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

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 Ri, which is 1.9 kpc (± 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 Ri, , 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.