13C Isotopic Ratios of HC3N on Titan Measured with ALMA

We present the first determination of the abundance ratios of C-13 substitutions of cyanoacetylene (HC3N), [(HCCCN)-C-13]:[(HCCCN)-C-13]:[(HCCCN)-C-13], in Titan's atmosphere measured using millimeter-wave spectra obtained by the Atacama Large Millimeter/submillimeter Array. To compare the line intensities precisely, data sets that include multiple molecular lines were extracted to suppress effects of Titan's environmental conditions and observation settings. The [(HCCCN)-C-13]:[(HCCCN)-C-13] and [(HCCCN)-C-13]:[(HCCCN)-C-13] ratios were obtained from 12 and 1 selected data sets, respectively. As a result, nearly the uniform [(HCCCN)-C-13]:[(HCCCN)-C-13]:[(HCCCN)-C-13] abundance ratios of 1.17 ( +/- 0.20) : 1.09 ( +/- 0.25) : 1 (1 sigma) were derived, whereas previously reported ratios for the interstellar medium (ISM) have shown large anomalies that may be caused by C-13 concentrations in precursors. The result obtained here suggests that C-13 concentration processes suggested in the ISM studies do not work effectively on precursors of HC3N and HC3N themselves owing to Titan's high atmospheric temperature and/or depletion of both C-13 and C-13+.