Abundance Determinations In HII Regions And Planetary Nebulae

The methods of abundance determinations in H ii regions and planetary nebulae are described, with emphasis on the underlying assumptions and inherent problems. Recent results on abundances in Galactic H ii regions and in Galactic and extragalactic Planetary Nebulae are reviewed. Introduction H ii regions are ionized clouds of gas associated with zones of recent star formation. They are powered by one, a few, or a cluster of massive stars (depending on the resolution at which one is working). The effective temperatures T * of the ionizing stars lie in the range 35 000 – 50 000 K. The nebular geometries result from the structure of the parent molecular cloud. Stellar winds, at evolved stages, may produce ring-like structures, but the morphology of H ii regions is generally rather complex on all scales. Typical hydrogen densities n are 10 3 – 10 4 cm –3 for compact H ii regions. The average densities in giant extragalactic H ii regions are lower, typically 10 2 cm –3 since giant H ii regions encompass also zones of diffuse material. The total supply of nebular gas is generally large, so that all (or at least a significant fraction) of the ionizing photons are absorbed. Planetary nebulae (PNe) are evolutionary products of so-called intermediate mass stars (initial masses of 1 – 8 M⊙) as they progress from the asymptotic giant branch (AGB) to the white dwarf stage.