DISPERSAL AND MIXING OF OXYGEN IN THE INTERSTELLAR-MEDIUM OF GAS-RICHGALAXIES

Stellar and nebular abundance indicators reveal that there exists sign ificant abundance fluctuations in the interstellar medium (ISM) of gas -rich galaxies. It is shown that at the present observed solar level o f O/H similar to 6 10(-4), abundance differences of a factor of two, s uch as existing between the Sun and the nearby Orion Nebula, are many times larger than expected. We examine a variety of hydrodynamical pro cesses operating at scales ranging from 1 pc to greater than 10 kpc, a nd show that the ISM should appear better homogenized chemically than it actually is: (i) on large galactic scales (1 greater than or equal to l greater than or equal to 10 kpc), turbulent diffusion of interste llar clouds in the shear flow of galactic differential rotation is abl e to wipe out azimuthal O/H fluctuations in less than 10(9) yr; (ii) a t the intermediate scale (100 greater than or equal to l greater than or equal to 1000 pc), cloud collisions and expanding supershells drive n by evolving associations of massive stars, differential rotation and triggered star formation will re-distribute and mix gas efficiently i n about 10(8) yr; (iii) at small scales (1 greater than or equal to l greater than or equal to 100 pc), turbulent diffusion may be the domin ant mechanism in cold clouds, while Rayleigh-Taylor and Kelvin-Helmhot z instabilities quickly develop in regions of gas ionized by massive s tars, leading to full mixing in less than or equal to 2 10(6) yr. It i s suggested that the relatively large O/H fluctuations observed in lar ge disk galaxies may be due to retention, in sites favored by triggere d star formation, of freshly enriched ejecta from SNR and supershells expanding in a differentially rotating disk, plus, possibly, infall of low metallicity material from individual clouds like high velocity cl ouds which splash on the disk on timescales shorter than the local mix ing time. In low-mass galaxies, stimulated star formation is much less efficient, and the most effective mixing mechanisms are absent; the e scape of newly enriched material due to galactic winds powered by the starburst events, the lack of large-scale stirring, and the long dorma nt phase between successive star forming episodes make possible the su rvival of large abundance discontinuities.