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

Abstract

We present a new constraint on the Galactic 12C/13C 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 12C/13C ratio in the solar neighborhood (66pm 5) is consistent with those obtained from CH$^+$. Two measurements toward the GC are 42.2pm 1.7 and 37.5pm 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 (sim 40) and those from CH$^+$ toward Sgr A$^*$ and Sgr B2(N) ($>$30). We have calculated a new Galactic 12C/13C gradient of (6.4pm 1.9)$R_ GC $/kpc+(25.9pm 10.5) and found an increasing trend of the 12C/13C 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.