The chemical compositions of gaseous nebulae

Abstract

Abundances of chemical elements in gaseous nebulae yield important clues to late stages of stellar evolution, as in planetary nebulae, supernova remnants, and nova ejects, and to the chemical evolution of galaxies as revealed by H n regions in our own and other stellar systems. In this article a description is given of the methods whereby quantitative chemical analyses of nebulae may be attempted. Planetary nebulae offer engaging opportunities because they often show symmetrical structures, rich spectra, and well-defined progenitor stars. The rich UV radiation from the central stars excites their spectra by processes that appear to be well understood. Analyses of supernova remnants are more difficult because of geometrical complexities and poorly defined boundary conditions, while lack of spatial resolution hinders studies of QSOs and active galactic nuclei.Many of the physical processes involved and required atomic parameters are common to all types of gaseous nebulae. In this review we describe methods of plasma diagnostics, i.e., determinations of electron density and temperature, evaluate the role of theoretical nebular models, and assess sources of error accruing from fluctuations in the electron density and temperature throughout the emitting volume. The irregularities of most gaseous nebulae impose severe limitations on modeling consequently, in many instances such models are most useful for estimating the concentration of ions in unobservable stages. The importance of extending the observed spectral range by infrared and UV measurements is emphasized. Individual elements such as He, C, N, O, Ne, S, Ar, and Cl offer their own special problems. Much attention has been paid to He because of its importance to cosmology. A final section discusses the relationship of gaseous nebular composition studies to stellar evolution problems.

The 3-D ionization structure of NGC 6818: A Planetary Nebula threatened by recombination

“…5 T e[N II] refers to the intensity peaks of the low ionization regions and is systematically below T e[O III], in agreement with both the previous reports for NGC 6818 (9500 K, Hyung et al 1999, to 11 280 K, McKenna et al 1996 and the general results for PNe (see Aller 1990;Gruenwald & Viegas 1995;Mathis et al 1998 Keenan et al 1999).…”

The relation between elemental abundances and morphology in planetary nebulae

“…Similarly, although the range of variation in O/H [ log(O/H) ∼ 0.9 dex] is perhaps the smallest of all of the elements considered here, the ratio log(O/H)/σ R is of the order of ∼5. (1983); A87: Aller & Keyes (1987); A90: Aller (1990); B78: Barker (1978); B83: Barker (1980); B86: Barker (1986); CO96: Costa et al (1996); CO97: Costa et al (1997); C96: Cuisinier, Acker & Koppen (1996) (1996); H: Hromov & Kohoutek (1968); C: Corradi & Schwarz (1995); C93: ; Z: Zuckerman & Aller (1986). The bottom line appears to be that observed trends in abundance are likely to be real, and not too much affected by observational uncertainties and differences between analyses.…”

High-resolution spectroscopy of globular cluster post-Asymptotic Giant Branch stars

“…However, such estimates may not provide reliable chemical compositions since: (i) much of the ejected matter is locked up in grains (especially the refractory elements Si, Fe etc.) and is not detectable via emission line spectroscopy, and (ii) there will be unobserved ionization stages in any nebular spectrum which may lead to underestimates of the total element abundances (Aller 1990).…”