Isotopes in the interstellar medium and circumstellar envelopes

Measurements of the present-day abundances of elements and isotopes, combin ed with model calculations, allow us to trace the history of nucleosynthesi s in the universe. Throughout this review, emphasis will be placed on descr iptions of the measurement processes and the interpretations needed to obta in actual isotope and element abundances from measurements. Comparisons of the abundances of isotopomers of a given element are less affected by syste matic effects than are comparisons of the abundances of different elements. Thus ratios of isotopomers should be given a greater weight when data and models are compared. As is generally accepted, the universe began with an explosive event, the B ig Bang. The nucleosynthesis associated with this event produced 'primordia l' abundances of the 'light elements', deuterium, He-3, He-4, and Li-7. Sub sequent stellar processing of the light elements has altered the relative a bundances, and also produced heavier elements such as carbon, nitrogen and oxygen. Stellar nucleosynthesis products from solar and larger mass stars a re expelled into the interstellar medium (ISM). The goal of studies of the abundances of the light elements is to estimate the primordial abundances, that is, the abundances produced in the Big Bang. It is believed that D is always net destroyed in stars; He-3 and Li-7 may be net produced, He-4 is c ertainly net produced. In the Solar System itself, results are obtained fro m in situ measurements with space probes to Jupiter, measurements of solar wind constituents, the analysis of the content of meteorites, and spectral line measurements of the solar photosphere. For sources outside the Solar S ystem, these data are based on spectral line measurements of gas-phase spec ies. The ratio of gas-phase abundances of elements, such as carbon to lithi um may be affected by differing amounts of condensation onto dust grains; h owever such a process will not affect the ratio of isotopes such as Li-6/Li -7. The most reliable measurements of D to H ratios are based on spectroscopic measurements of Lyman series ultraviolet absorption lines from foreground i nterstellar gas. Measurements of clouds in our galaxy have been obtained wi th satellites such as the International Ultraviolet Observatory, Copernicus , and the Hubble Space Telescope. The most interesting new development is t he measurement of distant clouds with large redshifted velocities. Such dat a can be taken with Earth-bound optical telescopes. In the near future the Far Ultra Violet Explorer will refine and extend measurements of D/H ratios in relatively nearby regions. Abundances of He-3 in the ISM have been measured using the hyperfine transi tion of He-3(+) in galactic H It regions which are ionized by high-mass sta rs. He-4 is the most abundant of the light elements. The primordial abundance m ust be very accurately determined if one wishes to use this quantity to est imate the baryon density in the early universe. Recently He-4/H ratios have been measured in a number of metal-poor compact blue galaxies. These sourc es seem to have had little stellar evolution, so the ratio should be close to the primordial value. Estimates of the primordial abundance of Li-7 are made for a population of old stars found far from the plane of our galaxy. A refinement of Li abunda nce estimates requires a more detailed understanding of the Li destruction processes in stars.