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Astron. Astrophys. 326, 139-142

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The evolution of the C, N, and O isotope ratios from an improved comparison of the interstellar medium with the Sun

R. Wielen1 and T.L. Wilson2

1Astronomisches Rechen-Institut, Mönchhofstrasse 12-14, D-69120 Heidelberg, Germany
2Max-Planck-Institut für Radioastronomie, Postfach 2024, D-53010 Bonn, Germany

Received 14 January 1997 / Accepted 21 March 1997


We present an improved comparison of the C, N, and O isotope ratios between the Sun and the interstellar medium, in which the birth-place of the Sun is taken into account. Such a comparison gives empirical information about the time evolution of the isotope ratios in the interstellar medium over the last 4.5.109 years. Wielen et al. (1996) have found that the Sun was born at a galactocentric distance FORMULA which is 1.9 kpc ( FORMULA 0.9 kpc) smaller than the present galactocentric distance R0 of the Sun. Therefore, we use the values of the present-day isotope ratios of the interstellar medium at this galactocentric distance FORMULA , instead of the conventional approach using local values at R0 for the comparison with the solar system. For three isotope ratios, the improved comparison produces results which are in better agreement with theoretical predictions than the results of a conventional comparison. The most important improvement is found for 16O/18O : This isotope ratio is found to decrease with time if the birth-place of the Sun is taken into account. The better agreement of the evolution in time of isotope ratios derived from our improved comparison with the theoretical expectations provides supporting evidence that the birth-place of the Sun is closer to the galactic center than R0, as found by Wielen et al.

Key words: interstellar medium: abundances - Sun : abundances - Galaxy: abundances - Galaxy: evolution

Send offprint requests to: R. Wielen

© European Southern Observatory (ESO) 1997

Online publication: September 9, 1997
Last change: April 20, 1998