Astronomy and Astrophysics – Astrophysics
Scientific paper
Jul 1980
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1980apj...239...78k&link_type=abstract
Astrophysical Journal, Part 1, vol. 239, July 1, 1980, p. 78-88.
Astronomy and Astrophysics
Astrophysics
87
Abundance, Gas Composition, Hydrogen, Interstellar Gas, Milky Way Galaxy, Oxygen, Planetary Nebulae, Galactic Evolution, Gas Ionization, Radial Velocity, Stellar Evolution, Tables (Data)
Scientific paper
Oxygen-to-hydrogen ratios are calculated for the 97 planetary nebulae for which there are sufficient data and for which the correction for unobserved ionization states is not too large. The ratios are strongly correlated with population type. Almost all planetaries with absolute LSR radial velocity greater than about 85 km/s and distance from the galactic plane greater than 1 kpc have low O/H; high values of O/H are confined to the galactic disk. The radial gradient within the disk is not easily discerned from the planetaries. An apparent gradient is seen because the planetaries with distances from the galactic center greater than about 11 kpc are dominantly halo objects with high velocity or large distance from the plane. The planetaries are divided into relative age groups which depend upon the nebular velocities, distances from the galactic plane, excitation levels, and He/H ratios. The mean O/H for each group increases with decreasing age. The available evidence shows that this effect is not due to nuclear processing in individual stars and is consequently an effect of galactic evolution. Galactic O/H has increased by a factor of between 5 and 10 from the time of the formation of the early halo to the present, consistent with earlier studies. The relative oxygen enrichment rate of the Galaxy is characterized by a large early increase from the old to the intermediate halo, followed by a smaller but steady relative enrichment rate among the identified age groups.
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