Annu. Rev. Astron. Astrophys. 1981. 19: 77-113
Copyright © 1981 by . All rights reserved

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3. LARGE-SCALE TRENDS WITHIN THE GALAXY

3.1. Abundance Gradients in Stars and the Interstellar Medium

Various investigations of stars, HII regions, and planetary nebulae (Table 2) indicate the existence of a radial gradient in metallicity of the order of -0.05 dex kpc-1 in the neighborhood of the Sun (see Peimbert 1979b for a discussion and earlier references). The gradient is fairly similar for oxygen, nitrogen, and metals, except in the case of planetary nebulae, for which nitrogen shows a significantly steeper gradient than oxygen (cf. Danziger et al. 1978), but the situation is complicated by the existence of planetaries of different types involving various degrees of self-enrichment; the relation between N and O abundances will be discussed further below. Most of the gradients are in fair agreement with one another, but there is disagreement on their age-dependence (if any) and on whether the logarithmic gradient is constant or flattens off towards the Galactic center, which would correspond to an approximately linear trend in the actual abundances within a few kpc of the solar galactic radius (cf. Pagel 1979b). Supergiants give larger abundances and larger gradients than do the other objects (Luck & Bond 1980) but they have been omitted from the table because of the possibility that their atmospheric compositions are affected by mixing with the interior (Luck & Lambert 1980). A further complication is that Talent & Dufour(1979) deduce significant abundance changes along each of the Sagittarius, Local, and Perseus arms, corresponding to a radial gradient of about -0.2 dex kpc-1 in any one arm - a result for which it is difficult to think of any theoretical explanation. Grenon (1972) and Janes (1975) have discussed the complications introduced by the radial gradient into the relationship between compositions and kinematic properties of disk population stars. A gradient at right angles to the galactic plane also exists (e.g. Trefzger 1981); it is of the order of -0.6 dex kpc-1 near the plane, but it is difficult to sort out age and purely height-dependent effects (Mayor 1976).

Table 2. Local abundance gradients in the galactic disk

Objects -partial[Fe/H]/partialR -partial log(0/H)/partialR -partial log(N/H)/partialR Ref.


       
Young disk stars        
     (dF + dG) 0.10 ± 0.02      
Old disk stars 0.04 ± 0.03     Mayor 1976
All Stars 0.05 ± 0.01      
gK and open clusters 0.05 ± 0.01     Janes 1979
Cepheids (from periods)   0.08:   Peimbert 1979b

       
HII regions   0.06 ± 0.02 0.08 ± 0.02 Talent & Dufour 1979
     (optical spectra)   0.13 ± 0.04 0.23 ± 0.06 Peimbert et al. 1978
    0.05 ± 0.03 0.10 ± 0.03 Hawley 1978
HII regions        
     (electron temperature)   0.05:   Mezger et al. 1979
Planetary nebulae       Torres-Peimbert &
     (optical spectra)   0.06 ± 0.02 0.18 ± 0.04 Peimbert 1977

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