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

Abstract

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$\pm$5) is consistent with those obtained from CH${}^{+}$. Two measurements toward the Galactic Center are 42.2$\pm$1.7 and 37.5$\pm$6.5. Though the values are a factor of 2$\sim$3 higher than those derived from dense gas tracers (e.g., H${}_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${}^{\ast }$ and Sgr B2(N) ($>$30). We calculate a new Galactic ${}^{12}$C/${}^{13}$C gradient of (6.4$\pm$1.9)$R_{\mathrm{GC}}$/kpc+(25.9$\pm$10.5), and find an increasing trend of ${}^{12}$C/${}^{13}$C gradient obtained from high-density to low-density gas tracers, suggesting opacity effects and chemical fractionation may have a strong impact on the isotopic ratios observed at high-density regions.

11 pages, 6 figures, accepted for publication in A&A