Annu. Rev. Astron. Astrophys. 1981. 19:
77-113 Copyright © 1981 by Annual Reviews. All rights reserved |
We regard the solar neighborhood, somewhat loosely, as the region within which fairly accurate abundances of elements can be determined by stellar spectroscopy, i.e. that within a distance of the order of a kiloparsec. Important topics are (a) the standard abundance distribution in stars and the interstellar medium (ISM) and its uniformity (or otherwise); (b) the age-metallicity relation; (c) the statistical distribution of metallicities in long-lived stars; and (d) abundance anomalies of individual elements or isotopes that are considered as population-related, rather than due to processing in the observed stars themselves.
2.1. Standard Abundance Distribution
Most of our "standard" abundances come from observations within the Solar System (e.g. Cameron 1980), but a comparison with abundances in the ISM and in hot young stars gives an idea of the degree to which solar abundances are truly "cosmic" and provides some insight into the uncertainties in all three sources of data. Recent abundance determinations for some common elements are compared in Table 1, which shows that there is little difference between the Sun, young stars, and the local present-day ISM. The most disturbing discrepancy is that oxygen and nitrogen seem to be underabundant in Orion (and other nearby galactic HII regions) and in the Cygnus Loop SNR by a factor of 2 or so, which needs to be borne in mind when one tries to put together stellar and nebular data in order to study large-scale trends within and between galaxies. The discrepancy in Orion has been discussed by Meyer (1979), who finds that condensation on grains could account for part (but probably not all) of the discrepancy. [There is evidence for a stronger depletion of iron (Peimbert 1979a) and presumably other refractory elements.] Isotope ratios of C, N, O, Si, and S in the local ISM (Penzias 1980, Wannier 1980) differ from solar-system values by factors that are significant in the first three cases, but always less than 2. The interstellar D/H ratio, while on average consistent with the solar-system value, shows large local variations (Vidal-Madjar et al. 1978 and references therein, Laurent, Vidal-Madjar & York 1979).
12+ | 12+ | 12+ | ||||||
log He/H | log O/H | log O/C | log O/N | log O/Ne | log O/S | log Fe/H | ||
Sun | ||||||||
Photosphere a | 8.9 | 0.2 | 0.9 | 1.7 | 7.6 | |||
Prominences b | 10.8 | |||||||
Corona c | 0.9 | 1.1 | ||||||
B stars | 11.0 d | 8.9 e | 0.5 f | 1.0 e | 0.9 g | 7.5 f | ||
Orion Nebula | 11.0 h | 8.6 i | 0.1 i, j | 0.9 h | 0.7 k | 1.3 l | 6.7 m | |
Cygnus Loopn | 8.5 | 0.2 | 0.7 | 1.8 | ||||
Disk planetary | ||||||||
nebulaeo | 8.7 | |||||||