A new measurement of the Galactic ¹²C/¹³C gradient from sensitive HCO⁺ absorption observations

We present a new constraint on the Galactic 12 C/ 13 C gradient with sensitive HCO + absorption observations against strong continuum sources. The new measurements suffer less from beam dilution, optical depths, and chemical fractionation, allowing us to derive the isotopic ratios precisely. The measured 12 C/ 13 C ratio in the solar neighborhood (66±5) is consistent with those obtained from CH + . Two measurements toward the GC are 42.2±1.7 and 37.5±6.5. Though the values are a factor of two to three higher than those derived from dense gas tracers (e.g., H 2 CO, complex organic molecules) toward Sagittarius (Sgr) B2 regions, our results are consistent with the absorption measurements from c-C 3 H 2 toward Sgr B2 (~40) and those from CH + toward Sgr A* and Sgr B2(N) (>30). We have calculated a new Galactic 12 C/ 13 C gradient of (6.4±1.9) R GC /kpc+(25.9±10.5) and found an increasing trend of the 12 C/ 13 C gradient obtained from high-density to low-density gas tracers, suggesting that opacity effects and chemical fractionation may have a strong impact on the isotopic ratios observed in high-density regions.