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Article Contents

ABSTRACT

INTRODUCTION

1.BASIC PHYSICS OF PHOTOIONIZED NEBULAE

1.1.Ionization and recombination

1.1.1.Global ionization budget

1.1.2.The ionization structure

1.2.Heating and cooling

1.3.Line intensities

2.BASICS OF ABUNDANCE DETERMINATIONS IN IONIZED NEBULAE

2.1.Empirical methods

2.1.1.Direct methods

2.1.2.Strong line or statistical methods

2.2.Model fitting

2.2.1.Philosophy of model fitting

2.2.2.Photoionization codes

3.MAIN PROBLEMS AND UNCERTAINTIES IN ABUNDANCE DETERMINATIONS

3.1.Atomic data

3.1.1.Ionization, recombination and charge exchange

3.1.2.Transition probabilities, collision strengths and effective recombination coefficients

3.2.Stellar atmospheres

3.3.Reddening correction

3.4.Aperture correction, nebular geometry and density inhomogeneities

3.5.Spatial temperature variations

3.5.1.Temperature gradients

3.5.2.Small scale temperature variations

3.6.The optical recombination lines mystery

3.7.The role of internal dust

3.7.1.Evidence for the presence of dust in the ionized regions

3.7.2.Heavy element depletion

3.7.3.The effect of dust on the ionization structure

3.7.4.The effect of dust obscuration on the emission line spectrum

3.7.5.The effects of grains on heating and cooling of the gas

3.8.The specific case of the helium abundance determination

4.OBSERVATIONAL RESULTS ON ABUNDANCES IN H II REGIONS OF THE MILKY WAY

4.1.The Orion nebula: a benchmark

4.2.Abundance patterns in the solar vicinity and the solar abundance discrepancy

4.3.Abundance gradients in the Galaxy from H II regions

4.4.The Galactic center

4.5.Nebulae around evolved massive stars

5.OBSERVATIONAL RESULTS ON ABUNDANCES IN PLANETARY NEBULAE

5.1.NGC 7027 and IC 418: two test cases

5.2.What do PN abundances tell us?

5.3.PNe as probes of the chemical evolution of galaxies

5.3.1.The universal Ne/H versus O/H relation

5.3.2.Abundance gradients from PNe in the Milky Way

5.3.3.PNe in the Galactic bulge

5.3.4.PNe in the Galactic halo

5.3.5.PNe probe the histories of nearby galaxies

5.4.PNe probe the nucleosynthesis in their progenitor stars

5.4.1Global abundance ratios

5.4.2.Abundance inhomogeneities

REFERENCES