The 12 C/ 13 C Ratio in Sgr B2(N): Constraints for Galactic Chemical Evolution and Isotopic Chemistry

Abstract

A study has been conducted of 12 C/ 13 C ratios in five complex molecules in the Galactic center. H 2 CS, CH 3 CCH, NH 2 CHO, CH 2 CHCN, and CH 3 CH 2 CN and their 13 C-substituted species have been observed in numerous transitions at 1, 2, and 3 mm, acquired in a spectral-line survey of Sgr B2(N), conducted with the telescopes of the Arizona Radio Observatory (ARO). Between 22 and 54 individual, unblended lines for the 12 C species and 2–54 for 13 C-substituted analogs were modeled in a global radiative transfer analysis. All five molecules were found to consistently exhibit two velocity components near V LSR ∼ 64 and 73 km s −1 , with column densities ranging from N tot ∼ 3 × 10 14  − 4 × 10 17 cm −2 and ∼2 × 10 13  − 1 × 10 17 cm −2 for the 12 C and 13 C species, respectively. Based on 14 different isotopic combinations, ratios were obtained in the range 12 C/ 13 C = 15 ± 5 to 33 ± 13, with an average value of 24 ± 7, based on comparison of column densities. These measurements better anchor the 12 C/ 13 C ratio at the Galactic center, and suggest a slightly revised isotope gradient of 12 C/ 13 C = 5.21(0.52) D GC + 22.6(3.3). As indicated by the column densities, no preferential 13 C enrichment was found on the differing carbon sites of CH 3 CCH, CH 2 CHCN, and CH 3 CH 2 CN. Because of the elevated temperatures in Sgr B2(N), 13 C isotopic substitution is effectively “scrambled,” diminishing chemical fractionation effects. The resulting ratios thus reflect stellar nucleosynthesis and Galactic chemical evolution, as is likely the case for most warm clouds.