! Alla Silvia per tutto il sostegno e ramore. Et77 pup pp Tpç Tiayiaç. A b s t r a c t Optical recombination-line (ORL) abundances for carbon, nitrogen, and oxygen, relative to hydrogen, are presented for 12 Galactic planetary nebulae (PN) and 3 Magellanic Cloud PN (LMC N141, LMC N66 & SMC N87) and an extensive comparison with the corresponding abundances derived from UV {lUE), optical and IR {IRAS, ISO) collisionally-excited lines (CELs) is performed. Two extreme nebulae, NGC2022 and LMC N66, are exposed that exhibit very discordant - ORL versus CEL - abundances; by a factor of > 10 for the O^"*" ion. The Type I PN NGC2440 is revealed to have discrepancy factors similar to those of the pre­ viously studied NGC7009 and M 2-36. Along with previous results it is shown that ORL/CEL ionic abundance enhancements differ among nebulae, spanning a range from ~ 1.5 to > 20. There are indications that the C /0, N /0 elemental ra­ tios derived from ORLs may be larger that the corresponding CEL ratios, hinting at an origin for the two types of line from nebular material of dissimilar chemical history. The abundance enhancements of doubly ionized C, N and O are positively correlated with the difference (AT) between the Baimer recombination continuum and [O III] forbidden-line temperatures, suggesting that temperature variations, real or induced, are partly to blame for the discrepancies. The relative unifor­ mity, however, of the overabundance patterns and lack of consistent correlation with CEL excitation energies point away from Peimbert-type, simple temperature fluctuations as the cause of the problem. Recent results by Garnett & Dinerstein (2000) are confirmed, showing that the abundance discrepancies are: i) anticor­ related with the intrinsic nebular surface brightness, and ii) positively correlated with PN absolute radii, i.e. young, bright nebulae display less abundance discrep­ ancies than older, more extended ones. It is further shown however, that very sim­ ilar correlations exist between AT and the nebular radii and surface brightnesses, suggesting that fainter, more extended objects are more likely to have temperature discrepancies than brighter, smaller ones. These findings strongly indicate that the long-standing ORL/CEL abundance discrepancy problem is associated with the evolution of PN. Optical spectra of five H II regions, two within the Galaxy (M 17 and NGC 3576) and three in the Magellanic Clouds (30 Doradus, LMC N llB and SMC N66) were analyzed. The ORL/CEL discrepancy factors for the 0^+ ion are found for the first time to be in the range of 2-5, thus placing these objects in the abundance discrepancy regime of PN. For the first time also ORL/CEL discrepancies are doc­ umented for extragalactic Hll regions. Accurate, temperature-insensitive ORL C2+ /o 2+ ratios are derived for all five nebulae, showing remarkable agreement within each galactic system. A cknowledgements I’d like to thank Mike Barlow for taking me on this project, ensuring that it was not left unfinished by sustaining me through my fourth year and then declaring that my thesis is a joy to read (I leave that to the reader); also for making possible the trip to ESO’s mountaintop on La Silla, Chile, where I checked for myself the round Earth hypothesis by observing the southern sky.. .only I forgot to put that in my thesis. He is also thanked for advice on the analysis of H II regions spectra and for other bits of astronomical information. Mike in short is a European student’s ideal supervisor, mainly because he won’t let you give up, once your studentship evaporates right on time when you thought you’ve got a thesis at last. I would therefore strongly recommend Mike in case he wanted to become Departmental chairman, provost or European Commissioner (since he’s not a euro-sceptic). Next I’d like to thank Xiaowei Liu and Pete Storey; the former mainly for the loan of his customized MIDAS line-fitting routine that made my life a lot simpler and the latter for letting me use his nebular optimization code that made Chapter 3 possible. Paul Crowther is also thanked for advice on the spectral characteristics of OB and WR stars. I hope all the people in Physics & Astronomy who did their PhDs at roughly the same time when I did mine, go on to do something even more rewarding after it’s all over; and that the Perren Fund continues as a financial support for non-UK students. Silvia wants to be acknowledged too, so let me say it here that I have no pity for her would be thesis advisor in Guildford whose project she dumped (after having pocketed his 1st semester grants in a typical Italian way) and so she came to cool atoms in UCL and to meet me. These last 2.5 years in London wouldn’t have been so cool - and hot! - otherwise. Finally, and most importantly, I thank my family in Thessaloniki, Greece for their love and support through all this experience. C o n t e n t s List of Figures 8 List of Tables 10 1 Introduction 13 1.1 Physical processes in photoionized nebulae ........................................... 14 1.1.1 Photoionization ................................................................................ 14 1.1.2 Radiative recombination and line emission ............................... 17 1.1.3 Dielectronic recombination ......................................................... 19 1.1.4 Collisionally excited line emission ................................................ 19 1.2 Determination of electron densities and temperatures in nebulae . 22 1.2.1 Ne, Te determinations from CELs ................................................ 22 1.2.2 Ne, Te determinations from HI Baimer lines and optical con­ tinuum measurements.................................................................. 26 1.3 Determination of elemental abundances in nebulae ......................... 28 1.3.1 Derivation from observations of C E L s ...................................... 28 1.3.2 Derivation from observations of O R L s ...................................... 29 1.3.2.1 H e liu m ........................................................................... 29 1.3.2.2 Heavier elements: Carbon, Nitrogen, Oxygen, and N e o n ............................................................................... 30 CONTENTS 1.3.3 Temperature and density variations in nebulae: implications on abundance determinations .................................................... 31 1.4 Interstellar extinction ............................................................................... 35 1.4.1 The derivation of c (H /3 ) .............................................................. 38 1.5 Aims of this thesis and outline ............................................................... 38 2 Elemental Abundances in Galactic and Magellanic Cloud Plane­ tary Nebulae 40 2.1 Optical Observations .................................................................................. 40 2.1.1 D ata re d u c tio n ............................................................................... 43 2.2 The lUE observations ............................................................................... 47 2.3 D ata a n a ly sis ............................................................................................... 48 2.4 Nebular A n aly sis ........................................................................................ 52 2.4.1 Reddening ......................................................................................... 52 2.4.2 Nebular Diagnostics ..................................................................... 54 2.4.2.1 Electron D en sities ....................................................... 55 2.4.2.2 Electron Temperatures .............................................. 57 2.4.2.3 Recombination excitation of the N II and O II au­ roral lines ........................................................................ 58 2.4.2.4 Baimer discontinuity electron temperatures .... 63 2.5 Elemental abundances from CELs ........................................................ 64 2.6 Ionic Abundances from O R L s .................................................................. 71 2.6.1 Helium ............................................................................................ 72 2.6.2 C2+/H+, C3+/H+ and C^+/H+ .................................................. 72 2.6.3 N2+/H+, N^+/H+ and /E + .................................................. 76 2.6.4 0 2 + /H + ...........................................................................................
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