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

¹ Institut de Radioastronomie Millimetrique, 300 rue de la Piscine, 38400 Saint-Martin d’Hères, France
² Centro de Astrobiología (CAB), CSIC-INTA, Ctra. de Ajalvir Km. 4, 28850 Torrejón de Ardoz, Madrid, Spain
³ Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Road, Shanghai 200030, China
⁴ Research Center for Astronomical Computing, Zhejiang Laboratory, Hangzhou 311100, China
⁵ Department of Astronomy, Tsinghua University, Beijing 100084, China
⁶ CAS Key Laboratory of FAST, National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, China
⁷ School of Astronomy and Space Science, Nanjing University, Nanjing 210093, China
⁸ Department of Physics, Anhui Normal University, Wuhu, Anhui 241002, China

^★ Corresponding author; luo@iram.fr

Received: 7 July 2024
Accepted: 17 September 2024

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 (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., H₂CO, complex organic molecules) toward Sagittarius (Sgr) B2 regions, our results are consistent with the absorption measurements from c-C₃H₂ toward Sgr B2 (~40) and those from CH⁺ toward Sgr A* and Sgr B2(N) (>30). We have calculated a new Galactic ¹²C/¹³C gradient of (6.4±1.9)R_GC/kpc+(25.9±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.