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

3.2. The Diffuse Interstellar Medium and the Galactic Center

Observations of element abundances in the diffuse interstellar medium and in molecular clouds are subject to depletion and fractionation effects (Spitzer & Jenkins 1975, D.G. York, in preparation), but some evidence on total abundances comes from X-ray absorption measurements (Ryter, Cesarsky & Audouze 1975), which are compatible with (but do not necessarily demand) an abundance gradient reaching an enhancement by a factor of 3 or so at the galactic center. Infrared observations of Sgr A(W) suggest an enhancement of this order in Ne and presumably other heavy elements relative to H (Aitken, Griffiths & Jones 1976). Molecular observations imply that the isotopic ratios ¹³C/¹²C, ¹⁴N/¹⁵N, ¹⁷O/¹⁶O (which are expected to be enhanced by the CNO cycle) increase by small but significant amounts in the order: Solar System, galactic disk (R ± 4 kpc where R is the solar galactocentric radius), galactic center by factors of up to 3 or 4, but without any significant gradient in the galactic disk (Penzias 1980, Wannier 1980).

If, as has been rather naturally supposed in some chemical evolution models (e.g. Talbot & Arnett 1974, Vigroux, Audouze & Lequeux 1976) ¹³C, ¹⁴N and ¹⁷O are secondary nucleosynthesis products from ¹²C and ¹⁶O initially present in the progenitor star, one expects their abundances relative to primary products like ¹²C and ¹⁶O to vary (in first approximation) as the total abundance of primary elements relative to hydrogen, assuming that the efficiency of secondary production is independent of stellar composition. (This assumption actually seems rather doubtful: cf. Sweigart & Mengel 1979, Rocca-Volmerange & Audouze 1979.) The galactic center isotopic abundances conform to this trend (N/O is unknown there), but the isotope and N/O ratios in the disk do not. Taking into account the N/O ratio in other galaxies (see below), several authors (e.g. Smith 1975, Edmunds & Pagel 1978, Alloin et al. 1979) have proposed that a substantial component of nitrogen is a primary product whose yield (relative to oxygen) depends on the age and/or initial mass function of the underlying stellar population and is fairly constant in any one galaxy, but Wannier (1980) argues that a more radical revision of ideas on galactic chemical evolution is needed. We believe that classical secondary production may well be taking place near the centers of our own and some other galaxies (like M81 where evidence for a large N abundance is good; M. Peimbert, in preparation), but that in most other places the evidence favors either a primary process - meaning that ¹³C, ¹⁴N, and ¹⁷O come from the CNO cycle in stars that are already self-enriched in carbon by helium burning and mixing - or a form of secondary processing in which the shortage of "seeds" in low-abundance stars is more or less compensated by an increase in the vigor of mixing processes. (This could also have some relevance to the s-process abundances discussed in Section 2.4.) The many anomalies encountered in globular cluster red giants (e.g. Kraft 1979) suggest that both of these effects may well be operating.

Several other isotope ratios (Penzias 1980, Wannier 1980) are also of interest. D/H has an abundance gradient decreasing inwards, which agrees with cosmological synthesis and a limited degree of destruction by astration; however, the nonzero value at the galactic center implies a modest inflow of unprocessed material or a separate production source (Audouze 1977). ¹⁸O/¹⁶O is enhanced at the galactic center while ¹⁵N/¹⁴N is depleted, both by about a factor 2. Sulphur and silicon isotopes are effectively unchanged.