Astron. Astrophys. 326, 139-142 (1997)

Astron. Astrophys. 326, 139-142 (1997)

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

R. Wielen ¹ and T.L. Wilson ²

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

Received 14 January 1997 / Accepted 21 March 1997

Abstract

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 [FORMULA] 10⁹ years. Wielen et al. (1996) have found that the Sun was born at a galactocentric distance which is 1.9 kpc ( 0.9 kpc) smaller than the present galactocentric distance 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 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 O/ O: 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 [FORMULA] , as found by Wielen et al.