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

A study has been conducted of ¹²C/¹³C ratios in five complex molecules in the Galactic center. H₂CS, CH₃CCH, NH₂CHO, CH₂CHCN, and CH₃CH₂CN and their ¹³C-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 ¹²C species and 2-54 for ¹³C-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 × 10¹⁴ -4 × 10¹⁷ cm^-2 and ∼2 × 10¹³ -1 × 10¹⁷ cm^-2 for the ¹²C and ¹³C species, respectively. Based on 14 different isotopic combinations, ratios were obtained in the range ¹²C/¹³C = 15 ±5 to 33 ±13, with an average value of 24 ±7, based on comparison of column densities. These measurements better anchor the ¹²C/¹³C ratio at the Galactic center, and suggest a slightly revised isotope gradient of ¹²C/¹³C = 5.21(0.52) D _GC + 22.6(3.3). As indicated by the column densities, no preferential ¹³C enrichment was found on the differing carbon sites of CH₃CCH, CH₂CHCN, and CH₃CH₂CN. Because of the elevated temperatures in Sgr B2(N), ¹³C 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.